Part I
The present paper reports the beginnings of an investigation designed to throw light on the psychological basis of our æsthetic pleasure in unequal division. It is confined to horizontal division. Owing to the prestige of the golden section, that is, of that division of the simple line which gives a short part bearing the same ratio to the long part that the latter bears to the whole line, experimentation of this sort has been fettered. Investigators have confined their efforts to statistical records of approximations to, or deviations from, the golden section. This exalts it into a possible æsthetic norm. But such a gratuitous supposition, by limiting the inquiry to the verification of this norm, distorts the results, tempting one to forget the provisional nature of the assumption, and to consider divergence from the golden section as an error, instead of another example, merely, of unequal division. We have, as a matter of fact, on one hand, investigations that do not verify the golden section, and, on the other hand, a series of attempts to account for our pleasure in it, as if it were, beyond dispute, the norm. In this way the statistical inquiries have been narrowed in scope, and interpretation retarded and misdirected. Statistically our aim should be to ascertain within how wide limits æsthetically pleasing unequal divisions fall; and an interpretative principle must be flexible enough to include persistent variations from any hypothetical norm, unless they can be otherwise accounted for. If it is not forced on us, we have, in either case, nothing to do with the golden section.
Since Fechner, the chief investigation in the æsthetics of simple forms is that of Witmer, in 1893.[1] Only a small part of his work relates to horizontal division, but enough to show what seems to me a radical defect in statistical method, namely, that of accepting a general average of the average judgments of the several subjects, as 'the most pleasing relation' or 'the most pleasing proportion.'[2] Such a total average may fall wholly without the range of judgments of every subject concerned, and tells us nothing certain about the specific judgments of any one. Even in the case of the individual subject, if he shows in the course of long experimentation that he has two distinct sets of judgments, it is not valid to say that his real norm lies between the two; much less when several subjects are concerned. If averages are data to be psychophysically explained, they must fall well within actual individual ranges of judgment, else they correspond to no empirically determinable psychophysical processes. Each individual is a locus of possible æsthetic satisfactions. Since such a locus is our ultimate basis for interpretation, it is inept to choose, as 'the most pleasing proportion,' one that may have no correspondent empirical reference. The normal or ideal individual, which such a norm implies, is not a psychophysical entity which may serve as a basis of explanation, but a mathematical construction.
[1] Witmer, Lightner: 'Zur experimentellen Aesthetik einfacher räumlicher Formverhältnisse,' Phil. Studien, 1893, IX., S. 96-144, 209-263.
This criticism would apply to judgments of unequal division on either side the center of a horizontal line. It would apply all the more to any general average of judgments including both sides, for, as we shall soon see, the judgments of individuals differ materially on the two sides, and this difference itself may demand its explanation. And if we should include within this average, judgments above and below the center of a vertical line, we should have under one heading four distinct sets of averages, each of which, in the individual cases, might show important variations from the others, and therefore require some variation of explanation. And yet that great leveller, the general average, has obliterated these vital differences, and is recorded as indicating the 'most pleasing proportion.'[3] That such an average falls near the golden section is immaterial. Witmer himself, as we shall see,[4] does not set much store by this coincidence as a starting point for explanation, since he is averse to any mathematical interpretation, but he does consider the average in question representative of the most pleasing division.
[2] op. cit., 212-215.
[3] Witmer: op. cit., S. 212-215.
[4] op. cit., S. 262.
I shall now, before proceeding to the details of the experiment to be recorded, review, very briefly, former interpretative tendencies. Zeising found that the golden section satisfied his demand for unity and infinity in the same beautiful object.[5] In the golden section, says Wundt,[6] there is a unity involving the whole; it is therefore more beautiful than symmetry, according to the æsthetic principle that that unification of spatial forms which occurs without marked effort, which, however, embraces the greater manifold, is the more pleasing. But to me this manifold, to be æsthetic, must be a sensible manifold, and it is still a question whether the golden section set of relations has an actual correlate in sensations. Witmer,[7] however, wrote, at the conclusion of his careful researches, that scientific æsthetics allows no more exact statement, in interpretation of the golden section, than that it forms 'die rechte Mitte' between a too great and a too small variety. Nine years later, in 1902, he says[8] that the preference for proportion over symmetry is not a demand for an equality of ratios, but merely for greater variety, and that 'the amount of unlikeness or variety that is pleasing will depend upon the general character of the object, and upon the individual's grade of intelligence and æsthetic taste.' Külpe[9] sees in the golden section 'a special case of the constancy of the relative sensible discrimination, or of Weber's law.' The division of a line at the golden section produces 'apparently equal differences' between minor and major, and major and whole. It is 'the pleasingness of apparently equal differences.'
[5] Zelsing, A.: 'Aesthetische Forschungen,' 1855, S. 172; 'Neue Lehre von den Proportionen des menschlichen Körpera,' 1854, S. 133-174.
[6] Wundt, W.: 'Physiologische Psychologie,' 4te Aufl., Leipzig, 1893, Bd. II., S. 240 ff.
[7] op. cit., S. 262.
[8] Witmer, L.: 'Analytical Psychology,' Boston, 1902, p. 74.
[9] Külpe, O.: 'Outlines of Psychology,' Eng. Trans., London, 1895, pp. 253-255.
These citations show, in brief form, the history of the interpretation of our pleasure in unequal division. Zeising and Wundt were alike in error in taking the golden section as the norm. Zeising used it to support a philosophical theory of the beautiful. Wundt and others too hastily conclude that the mathematical ratios, intellectually discriminated, are also sensibly discriminated, and form thus the basis of our æsthetic pleasure. An extension of this principle would make our pleasure in any arrangement of forms depend on the mathematical relations of their parts. We should, of course, have no special reason for choosing one set of relationships rather than another, nor for halting at any intricacy of formulæ. But we cannot make experimental æsthetics a branch of applied mathematics. A theory, if we are to have psychological explanation at all, must be pertinent to actual psychic experience. Witmer, while avoiding and condemning mathematical explanation, does not attempt to push interpretation beyond the honored category of unity in variety, which is applicable to anything, and, in principle, is akin to Zeising's unity and infinity. We wish to know what actual psychophysical functionings correspond to this unity in variety. Külpe's interpretation is such an attempt, but it seems clear that Weber's law cannot be applied to the division at the golden section. It would require of us to estimate the difference between the long side and the short side to be equal to that of the long side and the whole. A glance at the division shows that such complex estimation would compare incomparable facts, since the short and the long parts are separated, while the long part is inclosed in the whole. Besides, such an interpretation could not apply to divisions widely variant from the golden section.
This paper, as I said, reports but the beginnings of an investigation into unequal division, confined as it is to results obtained from the division of a simple horizontal line, and to variations introduced as hints towards interpretation. The tests were made in a partially darkened room. The apparatus rested on a table of ordinary height, the part exposed to the subject consisting of an upright screen, 45 cm. high by 61 cm. broad, covered with black cardboard, approximately in the center of which was a horizontal opening of considerable size, backed by opal glass. Between the glass and the cardboard, flush with the edges of the opening so that no stray light could get through, a cardboard slide was inserted from behind, into which was cut the exposed figure. A covered electric light illuminated the figure with a yellowish-white light, so that all the subject saw, besides a dim outline of the apparatus and the walls of the room, was the illuminated figure. An upright strip of steel, 1½ mm. wide, movable in either direction horizontally by means of strings, and controlled by the subject, who sat about four feet in front of the table, divided the horizontal line at any point. On the line, of course, this appeared as a movable dot. The line itself was arbitrarily made 160 mm. long, and 1½ mm. wide. The subject was asked to divide the line unequally at the most pleasing place, moving the divider from one end slowly to the other, far enough to pass outside any pleasing range, or, perhaps, quite off the line; then, having seen the divider at all points of the line, he moved it back to that position which appealed to him as most pleasing. Record having been made of this, by means of a millimeter scale, the subject, without again going off the line, moved to the pleasing position on the other side of the center. He then moved the divider wholly off the line, and made two more judgments, beginning his movement from the other end of the line. These four judgments usually sufficed for the simple line for one experiment. In the course of the experimentation each of nine subjects gave thirty-six such judgments on either side the center, or seventy-two in all.
In Fig. 1, I have represented graphically the results of these judgments. The letters at the left, with the exception of X, mark the subjects. Beginning with the most extreme judgments on either side the center, I have erected modes to represent the number of judgments made within each ensuing five millimeters, the number in each case being denoted by the figure at the top of the mode. The two vertical dot-and-dash lines represent the means of the several averages of all the subjects, or the total averages. The short lines, dropped from each of the horizontals, mark the individual averages of the divisions either side the center, and at X these have been concentrated into one line. Subject E obviously shows two pretty distinct fields of choice, so that it would have been inaccurate to condense them all into one average. I have therefore given two on each side the center, in each case subsuming the judgments represented by the four end modes under one average. In all, sixty judgments were made by E on each half the line. Letter E¹ represents the first thirty-six; E² the full number. A comparison of the two shows how easily averages shift; how suddenly judgments may concentrate in one region after having been for months fairly uniformly distributed. The introduction of one more subject might have varied the total averages by several points. Table I. shows the various averages and mean variations in tabular form.
TABLE I. Left. Right. Div. M.V. Div. M.V. A 54 2.6 50 3.4 B 46 4.5 49 5.7 C 75 1.8 71 1.6 D 62 4.4 56 4.1 E¹ 57 10.7 60 8.7 F 69 2.6 69 1.6 G 65 3.7 64 2.7 H 72 3.8 67 2.1 J 46 1.9 48 1.3 -- --- -- --- Total 60 3.9 59 3.5
Golden Section = 61.1.
¹These are E's general averages on 36 judgments. Fig. 1, however, represents two averages on each side the center, for which the figures are, on the left, 43 with M.V. 3.6; and 66 with M.V. 5.3. On the right, 49, M.V. 3.1; and 67, M.V. 2.7. For the full sixty judgments, his total average was 63 on the left, and 65 on the right, with mean variations of 9.8 and 7.1 respectively. The four that E² in Fig. 1 shows graphically were, for the left, 43 with M.V. 3.6; and 68, M.V. 5.1. On the right, 49, M.V. 3.1; and 69, M.V. 3.4.
Results such as are given in Fig. 1, appear to warrant the criticism of former experimentation. Starting with the golden section, we find the two lines representing the total averages running surprisingly close to it. This line, however, out of a possible eighteen chances, only twice (subjects D and G) falls wholly within the mode representing the maximum number of judgments of any single subject. In six cases (C twice, F, H, J twice) it falls wholly without any mode whatever; and in seven (A, B twice, E, F, G, H) within modes very near the minimum. Glancing for a moment at the individual averages, we see that none coincides with the total (although D is very near), and that out of eighteen, only four (D twice, G twice) come within five millimeters of the general average, while eight (B, C, J twice each, F, H) lie between ten and fifteen millimeters away. The two total averages (although near the golden section), are thus chiefly conspicuous in showing those regions of the line that were avoided as not beautiful. Within a range of ninety millimeters, divided into eighteen sections of five millimeters each, there are ten such sections (50 mm.) each of which represents the maximum of some one subject. The range of maximum judgments is thus about one third the whole line. From such wide limits it is, I think, a methodological error to pick out some single point, and maintain that any explanation whatever of the divisions there made interprets adequately our pleasure in unequal division. Can, above all, the golden section, which in only two cases (D, G) falls within the maximum mode; in five (A, C, F, J twice) entirely outside all modes, and in no single instance within the maximum on both sides the center--can this seriously play the role of æsthetic norm?
I may state here, briefly, the results of several sets of judgments on lines of the same length as the first but wider, and on other lines of the same width but shorter. There were not enough judgments in either case to make an exact comparison of averages valuable, but in three successively shorter lines, only one subject out of eight varied in a constant direction, making his divisions, as the line grew shorter, absolutely nearer the ends. He himself felt, in fact, that he kept about the same absolute position on the line, regardless of the successive shortenings, made by covering up the ends. This I found to be practically true, and it accounts for the increasing variation toward the ends. Further, with all the subjects but one, two out of three pairs of averages (one pair for each length of line) bore the same relative positions to the center as in the normal line. That is, if the average was nearer the center on the left than on the right, then the same held true for the smaller lines. Not only this. With one exception, the positions of the averages of the various subjects, when considered relatively to one another, stood the same in the shorter lines, in two cases out of three. In short, not only did the pair of averages of each subject on each of the shorter lines retain the same relative positions as in the normal line, but the zone of preference of any subject bore the same relation to that of any other. Such approximations are near enough, perhaps, to warrant the statement that the absolute length of line makes no appreciable difference in the æsthetic judgment. In the wider lines the agreement of the judgments with those of the normal line was, as might be expected, still closer. In these tests only six subjects were used. As in the former case, however, E was here the exception, his averages being appreciably nearer the center than in the original line. But his judgments of this line, taken during the same period, were so much on the central tack that a comparison of them with those of the wider lines shows very close similarity. The following table will show how E's judgments varied constantly towards the center:
AVERAGE. L. R. 1. Twenty-one judgments (11 on L. and 10 on R.) during experimentation on I¹, I², etc., but not on same days. 64 65
2. Twenty at different times, but immediately before judging on I¹, I², etc. 69 71
3. Eighteen similar judgments, but immediately after judging on I¹, I², etc. 72 71
4. Twelve taken after all experimentation with I¹, I², etc., had ceased. 71 69
The measurements are always from the ends of the line. It looks as if the judgments in (3) were pushed further to the center by being immediately preceded by those on the shorter and the wider lines, but those in (1) and (2) differ markedly, and yet were under no such influences.
From the work on the simple line, with its variations in width and length, these conclusions seem to me of interest. (1) The records offer no one division that can be validly taken to represent 'the most pleasing proportion' and from which interpretation may issue. (2) With one exception (E) the subjects, while differing widely from one another in elasticity of judgment, confined themselves severally to pretty constant regions of choice, which hold, relatively, for different lengths and widths of line. (3) Towards the extremities judgments seldom stray beyond a point that would divide the line into fourths, but they approach the center very closely. Most of the subjects, however, found a slight remove from the center disagreeable. (4) Introspectively the subjects were ordinarily aware of a range within which judgments might give equal pleasure, although a slight disturbance of any particular judgment would usually be recognized as a departure from the point of maximum pleasingness. This feeling of potential elasticity of judgment, combined with that of certainty in regard to any particular instance, demands--when the other results are also kept in mind--an interpretative theory to take account of every judgment, and forbids it to seize on an average as the basis of explanation for judgments that persist in maintaining their æsthetic autonomy.
I shall now proceed to the interpretative part of the paper. Bilateral symmetry has long been recognized as a primary principle in æsthetic composition. We inveterately seek to arrange the elements of a figure so as to secure, horizontally, on either side of a central point of reference, an objective equivalence of lines and masses. At one extreme this may be the rigid mathematical symmetry of geometrically similar halves; at the other, an intricate system of compensations in which size on one side is balanced by distance on the other, elaboration of design by mass, and so on. Physiologically speaking, there is here a corresponding equality of muscular innervations, a setting free of bilaterally equal organic energies. Introspection will localize the basis of these in seemingly equal eye movements, in a strain of the head from side to side, as one half the field is regarded, or the other, and in the tendency of one half the body towards a massed horizontal movement, which is nevertheless held in check by a similar impulse, on the part of the other half, in the opposite direction, so that equilibrium results. The psychic accompaniment is a feeling of balance; the mind is æsthetically satisfied, at rest. And through whatever bewildering variety of elements in the figure, it is this simple bilateral equivalence that brings us to æsthetic rest. If, however, the symmetry is not good, if we find a gap in design where we expected a filling, the accustomed equilibrium of the organism does not result; psychically there is lack of balance, and the object is æsthetically painful. We seem to have, then, in symmetry, three aspects. First, the objective quantitative equality of sides; second, a corresponding equivalence of bilaterally disposed organic energies, brought into equilibrium because acting in opposite directions; third, a feeling of balance, which is, in symmetry, our æsthetic satisfaction.
It would be possible, as I have intimated, to arrange a series of symmetrical figures in which the first would show simple geometrical reduplication of one side by the other, obvious at a glance; and the last, such a qualitative variety of compensating elements that only painstaking experimentation could make apparent what elements balanced others. The second, through its more subtle exemplification of the rule of quantitative equivalence, might be called a higher order of symmetry. Suppose now that we find given, objects which, æsthetically pleasing, nevertheless present, on one side of a point of reference, or center of division, elements that actually have none corresponding to them on the other; where there is not, in short, objective bilateral equivalence, however subtly manifested, but, rather, a complete lack of compensation, a striking asymmetry. The simplest, most convincing case of this is the horizontal straight line, unequally divided. Must we, because of the lack of objective equality of sides, also say that the bilaterally equivalent muscular innervations are likewise lacking, and that our pleasure consequently does not arise from the feeling of balance? A new aspect of psychophysical æsthetics thus presents itself. Must we invoke a new principle for horizontal unequal division, or is it but a subtly disguised variation of the more familiar symmetry? And in vertical unequal division, what principle governs? A further paper will deal with vertical division. The present paper, as I have said, offers a theory for the horizontal.
To this end, there were introduced, along with the simple line figures already described, more varied ones, designed to suggest interpretation. One whole class of figures was tried and discarded because the variations, being introduced at the ends of the simple line, suggested at once the up-and-down balance of the lever about the division point as a fulcrum, and became, therefore, instances of simple symmetry. The parallel between such figures and the simple line failed, also, in the lack of homogeneity on either side the division point in the former, so that the figure did not appear to center at, or issue from the point of division, but rather to terminate or concentrate in the end variations. A class of figures that obviated both these difficulties was finally adopted and adhered to throughout the work. As exposed, the figures were as long as the simple line, but of varying widths. On one side, by means of horizontal parallels, the horizontality of the original line was emphasized, while on the other there were introduced various patterns (fillings). Each figure was movable to the right or the left, behind a stationary opening 160 mm. in length, so that one side might be shortened to any desired degree and the other at the same time lengthened, the total length remaining constant. In this way the division point (the junction of the two sides) could be made to occupy any position on the figure. The figures were also reversible, in order to present the variety-filling on the right or the left.
If it were found that such a filling in one figure varied from one in another so that it obviously presented more than the other of some clearly distinguishable element, and yielded divisions in which it occupied constantly a shorter space than the other, then we could, theoretically, shorten the divisions at will by adding to the filling in the one respect. If this were true it would be evident that what we demand is an equivalence of fillings--a shorter length being made equivalent to the longer horizontal parallels by the addition of more of the element in which the two short fillings essentially differ. It would then be a fair inference that the different lengths allotted by the various subjects to the short division of the simple line result from varying degrees of substitution of the element, reduced to its simplest terms, in which our filling varied. Unequal division would thus be an instance of bilateral symmetry.
The thought is plausible. For, in regarding the short part of the line with the long still in vision, one would be likely, from the æsthetic tendency to introduce symmetry into the arrangement of objects, to be irritated by the discrepant inequality of the two lengths, and, in order to obtain the equality, would attribute an added significance to the short length. If the assumption of bilateral equivalence underlying this is correct, then the repetition, in quantitative terms, on one side, of what we have on the other, constitutes the unity in the horizontal disposition of æsthetic elements; a unity receptive to an almost infinite variety of actual visual forms--quantitative identity in qualitative diversity. If presented material resists objective symmetrical arrangement (which gives, with the minimum expenditure of energy, the corresponding bilateral equivalence of organic energies) we obtain our organic equivalence in supplementing the weaker part by a contribution of energies for which it presents no obvious visual, or objective, basis. From this there results, by reaction, an objective equivalence, for the psychic correlate of the additional energies freed is an attribution to the weaker part, in order to secure this feeling of balance, of some added qualities, which at first it did not appear to have. In the case of the simple line the lack of objective symmetry that everywhere meets us is represented by an unequal division. The enhanced significance acquired by the shorter part, and its psychophysical basis, will engage us further in the introspection of the subjects, and in the final paragraph of the paper. In general, however, the phenomenon that we found in the division of the line--the variety of divisions given by any one object, and the variations among the several subjects--is easily accounted for by the suggested theory, for the different subjects merely exemplify more fixedly the shifting psychophysical states of any one subject.
In all, five sets of the corrected figures were used. Only the second, however, and the fifth (chronologically speaking) appeared indubitably to isolate one element above others, and gave uniform results. But time lacked to develop the fifth sufficiently to warrant positive statement. Certain uniformities appeared, nevertheless, in all the sets, and find due mention in the ensuing discussion. The two figures of the second set are shown in Fig. 2. Variation No. III. shows a design similar to that of No. II., but with its parts set more closely together and offering, therefore, a greater complexity. In Table II. are given the average divisions of the nine subjects. The total length of the figure was, as usual, 160 mm. Varying numbers of judgments were made on the different subjects.
TABLE II.
No. I. No. II. No. I. (reversed). No. II. (reversed). L. R. L. R. R. L. R. L.
A 55 0 48 0 59 0 50 0 B 59 0 44 0 63 0 52 0 C 58 0 56 0 52 0 50 0 D 60 0 56 0 60 0 55 0 E 74 59 73 65 74 60 75 67 F 61 67 60 66 65 64 62 65 G 64 64 62 68 63 64 53 67 H 76 68 75 64 66 73 67 71 J 49 0 41 0 50 0 42 0 -- -- -- -- -- -- -- -- Total. 61 64 57 65 61 65 54 67
With the complex fillings at the left, it will be seen, firstly, that in every case the left judgment on No. III. is less than that on No. II. With the figures reversed, the right judgments on No. III. are less than on No. II., with the exception of subjects E and H. Secondly, four of the subjects only (E, F, G and H) had judgments also on the side which gave the complex filling the larger space; to E, F and G, these were secondary preferences; to H they were always primary. Thirdly, the judgments on No. III. are less, in spite of the fact that the larger component parts of No. II., might be taken as additional weight to that side of the line, and given, therefore, the shorter space, according to the principle of the lever.
The subjects, then, that appear not to substantiate our suggested theory are E and H, who in the reversed figures give the shorter space to the less complex filling, and F and G, who, together with E and H, have always secondary judgments that allot to either complex filling a larger space than to the simple horizontal. Consider, first, subjects E and H. For each, the difference in division of II. and III. is in any case very slight. Further, subject E, in judgments where the complex filling exceeds the horizontal parallels in length, still gives the more complex of the two fillings markedly the shorter space, showing, apparently, that its additional complexity works there in accord with the theory. There was, according to his introspection, another principle at work. As a figure, he emphatically preferred II. to III. The filling of II. made up, he found, by its greater interest, for lack of length. He here secured a balance, in which the interest of the complex material compensated for the greater extent of the simpler horizontals. This accounts for its small variation from III., and even for its occupying the smaller space. But in judgments giving the two complex fillings the larger space, the more interesting material exceeded in extent the less interesting. In such divisions the balance was no longer uppermost in mind, but the desire to get as much as possible of the interesting filling. To this end the horizontal parallels were shortened as far as they could be without becoming insignificant. But unless some element of balance were there (although not present to introspection) each complex filling, when up for judgment, would have been pushed to the same limit. It, therefore, does seem, in cases where the complex fillings occupied a larger space than the horizontals, that the subject, not trying consciously to secure a balance of interests, was influenced more purely by the factor of complexity, and that his judgments lend support to our theory.
Subject H was the only subject who consistently preferred to have all complex fillings occupy the larger space. Introspection invariably revealed the same principle of procedure--he strove to get as much of the interesting material as he could. He thought, therefore, that in every case he moved the complex filling to that limit of the pleasing range that he found on the simple line, which would yield him most of the filling. Balance did not appear prominent in his introspection. A glance, however, at the results shows that his introspection is contradicted. For he maintains approximately the same division on the right in all the figures, whether reversed or not, and similarly on the left. The average on the right for all four is 67; on the left it is 74. Comparing these with the averages on the simple line, we see that the right averages coincide exactly, while the left but slightly differ. I suspect, indeed, that the fillings did not mean much to H, except that they were 'interesting' or 'uninteresting'; that aside from this he was really abstracting from the filling and making the same judgments that he would make on the simple line. Since he was continually aware that they fell within the 'pleasing range' on the simple line, this conclusion is the more plausible.
Perhaps these remarks account for the respective uniformities of the judgments of E and H, and their departure from the tendency of the other subjects to give the more complex filling constantly the shorter space. But subjects F and G also had judgments (secondary with both of them) giving to the complex filling a larger extent than to the parallels. With them one of two principles, I think, applies: The judgments are either instances of abstraction from the filling, as with H, or one of simpler gravity or vertical balance, as distinguished from the horizontal equivalence which I conceive to be at the basis of the other divisions. With F it is likely to be the latter, since the divisions of the figures under discussion do not approach very closely those of the simple line, and because introspectively he found that the divisions giving the complex the larger space were 'balance' divisions, while the others were determined with 'reference to the character of the fillings.' From G I had no introspection, and the approximation of his judgments to those he gave for the simple line make it probable that with him the changes in the character of the filling had little significance. The average of his judgments in which the complex filling held the greater space is 66, while the averages on the simple line were 65 on the left, and 64 on the right. And, in general, abstraction from filling was easy, and to be guarded against. Subject C, in the course of the work, confessed to it, quite unsolicited, and corrected himself by giving thenceforth all complex fillings much smaller space than before. Two others noticed that it was particularly hard not to abstract. Further, none of the four subjects mentioned (with that possible exception of E) showed a sensitiveness similar to that of the other five.
With the exception of H, and in accord with the constant practice of the other five, these subjects, too, occasionally found no pleasing division in which the complex filling preponderated in length over the horizontals. It was uniformly true, furthermore, in every variation introduced in the course of the investigation, involving a complex and a simple filling, that all the nine subjects but H preferred the complex in the shorter space; that five refused any divisions offering it in the larger space; that these five showed more sensitiveness to differences in the character of fillings; and that with one exception (C) the divisions of the simple line which these subjects gave were nearer the ends than those of the others. It surely seems plausible that those most endowed with æsthetic sensitiveness would find a division near the center more unequal than one nearer the end; for one side only slightly shorter than the other would at once seem to mean the same thing to them, and yet, because of the obvious difference in length, be something markedly different, and they would therefore demand a part short enough to give them sharp qualitative difference, with, however, in some way, quantitative equivalence. When the short part is too long, it is overcharged with significance, it strives to be two things at once and yet neither in its fulness.
We thus return to the simple line. I have considered a series of judgments on it, and a series on two different figures, varying in the degree of complexity presented, in one of their fillings. It remains very briefly to see if the introspection on the simple line furnishes further warrant for carrying the complexities over into the simple line and so giving additional validity to the outlined theory of substitution. The following phrases are from introspective notes.
A. Sweep wanted over long part. More attention to short. Significance of whole in short. Certainly a concentration of interest in the short. Short is efficacious. Long means rest; short is the center of things. Long, an effortless activity; short, a more strenuous activity. When complex fillings are introduced, subject is helped out; does not have to put so much into the short division. In simple line, subject introduces the concentration. In complex figures the concentration is objectified. In equal division subject has little to do with it; the unequal depends on the subject--it calls for appreciation. Center of references is the division point, and the eye movements to right and left begin here, and here return. The line centers there. The balance is a horizontal affair.
B. Center a more reposing division. Chief attention to division point, with side excursions to right and left, when refreshment of perception is needed. The balance is horizontal and not vertical.
C. A balance with variety, or without symmetry. Centers at division point and wants sweep over long part. More concentration on short part. Subjective activity there--an introduction of energy. A contraction of the muscles used in active attention. Long side easier, takes care of itself, self-poised. Line centers at division point. Active with short division. Introduces activity, which is equivalent to the filling that the complex figures have; in these the introduced activity is objectified--made graphic.
D. Focal point at division point: wants the interesting things in a picture to occupy the left (when short division is also on left). Short division the more interesting and means greater complication. When the pleasing division is made, eyes move first over long and then over short. Division point the center of real reference from which movements are made.
E. No reference to center in making judgments; hurries over center. All portions of simple line of equal interest; but in unequal division the short gets a non-apparent importance, for the line is then a scheme for the representation of materials of different interest values. When the division is too short, the imagination refuses to give it the proportionally greater importance that it would demand. When too long it is too near equality. In enjoying line, the division point is fixed, with shifts of attention from side to side. An underlying intellectual assignment of more value to short side, and then the sense-pleasure comes; the two sides have then an equality.
F. Middle vulgar, common, prosaic; unequal lively. Prefers the lively. Eyes rest on division point, moving to the end of long and then of short. Ease, simplicity and restfulness are proper to the long part of complex figures. Short part of simple line looks wider, brighter and more important than long.
G. Unequal better than equal. Eye likes movement over long and then over short. Subject interested only in division point. Short part gives the æsthetic quality to the line.
H. Center not wanted. Division point the center of interest. (No further noteworthy introspection from H, but concerning complex figures he said that he wanted simple or the compact on the short, and the interesting on the long.)
These introspective notes were given at different times, and any repetitions serve only to show constancy. The subjects were usually very certain of their introspection. In general it appears to me to warrant these three statements: (1) That the center of interest is the division point, whence eye-movements, or innervations involving, perhaps, the whole motor system, are made to either side. (2) That there is some sort of balance or equivalence obtained (a bilateral symmetry), which is not, however, a vertical balance--that is, one of weights pulling downwards, according to the principle of the lever. All the subjects repudiated the suggestion of vertical balance. (3) That the long side means ease and simplicity, and represents graphically exactly what it means; that the short side means greater intensity, concentration, or complexity, and that this is substituted by the subject; the short division, unlike the long, means something that it does not graphically represent.
So much for the relation between what is objectively given and the significance subjectively attributed to it. There remains still the translation into psychophysical terms. The results on the complex figures (showing that a division may be shortened by making the innervations on that side increasingly more involved) lend plausibility to the interpretation that the additional significance is, in visual terms, a greater intricacy or difficulty of eye-movement, actual or reproduced; or, in more general terms, a greater tension of the entire motor system. In such figures the psychophysical conditions for our pleasure in the unequal division of the simple horizontal line are merely graphically symbolized, not necessarily duplicated. On page 553 I roughly suggested what occurs in regarding the unequally divided line. More exactly, this: the long section of the line gives a free sweep of the eyes from the division point, the center, to the end; or again, a free innervation of the motor system. The sweep the subject makes sure of. Then, with that as standard, the æsthetic impulse is to secure an equal and similar movement, from the center, in the opposite direction. It is checked, however, by the end point of the short side. The result is the innervation of antagonistic muscles, by which the impression is intensified. For any given subject, then, the pleasing unequal division is at that point which causes quantitatively equal physiological discharges, consisting of the simple movement, on one hand, and, on the other, the same kind of movement, compounded with the additional innervation of the antagonists resulting from the resistance of the end point. Since, when the characteristic movements are being made for one side, the other is always in simultaneous vision, the sweep receives, by contrast, further accentuation, and the innervation of antagonists doubtless begins as soon as movement on the short side is begun. The whole of the short movement is, therefore, really a resultant of the tendency to sweep and this necessary innervation of antagonists. The correlate of the equivalent innervations is equal sensations of energy of movement coming from the two sides. Hence the feeling of balance. Hence (from the lack of unimpeded movement on the short side) the feeling there of 'intensity,' or 'concentration,' or 'greater significance.' Hence, too, the 'ease,' the 'simplicity,' the 'placidity' of the long side.
As in traditional symmetry, the element of unity or identity, in unequal division, is a repetition, in quantitative terms, on one side, of what is given on the other. In the simple line the equal division gives us obviously exact objective repetition, so that the psychophysical correlates are more easily inferred, while the unequal offers apparently no compensation. But the psychophysical contribution of energies is not gratuitous. The function of the increment of length on one side, which in the centrally divided line makes the divisions equal, is assumed in unequal division by the end point of the short side; the uniform motor innervations in the former become, in the latter, the additional innervation of antagonists, which gives the equality. The two are separated only in degree. The latter may truly be called, however, a symmetry of a higher order, because objectively the disposition of its elements is not graphically obvious, and psychophysically, the quantitative unity is attained through a greater variety of processes. Thus, in complex works of art, what at first appears to be an unsymmetrical composition, is, if beautiful, only a subtle symmetry. There is present, of course, an arithmetically unequal division of horizontal extent, aside from the filling. But our pleasure in this, without filling, has been seen to be also a pleasure in symmetry. We have, then, the symmetry of equally divided extents and of unequally divided extents. They have in common bilateral equivalence of psychophysical processes; the nature of these differs. In both the principle of unity is the same. The variety through which it works is different.
* * * * *
STUDIES IN ANIMAL PSYCHOLOGY.
* * * * *
HABIT FORMATION IN THE CRAWFISH CAMBARUS AFFINIS.[1]
BY ROBERT M. YERKES AND GURRY E. HUGGINS.
[1] See also Yerkes, Robert: 'Habit-Formation in the Green Crab, Carcinus Granulalus,' Biological Bulletin, Vol. III., 1902, pp. 241-244.
This paper is an account of some experiments made for the purpose of testing the ability of the crawfish to profit by experience. It is well known that most vertebrates are able to learn, but of the invertebrates there are several classes which have not as yet been tested.
The only experimental study of habit formation in a crustacean which we have found is that of Albrecht Bethe[2] on the crab, Carcinus maenas. In his excellent paper on the structure of the nervous system of Carcinus Bethe calls attention to a few experiments which he made to determine, as he puts it, whether the crab possesses psychic processes. The following are the observations made by him. Experiment I. A crab was placed in a basin which contained in its darkest corner an Eledone (a Cephalopod). The crab at once moved into the dark region because of its instinct to hide, and was seized by the Eledone and drawn under its mantle. The experimenter then quickly freed the crab from its enemy and returned it to the other end of the basin. But again the crab returned to the dark and was seized. This was repeated with one animal five times and with another six times without the least evidence that the crabs profited by their experiences with the Eledone. Experiment 2. Crabs in an aquarium were baited with meat. The experimenter held his hand above the food and each time the hungry crab seized it he caught the animal and maltreated it, thus trying to teach the crabs that meat meant danger. But as in the previous experiment several repetitions of the experience failed to teach the crabs that the hand should be avoided. From these observations Bethe concludes that Carcinus has no 'psychic qualities' (i.e., is unable to profit by experience), but is a reflex machine.
[2] Bethe, Albrecht: 'Das Centralnervensystem von Carcinus maenas,' II. Theil., Arch. f. mikr. Anat., Bd. 51, 1898, S. 447.
Bethe's first test is unsatisfactory because the crabs have a strong tendency to hide from the experimenter in the darkest corner. Hence, if an association was formed, there would necessarily be a conflict of impulses, and the region in which the animal would remain would depend upon the relative strengths of its fear of the experimenter and of the Eledone. This objection is not so weighty, however, as is that which must obviously be made to the number of observations upon which the conclusions are based. Five or even twenty-five repetitions of such an experiment would be an inadequate basis for the statements made by Bethe. At least a hundred trials should have been made. The same objection holds in case of the second experiment. In all probability Bethe's statements were made in the light of long and close observation of the life habits of Carcinus; we do not wish, therefore, to deny the value of his observations, but before accepting his conclusions it is our purpose to make a more thorough test of the ability of crustaceans to learn.
For determining whether the crawfish is able to learn a simple form of the labyrinth method was employed. A wooden box (Fig. 1) 35 cm. long, 24 cm. wide and 15 cm. deep, with one end open, and at the other end a triangular compartment which communicated with the main portion of the box by an opening 5 cm. wide, served as an experiment box. At the open end of this box a partition (P) 6 cm. long divided the opening into two passages of equal width. Either of these passages could be closed with a glass plate (G), and the subject thus forced to escape from the box by the choice of a certain passage. This box, during the experiments, was placed in the aquarium in which the animals lived. In order to facilitate the movement of the crawfish toward the water, the open end was placed on a level with the water in the aquarium, and the other end was raised so that the box made an angle of 6° with the horizontal.
Experiments were made under uniform conditions, as follows. A subject was taken from the aquarium and placed in a dry jar for about five minutes, in order to increase the desire to return to the water; it was then put into the triangular space of the experiment box and allowed to find its way to the aquarium. Only one choice of direction was necessary in this, namely, at the opening where one of the passages was closed. That the animal should not be disturbed during the experiment the observer stood motionless immediately behind the box.
Before the glass plate was introduced a preliminary series of tests was made to see whether the animals had any tendency to go to one side on account of inequality of illumination, of the action of gravity, or any other stimulus which might not be apparent to the experimenter. Three subjects were used, with the results tabulated.
Exit by Exit by Right Passage Left Passage. No. 1 6 4 No. 2 7 3 No. 3 3 7 16 14
Since there were more cases of exit by the right-hand passage, it was closed with the glass plate, and a series of experiments made to determine whether the crawfish would learn to avoid the blocked passage and escape to the aquarium by the most direct path. Between March 13 and April 14 each of the three animals was given sixty trials, an average of two a day. In Table I. the results of these trials are arranged in groups of ten, according to the choice of passages at the exit. Whenever an animal moved beyond the level of the partition (P) on the side of the closed passage the trial was counted in favor of the closed passage, even though the animal turned back before touching the glass plate and escaped by the open passage.
TABLE I.
HABIT FORMATION IN THE CRAWFISH.¹
Experiments. No. 1 No. 2 No. 3 Totals Per cent Open Closed Open Closed Open Closed Open Closed Open 1-10 8 2 5 5 2 8 15 15 50.0 11-20 4 6 8 2 6 4 18 12 60.0 21-30 6 3² 8 2 8 2 22 7 75.8 31-40 9 1 8 2 8 2 25 5 83.3 41-50 8 2 8 2 7 3 23 7 76.6 51-60 10 0 8 2 9 1 27 3 90.0
TEST OF PERMANENCY OF HABIT AFTER 14 DAYS' REST.
61-70 6 4 8 2 8 2 22 8 73.3 (1-10) 71-80 6 4 8 2 7 3 21 9 70.0 (11-20)
¹The experiments of this table were made by F.D. Bosworth.
²One trial in which the subject failed to return to the water within thirty minutes.
In these experiments there is a gradual increase in the number of correct choices (i.e., choice of the 'open' passage) from 50 per cent. for the first ten trials to 90 per cent. for the last ten (trials 51-60). The test of permanency, made after two weeks, shows that the habit persisted.
Although the observations just recorded indicate the ability of the crawfish to learn a simple habit, it seems desirable to test the matter more carefully under somewhat different conditions. For in the experiments described the animals were allowed to go through the box day after day without any change in the floor over which they passed, and as it was noted that they frequently applied their antennae to the bottom of the box as they moved along, it is possible that they were merely following a path marked by an odor or by moistness due to the previous trips. To discover whether this was really the case experiments were made in which the box was thoroughly washed out after each trip.
The nature of the test in the experiments now to be recorded is the same as the preceding, but a new box was used. Fig. 2 is the floor plan and side view of this apparatus. It was 44.5 cm. long, 23.5 cm. wide and 20 cm. deep. The partition at the exit was 8.5 cm. in length. Instead of placing this apparatus in the aquarium, as was done in the previous experiments, a tray containing sand and water was used to receive the animals as they escaped from the box. The angle of inclination was also changed to 7°. For the triangular space in which the animals were started in the preceding tests a rectangular box was substituted, and from this an opening 8 cm. wide by 5 cm. deep gave access to the main compartment of the box.
A large healthy crawfish was selected and subjected to tests in this apparatus in series of ten experiments given in quick succession. One series a day was given. After each test the floor was washed; as a result the experiments were separated from one another by a three-minute interval, and each series occupied from thirty minutes to an hour. Table II. gives in groups of five these series of ten observations each. The groups, indicated by Roman numerals, run from I. to IX., there being, therefore, 450 experiments in all. Groups I. and II., or the first 100 experiments, were made without having either of the exit passages closed, in order to see whether the animal would develop a habit of going out by one side or the other. It did very quickly, as a matter of fact, get into the habit of using the left passage (L.). The last sixty experiments (Groups I. and II.) show not a single case of escape by the right passage. The left passage was now closed. Group III. gives the result. The time column (i.e., the third column of the table) gives for each series of observations the average time in seconds occupied by the animal in escaping from the box. It is to be noted that the closing of the Left passage caused an increase in the time from 30.9 seconds for the last series of the second group to 90 seconds for the first series of the third group. In this there is unmistakable evidence of the influence of the change in conditions. The animal after a very few experiences under the new conditions began going to the Right in most cases; and after 250 experiences it had ceased to make mistakes. Group VII. indicates only one mistake in fifty choices.
TABLE II.
HABIT FORMATION AND THE MODIFICATION OF HABITS IN THE CRAWFISH.
Results in Series of Ten. Avs. in Groups of 50. Series L. R. Time. L. R. L. R. Time. Group I. 1 9 1 45 Per Cent. 2 3 7 69 3 9 1 20 4 4 6 72 5 10 31 -- -- 35 15 70 30 47.4
II. 1 10 29 2 10 30 3 10 30 4 10 28.8 5 10 30.9 -- ---- 50 100 30 .... .... III. 1 4 6 90 2 2 2 8 89.2 1 3 1 9 36.7 1 4 2 8 51 2 5 1 9 43 2 -- -- -- 10 40 7 20 80 62 .... .... IV. 1 3 7 124 1 2 2 8 44 5 3 2 8 37 4 4 10 34 5 2 8 1 -- -- -- 9 41 11 18 82 60 .... .... V. 1 10 44 2 2 10 35 4 3 3 7 76 3 4 2 8 50 7 5 1 9 50 4 -- -- -- 6 44 20 12 88 51 .... .... VI. 1 2 8 45 2 2 10 41 5 3 1 9 41.8 7 4 10 32.7 7 5 10 8 -- -- -- 3 47 29 6 94 40 .... .... VII. 1 1 9 39 4 2 10 38 7 3 10 30.7 3 4 10 42 6 5 10 48 4 -- -- -- 1 49 24 2 98 39.5
R. L. .... .... VIII. 1 8 2 147 1 2 9 1 26 3 8 2 49 2 4 9 1 38 2 5 9 1 41 -- -- -- 43 7 5 86 14 60.2 .... .... IX. 1 1 9 41 2 2 8 39 1 3 10 29 4 1 9 47 5 1 9 32 1 10 90 38 -- -- -- 5 45 2
The dotted lines at the beginning of groups indicate the closed passage.
At the beginning of Group VIII. the Right instead of the Left passage was closed in order to test the ability of the animal to change its newly formed habit. As a result of this change in the conditions the animal almost immediately began going to the Left. What is most significant, however, is the fact that in the first trial after the change it was completely confused and spent over fifteen minutes wandering about, and trying to escape by the old way (Fig. 4 represents the path taken). At the end of the preceding group the time of a trip was about 48 seconds, while for the first ten trips of Group VIII. the time increased to 147 seconds. This remarkable increase is due almost entirely to the great length of time of the first trip, in which the animal thoroughly explored the whole of the box and made persistent efforts to get out by the Right passage as it had been accustomed to do. It is at the same time noteworthy that the average time for the second series of Group VIII. is only 26 seconds.
For Group IX. the conditions were again reversed, this time the Left passage being closed. Here the first trial was one of long and careful exploration, but thereafter no more mistakes were made in the first series, and in the group of fifty tests there were only five wrong choices.
The fifth column, R. L. and L. R., of Table II. contains cases in which the subject started toward one side and then changed its course before reaching the partition. In Group III., for instance, when the Left passage was closed, the subject started toward the Left seven times, but in each case changed to the Right before reaching the partition. This is the best evidence of the importance of vision that these experiments furnish.
The first experiments on habit formation proved conclusively that the crawfish is able to learn. The observations which have just been described prove that the labyrinth habit is not merely the following of a path by the senses of smell, taste or touch, but that other sensory data, in the absence of those mentioned, direct the animals. So far as these experiments go there appear to be at least four sensory factors of importance in the formation of a simple labyrinth habit: the chemical sense, touch, vision and the muscle sense. That the chemical sense and touch are valuable guiding senses is evident from even superficial observation, and of the importance of vision and the muscle sense we are certain from the experimental evidence at hand.
Of the significance of the sensations due to the 'direction of turning' in these habits the best evidence that is furnished by this work is that of the following observations. In case of the tests of Table II. the subject was, after 100 preliminary tests, trained by 250 experiences to escape by the Right-hand passage. Now, in Groups III. to VII., the subject's usual manner of getting out of the closed passage, when by a wrong choice it happened to get into it, was to draw back on the curled abdomen, after the antennae and chelæ had touched the glass plate, and then move the chelæ slowly along the Right wall of the partition until it came to the upper end; it would then walk around the partition and out by the open passage. Fig. 3 represents such a course. In Group VIII. the Right passage was closed, instead of the Left as previously. The first time the animal tried to get out of the box after this change in the conditions it walked directly into the Right passage. Finding this closed it at once turned to the Right, as it had been accustomed to do when it came in contact with the glass plate, and moved along the side of the box just as it did in trying to get around the end of the partition. The path taken by the crawfish in this experiment is represented in Fig. 4. It is very complex, for the animal wandered about more than fifteen minutes before escaping.
The experiment just described to show the importance of the tendency to turn in a certain direction was the first one of the first series after the change in conditions. When given its second chance in this series the subject escaped directly by the Left passage in 33 seconds, and for the three following trips the time was respectively 25, 25 and 30 seconds.
Upon the experimental evidence presented we base the conclusion that crawfish are able to profit by experience in much the same way that insects do, but far more slowly.
It was thought that a study of the way in which crawfish right themselves when placed upon their backs on a smooth surface might furnish further evidence concerning the ability of the animals to profit by experience.
Dearborn[3] from some observations of his concludes that there is no one method by which an individual usually rights itself, and, furthermore, that the animals cannot be trained to any one method. His experiments, like Bethe's, are too few to warrant any conclusions as to the possibility of habit formation.
[3] Dearborn, G.V.N.: 'Notes on the Individual Psychophysiology of the Crayfish,' Amer. Jour. Physiol., Vol. 3, 1900, pp. 404-433.
For the following experiments the subject was placed on its back on a smooth surface in the air and permitted to turn over in any way it could. Our purpose was to determine (1) whether there was any marked tendency to turn in a certain way, (2) whether if such was not the case a tendency could be developed by changing the conditions, and (3) how alteration in the conditions of the test would affect the turning.
A great many records were taken, but we shall give in detail only a representative series. In Table III., 557 tests made upon four subjects have been arranged in four groups for convenience of comparison of the conditions at different periods of the training process. Each of these groups, if perfect, would contain 40 tests for each of the four subjects, but as a result of accidents II., III., and IV. are incomplete.
TABLE III.
RE-TURNING OF CRAWFISH.
Group. Number of L. R. Time in Tests. Animal. Per cent. Seconds. I. 2 22.5 77.5 14.6 40 3 42.5 57.5 2.6 40 4 52.8 47.2 4.3 38 16 44.5 55.5 22.5 45 -- ---- ---- ---- --- 40.6 59.4 10.8 163
Group. Number of L. R. Time in Tests. Animal. Per cent. Seconds. II 2 28 72 50 43 3 32 68 6.2 50 4 -- 100 6.8 40 16 31.3 68.7 39.3 42 -- ---- ---- ---- --- 22.8 77.2 25.6 175
Group. Number of L. R. Time in Tests. Animal. Per cent. Seconds. III 2 2.5 97.5 46.5 40 -- -- -- -- -- 4 20 80 5.5 40 16 41 59 15 49 -- ---- ---- ---- --- 21.2 78.8 22 129
Group. Number of L. R. Time in Tests. Animal. Per cent. Seconds. IV. 2 2 98 41 50 -- -- -- -- -- 4 32.5 67.5 7.3 40 -- ---- ---- ---- --- 17 83 24 90
Group I., representing 163 tests, shows 59 per cent. to the right, with a time interval of 10.8 seconds (i.e., the time occupied in turning). Group II. shows 77 per cent. to the right; and so throughout the table there is an increase in the number of returnings to the right. These figures at first sight seem to indicate the formation of a habit, but in such case we would expect, also, a shortening of the time of turning. It may be, however, that the animals were gradually developing a tendency to turn in the easiest manner, and that at the same time they were becoming more accustomed to the unusual position and were no longer so strongly stimulated, when placed on their backs, to attempt to right themselves.
All the subjects were measured and weighed in order to discover whether there were inequalities of the two sides of the body which would make it easier to turn to the one side than to the other. The chelæ were measured from the inner angle of the joint of the protopodite to the angle of articulation with the dactylopodite. The carapace was measured on each side, from the anterior margin of the cephalic groove to the posterior extremity of the lateral edge. The median length of the carapace was taken, from the tip of the rostrum to the posterior edge, and the length of the abdomen was taken from this point to the edge of the telson. These measurements, together with the weights of three of the subjects, are given in the accompanying table.
TABLE IV.
MEASUREMENTS OF CRAWFISH.
Chelæ. Carapace. Abdomen. Weight. Left. Right. Left. Right. Median.
No. 2, 9.8 10.0 38.2 38.7 47.3 48.1 29.7 No. 4, 7.7 7.7 33.6 33.8 39.4 42.3 17.7 No. 16, 12.5 12.4 37.6 37.6 46.4 53.2 36.2
Since these measurements indicate slightly greater size on the right it is very probable that we have in this fact an explanation of the tendency to turn to that side.
To test the effect of a change in the conditions, No. 16 was tried on a surface slanted at an angle of 1° 12'. Upon this surface the subject was each time so placed that the slant would favor turning to the right. Under these conditions No. 16 gave the following results in two series of tests. In the first series, consisting of 46 turns, 82.6 per cent. were to the right, and the average time for turning was 17.4 seconds; in the second series, of 41 tests, there were 97.5 per cent, to the right, with an average time of 2.5 seconds. We have here an immediate change in the animal's method of re-turning caused by a slight change in the conditions. The subject was now tested again on a level surface, with the result that in 49 tests only 59 per cent. were toward the right, and the time was 15 seconds.
SUMMARY.
1. Experiments with crawfish prove that they are able to learn simple labyrinth habits. They profit by experience rather slowly, from fifty to one hundred experiences being necessary to cause a perfect association.
2. In the crawfish the chief factors in the formation of such habits are the chemical sense (probably both smell and taste), touch, sight and the muscular sensations resulting from the direction of turning. The animals are able to learn a path when the possibility of following a scent is excluded.
3. The ease with which a simple labyrinth habit may be modified depends upon the number of experiences the animal has had; the more familiar the animal is with the situation, the more quickly it changes its habits. If the habit is one involving the choice of one of two passages, reversal of the conditions confuses the subject much more the first time than in subsequent cases.
4. Crawfish right themselves, when placed on their backs, by the easiest method; and this is found to depend usually upon the relative weight of the two sides of the body. When placed upon a surface which is not level they take advantage, after a few experiences, of the inclination by turning toward the lower side.
* * * * *
THE INSTINCTS, HABITS, AND REACTIONS OF THE FROG.
BY ROBERT MEARNS YERKES.
I. THE ASSOCIATIVE PROCESSES OF THE GREEN FROG
I. SOME CHARACTERISTICS OF THE GREEN FROG.
The common green frog, Rana clamitans, is greenish or brownish in color, usually mottled with darker spots. It is much smaller than the bull frog, being from two to four inches in length ordinarily, and may readily be distinguished from it by the presence of prominent glandular folds on the sides of the back. In the bull frog, Rana catesbeana, these folds are very small and indistinct. The green frog is found in large numbers in many of the ponds and streams of the eastern United States, and its peculiar rattling croak may be heard from early spring until fall. It is more active, and apparently quicker in its reactions, than the bull frog, but they are in many respects similar in their habits. Like the other water frogs it feeds on small water animals, insects which chance to come within reach and, in times of famine, on its own and other species of frogs. The prey is captured by a sudden spring and the thrusting out of the tongue, which is covered with a viscid secretion. Only moving objects are noticed and seized; the frog may starve to death in the presence of an abundance of food if there is no movement to attract its attention. Most green frogs can be fed in captivity by swinging pieces of meat in front of them, and those that will not take food in this way can be kept in good condition by placing meat in their mouths, for as soon as the substance has been tasted swallowing follows.
The animals used for these experiments were kept in the laboratory during the whole year in a small wooden tank. The bottom of this tank was covered with sand and small stones, and a few plants helped to purify the water. An inch or two of water sufficed; as it was not convenient to have a constant stream, it was changed at least every other day. There was no difficulty whatever in keeping the animals in excellent condition.
Of the protective instincts of the green frog which have come to my notice during these studies two are of special interest: The instinctive inhibition of movement under certain circumstances, and the guarding against attack or attempt to escape by 'crouching' and 'puffing.' In nature the frog ordinarily jumps as soon as a strange or startling object comes within its field of vision, but under certain conditions of excitement induced by strong stimuli it remains perfectly quiet, as do many animals which feign death, until forced to move. Whether this is a genuine instinctive reaction, or the result of a sort of hypnotic condition produced by strong stimuli, I am not prepared to say. The fact that the inhibition of movement is most frequently noticed after strong stimulation, would seem to indicate that it is due to the action of stimuli upon the nervous system.
What appears to be an instinctive mode of guarding against attack and escaping an enemy, is shown whenever the frog is touched about the head suddenly, and sometimes when strong stimuli are applied to other parts of the body. The animal presses its head to the ground as if trying to dive or dodge something, and inflates its body. This kind of action is supposed to be a method of guarding against the attack of snakes and other enemies which most frequently seize their prey from the front. It is obvious that by pressing its head to the ground the frog tends to prevent any animal from getting it into its mouth, and in the few instants' delay thus gained it is able to jump. This is just the movement necessary for diving, and it is probable that the action should be interpreted in the light of that instinctive reflex. The 'puffing' also would seem to make seizure more difficult. Another fact which favors this interpretation is that the response is most commonly given to stimuli which seem to come from the front and which for this reason could not easily be escaped by a forward jump, while if the stimulus is so given that it appears to be from the rear the animal usually jumps away immediately. We have here a complex protective reaction which may be called a forced movement. It is, so far as one can see, very much like many reflexes, although it does not occur quite so regularly.
The machine-like accuracy of many of the frog's actions gives a basis for the belief that the animal is merely an automaton. Certain it is that one is safe in calling almost all the frog's actions reflex or instinctive. During months of study of the reaction-time of the frog I was constantly impressed with the uniformity of action and surprised at the absence of evidences of profiting by experience. In order to supplement the casual observations on the associations of the green frog made in the course of reaction-time experiments, the tests described in this paper were made. They do not give a complete view of the associative processes, but rather such a glimpse as will enable us to form some conception of the relation of the mental life of the frog to that of other animals. This paper presents the outlines of work the details of which I hope to give later.
II. EXPERIMENTAL STUDY OF HABITS.
A. The Chief Problems for which solutions were sought in the following experimental study were: (1) Those of associability in general, its characteristics, and the rapidity of learning; (2) of discrimination, including the parts played in associative processes by the different senses, and the delicacy of discrimination in each; (3) of the modifiability of associational reactions and general adaptation in the frog, and (4) of the permanency of associations.
B. Simple Associations, as studied in connection with reaction-time work, show that the green frog profits by experience very slowly as compared with most vertebrates. The animals have individual peculiarities in reaction which enable one in a short time to recognize any individual. To these characteristic peculiarities they stick tenaciously. One, for instance, always jumps upward when strongly stimulated; another has a certain corner of the tank in which it prefers to sit. Their habits are remarkably strong and invariable, and new ones are slowly formed. While using a large reaction box I noticed that the frogs, after having once escaped from an opening which could be made by pushing aside a curtain at a certain point in the box, tended to return to that place as soon as they were again put into the box. This appeared to be evidence of an association; but the fact that such stimuli as light and the relation of the opening to the place at which the animals were put into the box might in themselves be sufficient to direct the animals to this point without the help of any associations which had resulted from previous experience, makes it unsatisfactory. In addition to the possibility of the action being due to specific sensory stimuli of inherent directive value, there is the chance of its being nothing more than the well-known phenomenon of repetition. Frogs, for some reason, tend to repeat any action which has not proved harmful or unpleasant.
For the purpose of more carefully testing this kind of association, a small box with an opening 15 cm. by 10 cm. was arranged so that the animal could escape from confinement in it through the upper part of the opening, the lower portion being closed by a plate of glass 10 cm. by 10 cm., leaving a space 5 cm. by 10 cm. at the top. One subject placed in this box escaped in 5 minutes 42 seconds. After 5 minutes' rest it was given another trial, and this time got out in 2 minutes 40 seconds. The times for a few subsequent trials were: Third, 1 minute 22 seconds; fourth, 4 minutes 35 seconds; fifth, 2 minutes 38 seconds; sixth, 3 minutes 16 seconds. Although this seems to indicate some improvement, later experiments served to prove that the frogs did not readily form any associations which helped them to escape. They tended to jump toward the opening because it was light, but they did not learn with twenty or thirty experiences that there was a glass at the bottom to be avoided. Thinking that there might be an insufficient motive for escape to effect the formation of an association, I tried stimulating the subject with a stick as soon as it was placed in the box. This frightened it and caused violent struggles to escape, but instead of shortening the time required for escape it greatly lengthened it. Here was a case in which the formation of an association between the appearance of the upper part of the clear space and the satisfaction of escape from danger would have been of value to the frog, yet there was no evidence of adaptation to the new conditions within a reasonably short time. There can be little doubt that continuation of the training would have served to establish the habit. This very clearly shows the slowness of adaptation in the frog, in contrast with the rapidity of habit formation in the cat or chick; and at the same time it lends additional weight to the statement that instinctive actions are all-important in the frog's life. A few things it is able to do with extreme accuracy and rapidity, but to this list new reactions are not readily added. When put within the box described, an animal after having once escaped would sometimes make for the opening as if it knew perfectly the meaning of the whole situation, and yet the very next trial it would wander about for half an hour vainly struggling to escape.
A considerable number of simple experiments of this kind were tried with results similar to those just given. The frog apparently examines its surroundings carefully, and just when the observer thinks it has made itself familiar with the situation it reacts in such a way as to prove beyond doubt the absence of all adaptation. In all these experiments it should be said, for the benefit of any who may be trying similar work, that only animals of exceptional activity were used. Most green frogs when placed in the experiment box either sit still a great part of the time or jump about for only a short time. It is very important for studies of this kind, both on account of time saving and the obtaining of satisfactory records, to have animals which are full of energy and eager to escape when in confinement. By choosing such subjects one may pretty certainly avoid all unhealthy individuals, and this, it seems to me, counterbalances the disadvantage of taking animals which may be unusually quick in learning.
C. Complex Associations.
1. Labyrinth Habits.--A more thorough investigation of the associative processes, sensory discrimination and the permanency of impressions has been made by the labyrinth method. A wooden box, 72 cm. long, 28 cm. wide and 28 cm. deep, whose ground plan is represented by Fig. 1, served as the framework for a simple labyrinth. At one end was a small covered box, A, from which the frog was allowed to enter the labyrinth. This entrance passage was used in order that the animal might not be directed to either side by the disturbance caused by placing it in the box. E, the entrance, marks a point at which a choice of directions was necessary. P is a movable partition which could be used to close either the right or the left passage. In the figure the right is closed, and in this case if the animal went to the right it had to turn back and take the left passage in order to get out of the box. A series of interrupted electrical circuits, IC, covered the bottom of a portion of the labyrinth; by closing the key, K, the circuit could be made whenever a frog rested upon any two wires of the series. When the frog happened to get into the wrong passage the key was closed and the animal stimulated. This facilitated the experiment by forcing the animal to seek some other way of escape, and it also furnished material for an association. Having passed through the first open passage, which for convenience we may know as the entrance passage, the animal had to choose again at the exit. Here one of the passages was closed by a plate of glass (in the figure the left) and the other opened into a tank containing water. The box was symmetrical and the two sides were in all respects the same except for the following variable conditions. At the entrance the partition on one side changed the appearance, as it was a piece of board which cut off the light. On either side of the entrance there were grooves for holding card-boards of any desired color. The letters R, R mark sides which in this case were covered with red; W, W mark white spaces. These pieces of cardboard could easily be removed or shifted at any time. At the exit the glass plate alone distinguished the sides, and it is not likely that the animals were able to see it clearly. We have thus at the entrance widely differing appearances on the two sides, and at the exit similarity. The opening from A into the large box was provided with a slide door so that the animal could be prevented from returning to A after entering the labyrinth. The partitions and the triangular division at the entrance extended to the top of the box, 28 cm., so that the animals could not readily jump over them.
The experiments were made in series of ten, with ten-minute intervals between trials. In no case was more than one series a day taken, and wherever a day was missed the fact has been indicated in the tables. The only motive of escape from the box depended upon was the animal's desire to return to the water of the tank and to escape from confinement in the bright light of the room. The tank was one in which the frogs had been kept for several months so that they were familiar with it, and it was as comfortable a habitat as could conveniently be arranged. Usually the animals moved about almost constantly until they succeeded in getting out, but now and then one would remain inactive for long intervals; for this reason no record of the time taken for escape was kept. On account of the great amount of time required by experiments of this kind I have been unable to repeat this series of experiments in toto on several animals in order to get averages, but what is described for a representative individual has been proved normal by test observations on other animals. There are very large individual differences, and it may well be that the subject of the series of experiments herein described was above the average in ability to profit by experience. But, however that may be, what is demonstrated for one normal frog is thereby proved a racial characteristic, although it may be far from the mean condition.
Before beginning training in the labyrinth, preliminary observations were made to discover whether the animals had any tendencies to go either to the right or to the left. When the colored cardboards were removed it was found that there was usually no preference for right or left. In Table I. the results of a few preliminary trials with No. 2 are presented. For these the colors were used, but a tendency to the right shows clearly. Trials 1 to 10 show choice of either the right or the red throughout; that it was partly both is shown by trials 11 to 30, for which the colors were reversed. This individual has therefore, to begin with, a tendency to the right at the entrance. At the exit it went to the right the first time and continued so to do for several trials, but later it learned by failure that there was a blocked passage as well as an open one. In the tables the records refer to choices. It was useless to record time or to lay much stress upon the course taken, as it was sometimes very complicated; all that is given, therefore, is the action in reference to the passages. Right in every case refers to the choice of the open way, and wrong to the choice of the blocked passage. The paths taken improved steadily in that they became straighter. A few representative courses are given in this report. Usually if the animal was not disturbed a few jumps served to get it out of the labyrinth.
TABLE I.
PRELIMINARY TRIALS WITH FROG NO. 2.
Trials. Red on Right. White on Left. 1 to 10 10 times to red 0
Red on Left. White on Right. 11 to 20 4 times to red 6
Red on Left. White on Right. 21 to 30 3 times to red 7
To Red. To White. To Right. To Left. Totals. 17 13 23 7
This table indicates in trials 1 to 10 a strong tendency to the red cardboard. Trials 21 to 30 prove that there was also a tendency to the right.
Training was begun with the labyrinth arranged as shown in Fig. 1, that is, with the left entrance passage and the right exit passage open, and with red cardboard on the right (red was always on the side to be avoided) and white on the left. Table II. contains the results of 110 trials with No. 2, arranged according to right and wrong choice at the entrance and exit. Examination of this table shows a gradual and fairly regular increase in the number of right choices from the first series to the last; after 100 experiences there were practically no mistakes.
With another subject, No. 6a, the results of Table III. were obtained. In this instance the habit formed more slowly and to all appearances less perfectly. Toward the end of the second week of work 6a showed signs of sickness, and it died within a few weeks, so I do not feel that the experiments with it are entirely trustworthy. During the experiments it looked as if the animal would get a perfectly formed habit very quickly, but when it came to the summing up of results it was obvious that there had been little improvement.
TABLE II.
LABYRINTH HABIT. FROG NO. 2.
Entrance. Exit. Remarks. Trials. Right. Wrong. Right. Wrong. 1- 10 1 9 4 6 One day rest. 11- 20 2 8 5 5 21- 30 4 6 7 3 31- 40 5 5 6 4 41- 50 5 5 6 2 (17) (33) (30) (20) 51- 60 9 1 8 2 61- 70 6 4 10 0 71- 80 7 3 9 1 81- 90 9 1 8 2 91-100 10(50) 0(10) 10(52) 0( 8) --- --- --- --- 67 43 82 28
Other animals which were used gave results so similar to those for frog No. 2 that I feel justified in presenting the latter as representative of the rapidity with which the green frog profits by experience.
TABLE III.
LABYRINTH HABIT. FROG NO. 6a.
Entrance. Exit. Remarks. Trials Right. Wrong. Right. Wrong. 1- 10 6 4 5 5 One day rest. 11- 20 7 3 4 6 21- 30 2 8 1 9 31- 40 6 4 1 9 41- 50 7 3 8 2 (28) (22) (19) (31) 51- 60 5 5 7 3 61- 70 6 4 4 6 71- 80 4 6 3 7 One day rest. 81- 90 5 5 7 3 91-100 10(30) 0(20) 8(29) 2(21) ---- ---- ---- ---- (58) (44) (48) (52)
Preliminary Trials.
Red on Left Partition at Exit on Right 1- 5 5 times to Red 4 times to Partition.
Red on Right Partition at Exit on Left 6-10 3 times to Red 5 times to Partition.
2. Rapidity of Habit Formation.--As compared with other vertebrates whose rapidity of habit formation is known, the frog learns slowly. Experimental studies on the dog, cat, mouse, chick and monkey furnish excellent evidence of the ability of these animals to profit quickly by experience through the adapting of their actions to new conditions. They all show marked improvement after a few trials, and after from ten to thirty most of them have acquired perfect habits. But the comparison of the frog with animals which are structurally more similar to it is of greater interest and value, and we have to inquire concerning the relation of habit formation in the frog to that of fishes and reptiles. Few experimental studies with these animals have been made, and the material for comparison is therefore very unsatisfactory. E.L. Thorndike[1] has demonstrated the ability of fishes to learn a labyrinth path. In his report no statement of the time required for the formation of habit is made, but from personal observation I feel safe in saying that they did not learn more quickly than did the frogs of these experiments. Norman Triplett[2] states that the perch learns to avoid a glass partition in its aquarium after repeatedly bumping against it. Triplett repeated Moebius' famous experiment, and found that after a half hour's training three times a week for about a month, the perch would not attempt to capture minnows which during the training periods had been placed in the aquarium with the perch, but separated from them by a glass partition. Triplett's observations disprove the often repeated statement that fishes do not have any associative processes, and at the same time they show that the perch, at least, learns rapidly--not so rapidly, it is true, as most animals, but more so in all probability than the amphibia.
[1] Thorndike, Edward: 'A Note on the Psychology of Fishes,' American Naturalist. 1899, Vol. XXXIII., pp. 923-925.
[2] Triplett, Norman: 'The Educability of the Perch,' _Amer. Jour. Psy._, 1901, Vol. XII., pp. 354-360.
The only quantitative study of the associative processes of reptiles available is some work of mine on the formation of habits in the turtle.[3] In the light of that study I can say that the turtle learns much more rapidly than do fishes or frogs. Further observations on other species of turtles, as yet unpublished, confirm this conclusion.
[3] Yerkes, Robert Mearns: 'The Formation of Habits in the Turtle,' Popular Science Monthly, 1901, Vol. LVIII., pp. 519-535.
For the frog it is necessary to measure and calculate the improvement in order to detect it at first, while with the turtle or chick the most casual observer cannot fail to note the change after a few trials. In connection with the quickness of the formation of associations it is of interest to inquire concerning their permanency. Do animals which learn slowly retain associations longer? is a question to which no answer can as yet be given, but experiments may readily be made to settle the matter. I have tested the frog for permanency, and also the turtle, but have insufficient data for comparison.
3. Sensory Data Contributing to the Associations.--Among the most important of the sensory data concerned in the labyrinth habit are the visual impressions received from the different colored walls, the slight differences in brightness of illumination due to shadows from the partitions and the contrast in form of the two sides of the labyrinth resulting from the use of the partitions, and the muscular sensations dependent upon the direction of turning. The experiments proved beyond question that vision and the direction of turning were the all-important factors in the establishment of the habit. At first it seemed as if the direction of turning was the chief determinant, and only by experimenting with colors under other conditions was I able to satisfy myself that the animals did notice differences in the appearance of their surroundings and act accordingly. In Table IV. some results bearing on this point have been arranged. To begin with, the habit of going to the left when the red was on the right at the entrance had been established; then, in order to see whether the colors influenced the choice, I reversed the conditions, placing the red on the left, that is, on the open-passage side. The results as tabulated in the upper part of Table IV. show that the animals were very much confused by the reversal; at the entrance where there were several guiding factors besides the colors there were 50 per cent. of mistakes, while at the exit where there were fewer differences by which the animal could be directed it failed every time. This work was not continued long enough to break up the old habit and replace it by a new one, because I wished to make use of the habit already formed for further experiments, and also because the animals remained so long in the labyrinth trying to find their way out that there was constant danger of losing them from too prolonged exposure to the dry air.
TABLE IV.
INFLUENCE OF CHANCES OF CONDITIONS. FROG NO. 2.
Habit perfectly formed of going to Left (avoiding Red) at entrance and to Right at exit. Conditions now reversed. Red on Left. Partition at Exit on Right.
Trials. Entrance. Exit. Remarks. Right. Wrong. Right. Wrong. 1- 5 3 2 0 5 6-10 2 3 0 5
Discontinued because animal remained so long in labyrinth that there was danger of injuring it for further work. This shows that the habit once formed is hard to change.
Given 20 trials with conditions as at first in order to establish habit again.
1-10 9 1 8 2 11-20 10 0 9 1
Colors reversed, no other change. To test influence of colors.
1-10 6 4 10 0
INFLUENCE OF DISTURBANCE WHEN ANIMAL IS ENTERING BOX.
No Disturbance. Animal Touched.
To Red (Right). To White (Left). To Red. To White. 2 8 5 5
This was after the tendency to go to the Left at the entrance had been established.
These experiments to test the effect of changing colors are also of interest in that they show in a remarkable way the influence of the direction of turning. The animal after succeeding in getting around the first part of the labyrinth failed entirely to escape at the exit. Here it should have turned to the left, instead of the right as it was accustomed to, but it persisted in turning to the right. Fig. 3 represents approximately the path taken in the first trial; it shows the way in which the animal persisted in trying to get out on the right. From this it is clear that both vision and the complex sensations of turning are important.
The latter part of Table IV. presents further evidence in favor of vision. For these tests the colors alone were reversed. Previous to the change the animal had been making no mistakes whatever, thereafter there were four mistakes at the entrance and none at the exit. Later, another experiment under the same conditions was made with the same animal, No. 2, with still more pronounced results. In this case the animal went to the white, that is, in this instance, into the blind alley, and failed to get out; several times it jumped over to the left side (the open-passage side) of the box but each time it seemed to be attracted back to the white or repelled by the red, more probably the latter, as the animal had been trained for weeks to avoid the red. Concerning the delicacy of visual discrimination I hope to have something to present in a later paper.
The tactual stimuli given by contact with the series of wires used for the electrical stimulus also served to guide the frogs. They were accustomed to receive an electrical shock whenever they touched the wires on the blocked side of the entrance, hence on this side the tactual stimulus was the signal for a painful electrical stimulus. When the animal chose the open passage it received the tactual stimulus just the same, but no shock followed. After a few days' experimentation it was noted that No. 2 frequently stopped as soon as it touched the wires, whether on the open or the closed side. If on the closed side, it would usually turn almost immediately and by retracing its path escape by the open passage; if on the open side, it would sometimes turn about, but instead of going back over the course it had just taken, as on the other side, it would sit still for a few seconds, as if taking in the surroundings, then turn again and go on its way to the exit. This whole reaction pointed to the formation of an association between the peculiar tactual sensation and the painful shock which frequently followed it. Whenever the tactual stimulus came it was sufficient to check the animal in its course until other sensory data determined the next move. When the wrong passage had been chosen the visual data gotten from the appearance of the partition which blocked the path and other characteristics of this side of the labyrinth determined that the organism should respond by turning back. When, on the other hand, the open passage had been selected, a moment's halt sufficed to give sensory data which determined the continuation of the forward movement. Although this reaction did not occur in more than one tenth of the trials, it was so definite in its phases as to warrant the statements here made. Fig. 4 gives the path taken by No. 2 in its 123d trial. In this experiment both choices were correctly made, but when the frog touched the wires on the open side it stopped short and wheeled around; after a moment it turned toward the exit again, but only to reverse its position a second time. Soon it turned to the exit again, and this time started forward, taking a direct course to the tank. The usual course for animals which had thoroughly learned the way to the tank is that chosen in Fig. 5.
An interesting instance of the repetition of a reaction occurred in these experiments. A frog would sometimes, when it was first placed in the box, by a strong jump get up to the edge; it seldom jumped over, but instead caught hold of the edge and balanced itself there until exhaustion caused it to fall or until it was taken away. Why an animal should repeat an action of the nature of this is not clear, but almost invariably the second trial resulted in the same kind of reaction. The animal would stop at the same point in the box at which it had previously jumped, and if it did not jump, it would look up as if preparing to do so. Even after a frog had learned the way to the tank such an action as this would now and then occur, and almost always there would follow repetition in the manner described.
4. The Effect of Fear upon Habit Formation.--A certain amount of excitement undoubtedly promotes the formation of associations, but when the animal is frightened the opposite is true. I have no hesitation in stating that, in case of the green frog, any strong disturbing stimulus retards the formation of associations. Although the frogs gave little evidence of fear by movements after being kept in the laboratory for a few weeks, they were really very timid, and the presence of any strange object influenced all their reactions. Quiescence, it is to be remembered, is as frequently a sign of fear as is movement, and one is never safe in saying that the frog is not disturbed just because it does not jump. The influence of the experimenter's presence in the room with the frogs which were being tried in the labyrinth became apparent when the animals were tried in a room by themselves. They escaped much more quickly when alone. In order to keep records of the experiments it was necessary for me to be in the room, but by keeping perfectly quiet it was possible to do this without in any objectionable way influencing the results. It may be, however, that for this reason the learning is somewhat slower than it would have been under perfectly natural conditions. Early in this paper reference was made to the fact that the frog did not learn to escape from a box with a small opening at some distance from the floor if it was prodded with a stick. I do not mean to say that the animal would never learn under such conditions, but that they are unfavorable for the association of stimuli and retard the process. This conclusion is supported by some experiments whose results are tabulated at the bottom of Table IV. In these trials the animal had been trained to go to the left and to avoid red. At first ten trials were given in which the frog was in no way disturbed. The result was eight right choices and two wrong ones. For the next ten trials the frog was touched with a stick and thus made to enter the labyrinth from the box, A. This gave five right and five wrong choices, apparently indicating that the stimulus interfered with the choice of direction. Several other observations of this nature point to the same conclusion, and it may therefore be said that fright serves to confuse the frog and to prevent it from responding to the stimuli which would ordinarily determine its reaction.
5. The Permanency of Associations.--After the labyrinth habit had been perfectly formed by No. 2, tests for permanency were made, (1) after six days' rest and (2) after thirty days. Table V. contains the results of these tests. They show that for at least a month the associations persist. And although there are several mistakes in the first trials after the intervals of rest, the habit is soon perfected again. After the thirty-day interval there were forty per cent. of mistakes at the exit for the first series, and only 20 per cent. at the entrance. This in all probability is explicable by the fact that the colors acted as aids at the entrance, whereas at the exit there was no such important associational material.
TABLE V.
PERMANENCY OF ASSOCIATIONS. FROG NO. 2.
Tests after six days' rest (following the results tabulated in Table III.).
Trial. Entrance. Exit. Right. Wrong. Right. Wrong 1-10 7 3 8 2 (110-120) 11-20 10 0 10 0
Tests after THIRTY days' rest. 1-10 8 2 6 4 10-20 10 0 10 0
D. Association of Stimuli.--In connection with reaction-time work an attempt was made to form an association between a strong visual stimulus and a painful electrical shock, with negative results. A reaction box, having a series of interrupted circuits in the bottom like those already described for other experiments, and an opening on one side through which a light could be flashed upon the animal, served for the experiments. The tests consisted in the placing of a frog on the wires and then flashing an electric light upon it: if it did not respond to the light by jumping off the wires, an electrical stimulus was immediately given. I have arranged in Table VI. the results of several weeks' work by this method. In no case is there clear evidence of an association; one or two of the frogs reacted to the light occasionally, but not often enough to indicate anything more than chance responses. At one time it looked as if the reactions became shorter with the continuation of the experiment, and it was thought that this might be an indication of the beginning of an association. Careful attention to this aspect of the results failed to furnish any satisfactory proof of such a change, however, and although in the table statements are given concerning the relative numbers of short and long reactions I do not think they are significant.
TABLE VI.
ASSOCIATION OF ELECTRICAL AND VISUAL STIMULI. FROG No. 1a, 2a, 3a, 4a, 5a, A and Z.
Frog. Total No. Days. Result. Trials.
No. 1a 180 18 Increase in number of long reaction toward end. No evidence of association.
No. 2a 180 17 Increase in number of short reactions toward end. No evidence of association.
No. 3a 180 17 Marked increase in the number of short reactions toward end. No other evidence of association.
No. 4a 200 19 Slight increase in the short reactions. There were a few responses to the light on the third day.
No. 5a 200 20 No increase in the number of short reactions. Few possible responses to light on second and third days.
Frog A 250 20 No evidence of association.
Frog Z 450 28 No evidence of association.
To all appearances this is the same kind of an association that was formed, in the case of the labyrinth experiments, between the tactual and the electrical stimuli. Why it should not have been formed in this case is uncertain, but it seems not improbable that the light was too strong an excitement and thus inhibited action. There is also the probability that the frog was constrained by being placed in a small box and having the experimenter near.
III. SUMMARY.
1. The green frog is very timid and does not respond normally to most stimuli when in the presence of any strange object. Fright tends to inhibit movement.
2. That it is able to profit by experience has been proved by testing it in simple labyrinths. A few experiences suffice for the formation of simple associations; but in case of a series of associations from fifty to a hundred experiences are needed for the formation of a perfect habit.
3. Experiment shows that the frog is able to associate two kinds of stimuli, e.g., the peculiar tactual stimulus given by a wire and a painful electric stimulus which in the experiments followed the tactual. In this case the animal learns to jump away, upon receiving the tactual stimulus, before the experimenter gives the electric stimulus.
4. Vision, touch and the organic sensations (dependent upon direction of turning) are the chief sensory factors in the associations. The animals discriminate colors to some extent.
5. Perfectly formed habits are hard to change.
6. Fear interferes with the formation of associations.
7. Associations persist for at least a month.
II. REACTION TIME OF THE GREEN FROG TO ELECTRICAL AND TACTUAL STIMULI
IV. THE PROBLEMS AND POSSIBILITIES OF COMPARATIVE REACTION-TIME STUDIES.
Animal reaction time is at present a new field of research of evident importance and full of promise. A great deal of time and energy has been devoted to the investigation of various aspects of the time relations of human neural processes; a multitude of interesting facts have been discovered and a few laws established, but the results seem disproportionate to the amount of patient labor expended. Physiologists have determined the rate of transmission of the neural impulse for a few animals, and rough estimates of the time required for certain changes in the nervous system have been made, but this is all we have to represent comparative study. Just the path of approach which would seem most direct, in case of the time of neural changes, has been avoided. Something is known of the ontogenetic aspect of the subject, practically nothing of the phylogenetic; yet, in the study of function the comparative point of view is certainly as important as it is in the study of structure. In calling attention to the importance of the study of animal reaction time I would not detract from or minimize the significance of human investigations. They are all of value, but they need to be supplemented by comparative studies.
It is almost impossible to take up a discussion of the time relations of neural processes without having to read of physiological and psychological time. The time of nerve transmission, we are told, is pure physiological time and has nothing whatever to do with psychic processes; the time occupied by the changes in brain centers is, on the contrary, psychological time. At the very beginning of my discussion of this subject I wish to have it clearly understood that I make no such distinction. If one phase of the neural process be called physiological time, with as good reason may all be so named. I prefer, therefore, to speak of the time relations of the neural process.
Of the value of reaction-time studies, one may well believe that it lies chiefly in the way of approach which they open to the understanding of the biological significance of the nervous system. Certainly they are not important as giving us knowledge of the time of perception, cognition, or association, except in so far as we discover the relations of these various processes and the conditions under which they occur most satisfactorily. To determine how this or that factor in the environment influences the activities of the nervous system, and in what way system may be adjusted to system or part-process to whole, is the task of the reaction-time investigator.
The problems of reaction time naturally fall within three classes: Those which deal with (1) nerve transmission rates; (2) the time relations of the spinal center activities, and (3) brain processes. Within each of these groups there are innumerable special problems for the comparative physiologist or psychologist. Under class 1, for instance, there is the determining of the rates of impulse transmission in the sensory and the motor nerves, (a) for a variety of stimuli, (b) for different strengths of each stimulus, (c) for different conditions of temperature, moisture, nourishment, fatigue, etc., in case of each stimulus, (d) and all this for hundreds of representative animals. From this it is clear that lines of work are not lacking.
Closely related to these problems of rate of transmission are certain fundamental problems concerning the nature of the nerve impulse or wave. Whether there is a nerve wave, the reaction-time worker has as favorable an opportunity to determine as anyone, and we have a right to expect him to do something along this line. The relations of the form of the nerve impulse to the rhythm of vital action, to fatigue and to inhibition are awaiting investigation. Some of the most important unsettled points of psychology depend upon those aspects of neural activities which we ordinarily refer to as phenomena of inhibition, and which the psychologist is helpless to explain so long as the physiological basis and conditions are not known.
Then, too, in the study of animals the relation of reaction time to instincts, habits, and the surroundings of the subject are to be noted. Variability and adaptability offer chances for extended biological inquiries; and it is from just such investigations as these that biology has reason to expect much. The development of activity, the relation of reflex action to instinctive, of impulsive to volitional, and the value of all to the organism, should be made clear by reaction-time study. Such are a few of the broad lines of inquiry which are before the comparative student of animal reaction time. It is useless to dwell upon the possibilities and difficulties of the work, they will be recognized by all who are familiar with the results of human studies.
In the study of the time relations of neural processes Helmholtz was the pioneer. By him, in 1850, the rate of transmission of the nerve impulse in the sciatic nerve of the frog was found to be about 27 meters per second[4]. Later Exner[5] studied the time occupied by various processes in the nervous system of the frog by stimulating the exposed brain in different regions and noting the time which intervened before a contraction of the gastrocnemius in each case. Further investigation of the frog's reflex reaction time has been made by Wundt[6], Krawzoff and Langendorff[7], Wilson[8] and others, but in no case has the method of study been that of the psychologist. Most of the work has been done by physiologists who relied upon vivisectional methods. The general physiology of the nervous system of the frog has been very thoroughly worked up and the papers of Sanders-Ezn[9], Goltz[10] Steiner[11] Schrader[12] and Merzbacher[13],[14] furnish an excellent basis for the interpretation of the results of the reaction-time studies.
[4] Helmholtz, H.: 'Vorläufiger Bericht über die Portpflanzungsgeschwindigkeit der Nervenreizung.' _Arch. f. Anal. u. Physiol._, 1850, S. 71-73.
[5] Exner, S.: 'Experimentelle Untersuchung der einfachsten psychischen Processe.' Pflüger's Arch., Bd. 8. 1874, S. 526-537.
[6] Wundt, W.: 'Untersuchungen zur Mechanik der Nerven und Nervencentren.' Stuttgart, 1876.
[7] Krawzoff, L., und Langendorff, O.: 'Zur elektrischen Reizung des Froschgehirns.' Arch. f. Anal. u. Physiol., Physiol. Abth., 1879, S. 90-94.
[8] Wilson, W.H.: 'Note on the Time Relations of Stimulation of the Optic Lobes of the Frog.'Jour. of Physiol., Vol. XI., 1890, pp. 504-508.
[9] Sanders-Ezn: 'Vorarbeit für die Erforschung des Reflexmechanismus in Lendentmark des Frosches.' Berichte über die Verhandlungen der Kgl. sächs. Gesellsch. d. Wissensch. zu Leipzig, 1867, S. 3.
[10] Goltz, F.: 'Beiträge zur Lehre von den Functionen der Nervencentren des Frosches.' Berlin, 1869, 130 S.
[11] Steiner, J.: 'Untersuchungen über die Physiologie des Froschhirns.' Braunschweig, 1885, 127 S.
[12] Schrader, M.G.: 'Zur Physiologie des Froschgehirns.' Pflüger's Arch., Bd. 41, 1887, S. 75-90.
[13] Merzbacher, L.: 'Ueber die Beziebungen der Sinnesorgane zu den Reflexbewegungen des Frosches.' Pflüger's Arch., Bd. 81, 1900, S. 223-262.
[14] Merzbacher, L.: 'Untersuchungen über die Regulation der Bewegungen der Wirbelthiere. I. Beobachtungen an Fröschen.' Pflüger's Arch., Bd. 88, 1901, S. 453-474, 11 Text-figuren.
In the present investigation it has been my purpose to study the reactions of the normal frog by the reaction-time methods of the psychologist. Hitherto the amount of work done, the extent of movements or some other change has been taken as a measure of the influence of a stimulus. My problem is, What are the time relations of all these reactions? With this problem in mind I enter upon the following program: (1) Determination of reaction time to electrical stimuli: (a) qualitative, (b) quantitative, (c) for different strengths of current; (2) Determination of reaction time to tactual stimuli (with the same variations); (3) Auditory: (a) qualitative, (b) quantitative, with studies on the sense of hearing; (4) Visual: (a) qualitative, (b) quantitative, with observations concerning the importance of this sense in the life of the frog, and (5) Olfactory: (a) qualitative, (b) quantitative.
The present paper presents in rather bare form the results thus far obtained on electrical, tactual, and auditory reaction time; discussion of them will be deferred until a comparison of the results for the five kinds of stimuli can be given.
V. METHOD OF STUDY.
The measurements of reaction time herein considered were made with the Hipp Chronoscope. Cattell's 'Falling Screen' or 'Gravity Chronoscope' was used as a control for the Hipp. The Gravity Chronoscope consists of a heavy metal plate which slides easily between two vertical posts, with electrical connections so arranged that the plate, when released from the magnet at the top of the apparatus, in its fall, at a certain point breaks an electric circuit and at another point further down makes the same circuit. The rate of fall of the plate is so nearly constant that this instrument furnishes an accurate standard time with which Hipp readings may be compared, and in accordance with which the Hipp may be regulated. For, since the rate of a chronoscope varies with the strength of the current in use, with the variations in temperature and with the positions of the springs on the magnetic bar, it is always necessary to have some standard for corrections. In these experiments the time of fall of the gravity chronoscope plate, as determined by the graphic method with a 500 S.V. electric tuning fork, was 125[sigma] (i.e., thousandths of a second).
This period, 125[sigma], was taken as a standard, and each hour, before the beginning of reaction-time experiments, the time of the plate's fall was measured ten times with the Hipp, and for any variation of the average thus obtained from 125[sigma], the standard, the necessary corrections were made by changing the position of the chronoscope springs or the strength of the current.
The standard of comparison, 125[sigma], is shorter than most of the reaction times recorded, but since the time measured was always that from the breaking to the making of the circuit passing through the chronoscope it cannot be urged that there were errors resulting from the difference of magnetization which was caused by variations in the reaction time. But it is evident that the danger from differences in magnetization, if such exists, is not avoided in this way; instead, it is transferred from the reaction time proper to the period of preparation immediately preceding the reaction; for, from the moment the chronoscope is started until the stimulus is given a current is necessarily passing through the instrument. At a verbal signal from the operator the assistant started the chronoscope; the stimulus was then given by the operator, and the instrument recorded the time from the breaking of the circuit, effected by the stimulating apparatus, to the making of the circuit by the reaction of the animal. Despite precautions to prevent it, the period from the starting of the chronoscope to the giving of the stimulus was variable, and errors were anticipated, but a number of the tests proved that variations of even a second did not cause any considerable error.
A fairly constant current for the chronoscope was supplied by a six-cell 'gravity battery' in connection with two storage cells, GB (Fig. 6). This current could be used for two hours at a time without any objectionable diminution in its strength. The introduction of resistance by means of the rheostat, R, was frequently a convenient method of correcting the chronoscope.
Fig. 6 represents the general plan of the apparatus used in these experiments.
The general method of experimentation is in outline as follows:
1. At a 'ready' signal from the operator the assistant makes the chronoscope circuit by closing a key, K (Fig. 6), and then immediately starts the chronoscope.
2. Stimulus is given by the operator as soon as the chronoscope is started, and by this act the chronoscope circuit is broken and the record begun.
3. Animal reacts and by its movements turns a key, RK (Fig. 6), thus making the chronoscope circuit and stopping the record.
4. Assistant stops chronoscope and takes reading.
The steps of this process and the parts of the apparatus concerned in each may be clearly conceived by reference to the diagram given in Fig. 6. The various forms of stimulating apparatus used and the modification of the method will be described in the sections dealing with results. The same reaction key was used throughout (see Fig. 7). Its essential features are a lever l, pivoted in the middle and bearing a post at either end, p, p. From the middle of this lever there projected upward a small metal bar, b, through the upper part of which a string to the animal ran freely except when it was clamped by the spring, s. This string, which was attached to the subject's leg by means of a light elastic band, after passing through the bar ran over a wheel, w, and hung tense by reason of a five-gram weight attached to the end. Until everything was in readiness for an experiment the string was left free to move through the bar so that movement of the animal was not hindered, but the instant before the ready-signal was given it was clamped by pressure on s. The diagram shows the apparatus arranged for a reaction. The current is broken, since 1 and 2 are not in contact, but a slight movement of the animal turns the lever enough to bring 1 against 2, thus making the circuit and stopping the chronoscope. When the motor reaction of the subject was violent the string pulled out of the clamp so that the animal was free from resistance, except such as the string and weight offered. The five-gram weight served to give a constant tension and thus avoided the danger of error from this source. Between experiments the weight was placed on the table in order that there might be no strain upon the subject.
That the subject might be brought into a favorable position for an experiment without being touched by the operator a special reaction box was devised.
The animals used in these studies were specimens of Rana clamitans which were kept in a tank in the laboratory throughout the year.
VI. ELECTRIC REACTION TIME.
The reaction time to electrical stimuli was determined first because it seemed probable that this form of the pain reaction would be most useful for comparison with the auditory, visual, olfactory and tactual reactions. In this paper only the electrical and the tactual reaction times will be considered. The former will be divided into two groups: (1) Those resulting from a stimulus given by touching electrodes to the leg of the frog, and (2) those gotten by having the frog resting upon wires through which a current could be passed at any time.
Group 1 of the electrical reactions were taken under the following conditions. A reaction box about 40 cm. in diameter was used. The mean temperature of the experimenting room was about 20° C. In all cases the string was attached to the left hind leg of the frog, and the stimulus applied to the middle of the gastrocnemius muscle of the right hind leg. Reaction times were taken in series of ten, excluding those which were imperfect. As the moistness of the skin affects the strength of the electric stimulus received, it was necessary to moisten the animal occasionally, but as it did not seem advisable to disturb it after each experiment this was done at intervals of five minutes throughout the series. Were it not for this precaution it might be said that lengthening of the reaction times toward the end of a series simply indicated the weakening of the stimulus which resulted from the gradual drying of the skin. The stimulus in this group was applied by means of the stimulating apparatus of Fig. 6. It is merely two wire electrodes which could be placed upon the animal, with the additional device of a key for the breaking of the chronoscope circuit the instant the stimulus was given. The most serious objection to this method of stimulating is that there is a tactual as well as an electrical stimulus.
Before presenting averages, two representative series of reactions may be considered.
SERIES I. FROG B. APRIL 9, 1900. 10 A.M.
Temperature 19° C. String to left hind leg. Stimulus to right hind leg.
Strength of stimulating current 1.0 volt, .0001 ampère.
Number of Experiment. Hour. Reaction Time. Remarks.
1 10.25 No reaction. 2 10.27 No reaction. 3 10.30 139[sigma] 4 10.34 164 5 10.35 102 6 10.37 169 7 10.39 151 8 10.40 152 9 10.42 144 10 10.43 152 11 10.45 122 12 10.51 179 13 10.54 No reaction.
Average of 10, 147.4[sigma]
SERIES 2. FROG F. ELECTRICAL STIMULUS.
No. Hour. Reaction Time. Remarks. Deviation from Mean.
1 10.19 35[sigma] Probable reaction to visual stim. 2 10.22 173 4.7 3 10.24 161 - 7.3 4 10.25 133 -35.3 5 10.26 199 30.7 6 10.28 130 -38.3 7 10.32 179 10.7 8 10.34 187 18.7 9 10.35 60 Probable reflex. 10 10.37 183 14.7 11 10.38 166 - 2.3 12 10.39 172 3.7
Average of 10, 168.3[sigma] Average of first 5, 159.2[sigma] Average Variation, 16.64[sigma] Average of second 5, 177.4[sigma]
Both are fairly representative series. They show the extremely large variations, in the case of series 1, from 102 to 179[sigma]. In all these experiments such variation is unavoidable because it is impossible to have the conditions uniform. A very slight difference in the frog's position, which could not be detected by the operator, might cause considerable difference in the time recorded. Efforts were made to get uniform conditions, but the results seem to show that there is still much to be desired in this direction.
Tables VII. contains the results of four series of ten reactions each for frog A. It will be noticed that the time for the first five in each series is much shorter than that for the last five; this is probably indicative of fatigue.
TABLE VII.
REACTION TIME OF FROG A TO ELECTRICAL STIMULI.
Series of Averages Averages of Averages of ten reactions. of series. first five. second five. 1 163.1[sigma] 134.6[sigma] 191.6[sigma] 2 186.2 176.2 196.2 3 161.1 125.2 197.0 4 158.3 101.6 215.0 General averages 167.2[sigma] 134.4[sigma] 199.9[sigma]
TABLE VIII.
REACTION TIME OF FROG B TO ELECTRICAL STIMULI.
1 132.7[sigma] 118.2[sigma] 147.4[sigma] 2 196.6 167.8 225.4 3 147.4 145.5 149.8 4 157.5 152.0 163.0 General averages 158.6[sigma] 145.9[sigma] 171.4[sigma]
TABLE IX.
NORMAL AND REFLEX REACTION TIME OF SIX ANIMALS TO ELECTRICAL STIMULUS.
Normal. Reflex. Average for 20 Average for 20 Frog. reactions. Mean Var. reactions. Mean Var. A 149.5[sigma] 24.0[sigma] B 158.3 16.0 51.5[sigma] 8.0[sigma] C 191.0 24.3 D 167.0 10.1 E 182.4 28.0 45.1 5.5 F 176.3 10.2 46.0 4.5 General Average. 167.9[sigma] 18.8[sigma] 47.5[sigma] 6.0[sigma]
For D the average is for ten reactions.
B and E were males, F a female; the sex of the others was not determined by dissection and is uncertain.
Early in the experiments it became evident that there were three clearly defined types of reactions: there were a number of reactions whose time was shorter than that of the ordinary quick voluntary pain reaction, and there were also many whose time was considerably longer. The first type it was thought might represent the spinal reflex reaction time. For the purpose of determining whether the supposition was true, at the end of the series of experiments three of the frogs were killed and their reflex reaction time noted. This was done by cutting the spinal cord just back of the medulla, placing the animal on an experimenting board close to the reaction key with the thread from the key fastened to the left leg as in case of the previous work and stimulating the gastrocnemius with an induced current by the application of wire electrodes.
In Table IX. the reflex reaction times for the three animals are given.
The following results obtained with frog E show that the time of reaction increases with the increase in the time after death. The average of 20 reactions by E taken an hour after the cord had been cut was 45.5[sigma]; the average of 20 taken twenty hours later was 55.85[sigma].
As a rule the reflex reactions were but slightly variable in time as is indicated by the accompanying series.
SERIES OF REFLEX REACTIONS OF FROG F. Taken at rate of one per minute.
1 50[sigma] 2 58 3 55 4 59 5 48 6 46 7 45 8 51 9 42 10 44
Throughout these experiments it was noticed that any stimulus might cause (1) a twitch in the limb stimulated, or (2) a twitch followed by a jump, or (3) a sudden jump previous to which no twitch could be detected. And it soon appeared that these types of reaction, as it seems proper to call them, would have to be considered in any determination of the mean reaction time. As proof of the type theory there is given (Fig. 8) a graphic representation of 277 reactions to the electrical stimulus.
The column of figures at the left indicates the number of reactions at any point. Below the base line are the classes. For convenience of plotting the reactions have been grouped into classes which are separated by 25[sigma]. Class 1 includes all reactions between 1[sigma] and 25[sigma], class 2 all from 25[sigma] to 50[sigma], and so on to 400[sigma], thereafter the classes are separated by 100[sigma]. It is noticeable that there is one well-marked mode at 75[sigma]. A second mode occurs at 175[sigma]. This is the primary and in our present work the chiefly significant mode, since it is that of the quick instinctive reaction to a stimulus. At 500[sigma] there is a third mode; but as such this has little meaning, since the reactions are usually pretty evenly distributed from 300[sigma] on to 2000[sigma]; if there is any grouping, however, it appears to be about 500[sigma] and 800[sigma].
The first mode has already been called the reflex mode. The short reactions referred to usually lie between 40[sigma] and 80[sigma], and since experiment has shown conclusively that the spinal reflex occupies about 50[sigma], there can be little doubt that the first mode is that of the reflex reaction time.
The second mode represents those reactions which are the result of central activity and control. I should be inclined to argue that they are what we usually call the instinctive and impulsive actions. And the remaining reactions represent such as are either purely voluntary, if any frog action can be so described, or, in other words, depend upon such a balancing of forces in the brain as leads to delay and gives the appearance of deliberate choice.
Everything points to some such classification of the types as follows: (1) Stimuli strong enough to be injurious cause the shortest possible reaction by calling the spinal centers into action, or if not spinal centers some other reflex centers; (2) slightly weaker stimuli are not sufficient to affect the reflex mechanism, but their impulse passes on to the brain and quickly discharges the primary center. There is no hesitation, but an immediate and only slightly variable reaction; just the kind that is described as instinctive. As would be expected, the majority of the frog's responses are either of the reflex or of this instinctive type. (3) There is that strength of stimulus which is not sufficient to discharge the primary center, but may pass to centers of higher tension and thus cause a response. This increase in the complexity of the process means a slower reaction, and it is such we call a deliberate response. Precisely this kind of change in neural action and in reaction time is at the basis of voluntary action. And (4) finally, the stimulus may be so weak that it will not induce a reaction except by repetition. Just above this point lies the threshold of sensibility, the determination of which is of considerable interest and importance.
Group 2 of the electrical reactions consists of three series taken to determine the relation of strength of stimulus to reaction time. The conditions of experimentation differed from those for group 1 in the following points: (1) The stimulus was applied directly by the making of a circuit through wires upon which the subject rested (Fig. 9); (2) the thread was attached to the right hind leg; (3) the thread, instead of being kept at the tension given by the 5-gram weight as in the former reactions, was slackened by pushing the upright lever of the reaction key one eighth of an inch toward the animal. This was done in order to avoid the records given by the slight twitches of the legs which precede the motor reaction proper. For this reason the reactions of group 2 are not directly comparable with those of group 1. Fig. 9 is the plan of the bottom of a reaction box 15 cm. at one end, 30 cm. at the other, 60 cm. long and 45 cm. deep. On the bottom of this, at one end, a series of interrupted circuits were arranged as shown in the figure. The wires were 1.2 cm. apart, and an animal sitting anywhere on the series necessarily touched two or more, so that when the stimulus key, X, was closed the circuit was completed by the animal's body; hence, a stimulus resulted. The stimulus key, X, was a simple device by which the chronoscope circuit, c, c, was broken at the instant the stimulus circuit, s, c, was made.
Cells of 'The 1900 Dry Battery' furnished the current used as a stimulus. Three different strengths of stimulus whose relative values were 1, 2 and 4, were employed in the series 1, 2 and 3. Careful measurement by means of one of Weston's direct-reading voltmeters gave the following values: 1 cell, 0.2 to 0.5 volt, 0.00001 to 0.00003 ampère. This was used as the stimulus for series 1. 2 cells, 0.5 to 1.0 volt, 0.00003 to 0.00006 ampère. This was used for series 2. 4 cells, 1.2 to 1.8 volt, 0.00007 to 0.0001 ampère. This was used for series 3.
The reactions now under consideration were taken in sets of 24 in order to furnish evidence on the problem of fatigue. The stimulus was given at intervals of one minute, and the subject was moistened at intervals of ten minutes. To obtain 24 satisfactory reactions it was usually necessary to give from thirty to forty stimulations. Five animals, numbers 1, 2, 4, 5, and 6, served as subjects. They were green frogs whose size and sex were as follows:
Length. Weight. Sex. Number 1 7.5 cm. 35 grams. Male. Number 2 7.3 " 37 " Male. Number 4 8.2 " 50.4 " Female? Number 5 7.1 " 25 " Female. Number 6 7.8 " 42 " Male.
For most of these frogs a one-cell stimulus was near the threshold, and consequently the reaction time is extremely variable. In Table X. an analysis of the reactions according to the number of repetitions of the stimulus requisite for a motor reaction has been made. Numbers 1 and 5 it will be noticed reacted most frequently to the first stimulus, and for them 48 satisfactory records were obtained; but in case of the others there were fewer responses to the first stimulus, and in the tabulation of series 1 (Table XI.) averages are given for less than the regular sets of 24 reactions each.
TABLE X.
ANALYSIS OF REACTIONS TO ONE-CELL STIMULUS.
Frog. Reactions to To 2d. To 3d. To 4th. To 5th. More. Total No. first Stimulus. of Reactions. 1 53 2 1 0 0 1 57 2 20 12 5 5 4 12 58 4 31 15 1 0 2 8 57 5 51 11 1 2 0 1 66 6 45 15 6 3 1 5 75 Totals, 200 55 14 10 7 27 313
Table XI. is self-explanatory. In addition to the usual averages, there is given the average for each half of the sets, in order that the effect of fatigue may be noted. In general, for this series, the second half is in its average about one third longer than the first half. There is, therefore, marked evidence of tiring. The mean reaction time for this strength of stimulus is difficult to determine because of the extremely great variations. At one time a subject may react immediately, with a time of not over a fifth of a second, and at another it may hesitate for as much as a second or two before reacting, thus giving a time of unusual length. Just how many and which of these delayed responses should be included in a series for the obtaining of the mean reaction time to this particular stimulus is an extremely troublesome question. It is evident that the mode should be considered in this case rather than the mean, or at least that the mean should be gotten by reference to the mode. For example, although the reaction times for the one-cell stimulus vary all the way from 150[sigma] to 1000[sigma] or more, the great majority of them lie between 200[sigma] and 400[sigma]. The question is, how much deviation from the mode should be allowed? Frequently the inclusion of a single long reaction will lengthen the mean by 10[sigma] or even 20[sigma]. What is meant by the modal condition and the deviation therefrom is illustrated by the accompanying curve of a series of reaction times for the electric stimulus of group I.
__________________________________________________________________________ 8|______________________________________________________________________ 7|_____________________________________|________________________________ 6|_____________________________________|________________________________ 5|_____________________________________|________________________________ 4|________________________________|____|____|___________________________ 3|____________|___________________|____|____|___________________________ 2|_______|____|____|_________|____|____|____|____|______________________ 1|__|____|____|____|_________|____|____|____|____|____|____|____|____|__ 100 110 120 130 140 150 160 170 180 190 200 210 220 230
The column of figures at the left indicates the number of reactions; that below the base line gives the reaction times in classes separated by 10[sigma]. Of thirty-one reactions, seven are here in the class 170[sigma]. This is the model class, and the mean gotten by taking the average of 31 reactions is 162[sigma]. If the mode had been taken to represent the usual reaction time in this case, there would have been no considerable error. But suppose now that in the series there had occurred a reaction of 800[sigma]. Should it have been used in the determination of the mean? If so, it would have made it almost 30[sigma] greater, thus removing it considerably from the mode. If not, on what grounds should it be discarded? The fact that widely varying results are gotten in any series of reactions, points, it would seem, not so much to the normal variability as to accidental differences in conditions; and the best explanation for isolated reactions available is that they are due to such disturbing factors as would decrease the strength of the stimulus or temporarily inhibit the response. During experimentation it was possible to detect many reactions which were unsatisfactory because of some defect in the method, but occasionally when everything appeared to be all right an exceptional result was gotten. There is the possibility of any or all such results being due to internal factors whose influence it should be one of the objects of reaction-time work to determine; but in view of the fact that there were very few of these questionable cases, and that in series I, for instance, the inclusion of two or three reactions which stood isolated by several tenths of a second from the mode would have given a mean so far from the modal condition that the results would not have been in any wise comparable with those of other series, those reactions which were entirely isolated from the mode and removed therefrom by 200[sigma] have been omitted. In series I alone was this needful, for in the other series there was comparatively little irregularity.
The results of studies of the reaction time for the one-cell electric stimulus appear in Table XI. The first column of this table contains the average reaction time or mean for each subject. Nos. 2 and 4 appeared to be much less sensitive to the current than the others, and few responses to the first stimulus could be obtained. Their time is longer than that of the others, and their variability on the whole greater. Individual differences are very prominent in the studies thus far made on the frog. The one-cell stimulus is so near the threshold that it is no easy matter to get a mean which is significant. Could the conditions be as fully controlled as in human reaction time it would not be difficult, but in animal work that is impossible. No attempt has thus far been made to get the reaction time in case of summation effects except in occasional instances, and in so far as those are available they indicate no great difference between the normal threshold reaction and the summation reaction, but on this problem more work is planned.
There are large mean variations in Table XI., as would be anticipated. Since the reactions were taken in sets of 24, the means of each set as well as that of the total are given, and also, in columns 4 and 5, the means of the first half and the last half of each set.
A comparison of Tables XI., XII. and XIII. makes clear the differences in reaction time correlated with differences in the strength of an electric stimulus. For Table XI., series I, the relative value of the stimulus was I; for Table XII., series 2, it was 2, and for Table XIII., series 3, it was 4. Throughout the series from I to 3 there is a rapid decrease in the reaction time and in the variability of the same. The reaction time for stimulus I, the so-called threshold, is given as 300.9[sigma]; but of the three it is probably the least valuable, for reasons already mentioned. The mean of the second series, stimulus 2, is 231.5[sigma] while that of the third, stimulus 4, is only 103.1[sigma]. This great reduction in reaction time for the four-cell stimulus apparently shows the gradual transition from the deliberate motor reaction, which occurs only after complex and varied central neural activities, and the purely reflex reaction, which takes place as soon as the efferent impulse can cause changes in the spinal centers and be transmitted as an afferent impulse to the muscular system.
TABLE XI.
ELECTRICAL STIMULUS REACTION TIME. SERIES 1.
Average Average of Average Average Mean Var Frog. of all. Mean Var. Sets. of 1st h. of 2d h. of Sets.
1 238.5* 33.3* 216.0* 205.6* 226.7* 33.2* 261.0 248.0 274.1 33.3 2 458.0 219.0 458.0 270.4 643.8 219.0 4 273.4 59.9 273.4 245.7 301.1 59.9 5 263.9 50.5 268.6 244.7 292.5 44.9 259.2 236.0 282.4 56.1 6 271.1 65.1 322.6 273.2 372.0 87.9 219.6 208.5 230.6 42.3 Gen Av. 300.9 85.5 300.9 244.8 356.8 85.5
Totals. For No. 1 the averages are for 2 sets of 24 reactions each, 48 " 2 " " one set of 12 " " 12 " 4 " " one set of 24 " " 24 " 5 " " two sets of 24 " " 48 " 6 " " two sets of 24 and 12 reactions, respectively, 36
*Transcriber's Note: All values in [sigma], 1/1000ths of a second.
TABLE XII.
ELECTRICAL STIMULUS REACTION TIME. SERIES 2.
Average Average of Average Average Mean Var Frog. of all. Mean Var. Sets. of 1st h. of 2d h. of Sets.
1 227.3* 33.7* 229.4* 209.1* 249.6* 25.5* 225.2 207.3 243.0 42.1 2 240.1 30.9 239.0 222.3 255.1 29.0 241.3 220.2 262.4 32.8 4 270.3 56.5 298.5 285.3 311.4 62.8 242.2 206.0 278.4 50.2 198.5 26.2 195.0 197.5 193.0 33.5 202.0 195.2 209.0 18.8 6 224.4 24.4 221.6 209.7 233.7 23.6 227.2 213.5 241.0 25.1 Gen. Av. 231.5 34.3 231.0 216.6 246.6 34.3
For No. 5 the averages are for two sets of 18 each; for all the others there are 24 in each set.
*Transcriber's Note: All values in [sigma], 1/1000ths of a second.
TABLE XIII.
ELECTRICAL STIMULUS REACTION TIME. SERIES 3.
Average Average Average Average Mean Var. Frog. of all. Mean Var. of all. of 1st h. of 2d h. of Sets. 1 93.6* 13.5* 91.8* 93.2* 90.4* 13.5* 95.4 91.8 99.0 13.5 2 99.9 12.8 92.2 89.4 95.0 17.4 107.5 105.9 109.0 8.2 4 125.2 16.3 113.5 106.5 120.5 13.6 136.0 135.7 138.2 19.0 5 94.4 8.0 88.6 90.5 88.6 8.2 100.2 97.8 102.7 7.9 6 102.5 12.2 104.2 98.6 109.9 12.8 100.9 101.0 108.3 11.6 Gen. Avs. 103.1 12.5 103.1 101.0 105.9 12.5
For each animal there are two sets of 24 reactions each.
*Transcriber's Note: All values in [sigma], 1/1000ths of a second.
The spinal reflex for a decapitated frog, as results previously discussed show, is approximately 50[sigma]; and every time the four-cell stimulus is given this kind of a reaction results. There is a slight twitch of the legs, immediately after which the animal jumps. Now for all these series the thread was slackened by one eighth of an inch, but the reflex time was determined without this slack. Calculation of the lengthening of the reaction time due to the slack indicated it to be between 20 and 30[sigma], so if allowance be made in case of the reactions to the four-cell stimulus, the mean becomes about 70[sigma], or, in other words, nearly the same as the spinal reflex. The conclusion seems forced, therefore, that when a stimulus reaches a certain intensity it produces the cord response, while until that particular point is reached it calls forth central activities which result in much longer and more variable reaction times. It was said above that the series under consideration gave evidence of the gradual transition from the reflex to the volitional in reaction time. Is this true, or do we find that there are well-marked types, between which reactions are comparatively rare? Examination of the tables VII., VIII., IX., XI., XII. and XIII. will show that between 70[sigma] and 150[sigma] there is a break. (In tables XI., XII. and XIII., allowance must always be made for the slack in the thread, by subtracting 30[sigma].) All the evidence furnished on this problem by the electrical reaction-time studies is in favor of the type theory, and it appears fairly clear that there is a jump in the reaction time from the reflex time of 50-80[sigma], to 140 or 150[sigma], which may perhaps be taken as the typical instinctive reaction time. From 150[sigma] up there appears to be a gradual lengthening of the time as the strength of the stimulus is decreased, until finally the threshold is reached, and only by summation effect can a response be obtained.
The most important averages for the three series have been arranged in Table XIV. for the comparison of the different subjects. Usually the reaction time for series 3 is about one half as long as that for series 2, and its variability is also not more than half as large. In the small variability of series 3 we have additional reason for thinking that it represents reflexes, for Table IX. gives the mean variation of the reflex as not more than 8[sigma], and the fact that the means of this series are in certain cases much larger is fully explained by the greater opportunity for variation afforded by the slack in the thread.
TABLE XIV.
MEANS, ETC., FOR EACH SUBJECT FOR THE THREE SERIES. (TIME IN [sigma])
Mean First Second Mean Frog. Half. Half. Variation. Series 1 238.5 226.8 259.4 33.3 Series 2 227.3 208.2 246.3 33.7 No. 1 Series 3 93.6 92.5 94.7 13.5
Series 1 458.0 270.4 643.8 219.0 Series 2 240.1 221.2 258.8 30.9 No. 2 Series 3 99.9 97.6 102.0 12.8
Series 1 273.4 245.7 301.1 59.9 Series 2 270.3 245.6 294.9 56.5 No. 4 Series 3 125.2 121.1 129.3 16.3
Series 1 263.9 240.4 287.4 50.5 Series 2 198.5 196.4 201.0 26.2 No. 5 Series 3 94.4 94.2 94.7 8.0
Series 1 271.1 240.8 301.3 65.1 Series 2 224.4 211.6 237.3 24.4 No. 6 Series 3 102.5 99.8 109.1 12.2
A striking fact is that the averages for the first and last half of sets of reactions differ more for the weak than for the strong stimulus. One would naturally expect, if the increase were a fatigue phenomenon purely, that it would be greatest for the strongest stimulus; but the results force us to look for some other conditions than fatigue. A stimulus that is sufficiently strong to be painful and injurious to an animal forces an immediate response so long as the muscular system is not exhausted; but where, as in series 1 and 2 of the electrical stimulus, the stimulus is not harmful, the reason for a sudden reaction is lacking unless fear enters as an additional cause. Just as long as an animal is fresh and unfamiliar with the stimulus there is a quick reaction to any stimulus above the threshold, and as soon as a few experiences have destroyed this freshness and taught the subject that there is no immediate danger the response becomes deliberate. In other words, there is a gradual transition from the flash-like instinctive reaction, which is of vast importance in the life of such an animal as the frog, to the volitional and summation responses. The threshold electrical stimulus does not force reactions; it is a request for action rather than a demand, and the subject, although startled at first, soon becomes accustomed to the experience and responds, if at all, in a very leisurely fashion. The reaction time to tactual stimuli, soon to be considered, was determined by giving a subject only three or four stimulations a day; if more were given the responses failed except on repetition or pressure; for this reason the data on fatigue, or lengthening of reaction time toward the end of a series, are wanting in touch. A few tests for the purpose of discovering whether the time would lengthen in a series were made with results very similar to those of the threshold electrical stimulus; the chief difference lies in the fact that the responses to touch fail altogether much sooner than do those to the electrical stimulus. This, however, is explicable on the ground that the latter is a stimulus to which the animal would not be likely to become accustomed so soon as to the tactual.
First Half. Second Half. Second % Greater. Series 1 244.8[sigma] 356.8[sigma] 46 per cent Series 2 216.6 246.6 14 " Series 3 101.0 105.9 5 "
If pure fatigue, that is, the exhaustion of the nervous or muscular system, appears anywhere in this work, it is doubtless in series 3, for there we have a stimulus which is so strong as to force response on penalty of death; the reaction is necessarily the shortest possible, and, as a matter of fact, the motor reaction (jump forward) here occupies little more time than the leg-jerk of a decapitated frog. This probably indicates that the reaction is a reflex, and that the slight increase in its length over that of the spinal reflex is due to occasional cerebellar origin; but of this there can be no certainly from the evidence herewith presented. At any rate, there is no possibility of a voluntary reaction to the strong current, and any changes in the general character of the reaction time in a series will have to be attributed to fatigue of the nervous or muscular systems. The second halves of the sets of series 3 are 5 per cent. longer than the first, and unless this is due to the partial exhaustion of the nervous system it is hard to find an explanation of the fact. Fatigue of the muscles concerned seems out of the question because the reactions occur at the rate of only one per minute, and during the rest interval any healthy and well-nourished muscle would so far recover from the effect of contraction that it would be able to continue the rhythmic action for long periods.
To the inquiry, Does fatigue in the experiments mean tiring by the exhaustion of nerve energy, or is the lengthening in reaction time which would naturally be attributed to tiring due to the fact that experience has shown quick reaction to be unnecessary? we shall have to reply that there is evidence in favor of both as factors. There can be little doubt that in case of the strong stimuli there is genuine fatigue which makes quick reaction impossible; but at the same time it is certain that the 40 to 50 per cent. increase of the second half of sets in series 1 over the first half can not be due to fatigue, for the strain is here evidently much less than for series 3. Rather, it would seem that habituation instead of exhaustion is the all-important cause of the difference in series 1 and 2. It becomes clear from these considerations that the repetition of a stimulus can never mean the repetition of an effect.
VII. TACTUAL REACTION TIME.
In the following work on the reactions to tactual stimulation the subject was placed in a large reaction box with a thread attached to one of its legs and passing to a reaction key, as in the experiments already described. The box in which the subject was confined was surrounded by movable cloth curtains to prevent the animal's escape and at the same time permit the experimenter to work without being seen by the frog.
Tactual stimulation was given by means of a hand key[15] similar to that used for electrical stimulation which is represented in Fig. 6. The touch key ended in a hard-rubber knob which could be brought in contact with the skin of the subject. This key was fixed to a handle of sufficient length to enable the operator to reach the animal wherever it chanced to be sitting in the reaction box. Stimulation was given by allowing the rubber point of the touch key to come in contact with the skin in the middle region of the subject's back. As soon as the point touched the animal the chronoscope circuit was broken by the raising of the upper arm of the key.
[15] This apparatus was essentially the same as Scripture's device for the giving of tactual stimulation.
As a precaution against reactions to visual stimuli, which it might well be supposed would appear since the subject could not in every case be prevented from seeing the approaching apparatus, the frog was always placed with its head away from the experimenter so that the eyes could not readily be directed toward the touch apparatus. Notwithstanding care in this matter, a reaction occasionally appeared which was evidently due to some disturbance preceding the tactual stimulus which served as a warning or preparation for the latter. All such responses were at once marked as questionable visual reactions and were not included in the series of touch reactions proper.
As has been mentioned in connection with the discussion of fatigue, it was found absolutely necessary to have the subjects perfectly fresh and active, and for this purpose it was advisable to give not more than three or four stimulations at any one time. The subject was usually kept in the reaction box from 30 to 45 minutes, dependent upon the success of the experiments. As the work progressed it became evident that the responses to the stimulus were becoming less and less certain and slower, that the subjects were becoming accustomed to the novel experience and no longer suffered the surprise which had been the cause of the prompt reactions at first. It seemed best for this reason not to continue the work longer than two weeks, and as a consequence it was impossible to base the averages on more than twenty reactions for each subject.
So far as the tension of the thread is concerned, the condition for the tactual reaction time was the same as that for the first group of electrical reaction-time experiments. In comparing the tactual with the electrical of series 1, 2 and 3, allowance must be made for the slack in the latter cases.
Selection of the tactual reaction times upon which the mean is based, has been made with reference to the mode for each set of experiments. Inspection of the curves given by the reactions of each subject indicated that the great majority of the responses lay between 100 and 300[sigma], and that those which were beyond these limits were isolated and, in all probability, exceptional reactions due to some undetected variation in conditions which should throw them out of the regular series. On this account it was thought best to use only reactions between 100 and 300[sigma].
For convenience of comparison, again, the averages for the electrical reaction time of subjects A, B, C, D, E and F, and the same for the tactual reaction time of subjects 1, 2, 3, 4, 5 and 6 are herewith given together. All averages are for twenty reactions, except for D and 5, for which there are ten.
Besides the usual determination for the tactual reaction-time work on the six subjects named, there is given in Table XVI. the electrical reaction time of these animals to a two-cell current. Comparison of the electrical and tactual results are of interest in this case because the mean variation for each is about 34[sigma], being 34.3[sigma], for the electrical and 33.8[sigma], for the tactual.
TABLE XV.
Average of 20 Electrical Average of 20 Tactual Frog. Reactions. Frog. Reactions. A 149.5[sigma] 1 188.3[sigma] B 158.3 2 199.1 C 191.0 3 212.1 D 167.0 4 213.0 E 182.4 5 199.8 F 176.3 6 221.9 Gen. Avs. 167.9 205.7
TABLE XVI.
REACTION TIME FOR TACTUAL AND ELECTRICAL STIMULI.
Tactual Reaction Time. Electrical Reaction Time.
Frog. Average. Mean Variation. Average. Mean Variation.
1 188.3[sigma] 167.3[sigma] 2 199.1 180.1 3 212.1 4 213.0 210.3 5¹ 199.8 138.5 6 221.9 164.4 Gen. Avs. 205.7 33.8 172.1 34.3
¹For 5 the average of ten instead of twenty is given.
VIII. EQUAL VARIABILITY AS A CRITERION OF COMPARABILITY OF REACTION TIME FOR DIFFERENT KINDS OF STIMULI.
Since variability as indicated in the study of the influence of different strengths of electrical stimulus becomes less as the stimulus increases, parity in variability for different stimuli offers a basis for the comparison of reaction times. Certain it is that there is no use in comparing the reaction times for different senses or different qualities of stimuli unless the relative values of the stimuli are taken into consideration; but how are these values to be determined unless some such index as variability is available? If the reaction time to tactual stimuli as here presented is to be studied in its relation to the electrical reaction time, it will mean little simply to say that the former is longer than the latter, because the electrical reaction time for a one-cell stimulus happens to be somewhat less than that for the particular tactual stimulus used. For it is clear that this tactual reaction time is really shorter than the reaction time to a weak current. In making variability a basis of comparison it must be assumed that the strength of stimulus is the important factor, and that all other variable conditions are, so far as possible, excluded. If, now, on the basis of parity in variability we compare the tactual and electrical reaction times, it is apparent that the tactual is considerably longer. The tactual average of Table XV. is 205.7[sigma], while the electrical reaction time which has approximately the same variability is 172.1[sigma]. It may well be objected that I have no right to make variability the basis of my comparison in these experiments, because the work for the various kinds of stimuli was done under different conditions. Admitting the force of this objection, and at the same time calling attention to the fact that I do not wish to lay any stress on the results of the comparisons here made, I take this opportunity to call attention to the possibility of this criterion.
The use of variability as a basis of comparison would involve the assumptions (1) that a certain intensity of every stimulus which is to be considered is capable of producing the shortest possible, or reflex reaction, and that this reaction is at the same time the least variable; (2) that as the strength of a stimulus decreases the variability increases until the threshold is reached.
Suppose, now, it is our desire to compare the results of reactions to different intensities of electrical and tactual stimuli; let the figures be as follows:
Reaction Time. Variability. Stimulus Strength. Elect. Touch. Elect. Touch. 8 50[sigma] 50[sigma] 10[sigma] 10[sigma]. 4 130 155 25 30 2 175 220 40 40 1 300 320 50 60
In the double columns the results for electrical stimuli are given first, and in the second column are the tactual. Stimulus 8 is assumed to be of sufficient strength to induce what may be designated as forced movement, and whatever the quality of the stimulus this reaction time is constant. I make this statement theoretically, although all the evidence which this work furnishes is in support of it. So, likewise, is the variability of this type of reaction time small and nearly constant. At the other extreme, stimulus 1 is so weak as to be just sufficient to call forth a response; it is the so-called threshold stimulus. Whether all qualities of stimulus will give the same result here is a question to be settled by experimentation. Wundt contends that such is the case, but the observations I have made on the electrical and tactual reactions of the frog cause me to doubt this assumption. It seems probable that the 'just perceptible stimulus reaction time' is by no means the same thing for different qualities of stimulus. Those modifications of the vital processes which alone enable organisms to survive, make their appearance even in the response to the minimal stimulus. In one case the just perceptible stimulus may cause nothing more than slight local changes in circulation, excretion, muscular action; in another it may produce, just because of the particular significance of the stimulus to the life of the organism, a violent and sudden motor reaction. But grant, if you will, that the threshold reaction time is the same for all kinds of stimuli, and suppose that the variability is fairly constant, then, between the two extremes of stimuli, there are gradations in strength which give reaction times of widely differing variabilities. If, now, at some point in the series, as, for instance, to stimulus 2, the variability for different kinds of stimuli is the same either with reference to the reaction time (ratio) or absolutely, what interpretation is to be put upon the fact? Is it to be regarded as merely a matter of chance, and unworthy of any special attention, or should it be studied with a view to finding out precisely what variability itself signifies? It is obvious that any discussion of this subject, even of the possible or probable value of variability as a criterion for the comparative study of stimuli, can be of little value so long as we do not know what are the determining factors of variations of this sort. The only suggestion as to the meaning of such a condition (i.e., equal variability at some point)--and our studies seem to show it for touch and electrical stimulation--which I feel justified in offering at present, is that parity in variability indicates equality in strength of stimuli, that is, the electrical stimulus which has a reaction time of the same variability as a tactual stimulus has the same effect upon the peripheral nervous system as the tactual, it produces the same amplitude and perhaps the same form of wave, but the reaction times for the two stimuli differ because of the biological significance of the stimuli. The chances are that this is wholly dependent upon the central nervous system.
IX. SUMMARY.
1. This paper gives the results of some experiments on the frog to determine its electrical and tactual reaction time. It is the beginning of comparative reaction-time studies by which it is hoped important information may be gained concerning the significance and modes of action of the nervous system. Comparative physiology has already made clear that the time relations of neural processes deserve careful study.
2. According to the strength of the stimulus, electric stimulation of the frog causes three types of reaction: (1) A very weak or threshold stimulus results in a deliberate or delayed reaction, the time of which may be anywhere from 300[sigma] (thousandths of a second) to 2,000[sigma]. (2) A very strong stimulus causes a spinal reflex, whose time is from 50 to 80[sigma]; and (3) a stimulus of intermediate strength causes a quick instinctive reaction of from 150 to 170[sigma] in duration.
3. The reaction time for electric stimuli whose relative values were 1, 2 and 4 were found to be 300.9[sigma], 231.5[sigma] and 103.1[sigma].
4. The reaction time of the frog to a tactual stimulus (contact of a rubber point) is about 200[sigma].
5. The variability of reaction times of the frog is great, and increases as the strength of the stimulus decreases.
6. When two kinds of stimuli (e.g., electrical and tactual) give reaction times of equal variability, I consider them directly comparable.
7. According to this criterion of comparability the reaction time to electric stimulation which is comparable with that to tactual is 172.1[sigma]; and it is to be compared with 205.7[sigma]. Both of these have a variability of approximately 34[sigma]. On this basis one may say that the tactual reaction time is considerably longer than the electrical.