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.

[Ill.u.s.tration: FIG. 3. Labyrinth with Conditions the Reverse of the Usual. (Compare with FIG. 2.) The colors as well as the part.i.tions have been shifted. The path is, approximately, that taken by No. 2 in the first trial after the reversal of conditions.]

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 p.r.o.nounced 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-pa.s.sage 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 pa.s.sage 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 pa.s.sage; 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 a.s.sociation 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 pa.s.sage had been chosen the visual data gotten from the appearance of the part.i.tion 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 pa.s.sage 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.

[Ill.u.s.tration: FIG. 4. Path of No. 2 for 123d Trial. Showing the response to the tactual stimulus from wires.]

An interesting instance of the repet.i.tion 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 repet.i.tion in the manner described.

[Ill.u.s.tration: FIG. 5. Path Usually Taken by Animal Having Perfectly-formed Habit.]

4. _The Effect of Fear upon Habit Formation._--A certain amount of excitement undoubtedly promotes the formation of a.s.sociations, 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 r.e.t.a.r.ds the formation of a.s.sociations. 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 a.s.sociation of stimuli and r.e.t.a.r.d 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 a.s.sociations._--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 a.s.sociations 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 a.s.sociational material.

TABLE V.

PERMANENCY OF a.s.sOCIATIONS. 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. a.s.sociation of Stimuli.--In connection with reaction-time work an attempt was made to form an a.s.sociation 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 a.s.sociation; 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 a.s.sociation. 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.

a.s.sOCIATION 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 a.s.sociation.

No. 2a 180 17 Increase in number of short reactions toward end. No evidence of a.s.sociation.

No. 3a 180 17 Marked increase in the number of short reactions toward end. No other evidence of a.s.sociation.

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 a.s.sociation.

Frog Z 450 28 No evidence of a.s.sociation.

To all appearances this is the same kind of an a.s.sociation 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 a.s.sociations; but in case of a series of a.s.sociations from fifty to a hundred experiences are needed for the formation of a perfect habit.

3. Experiment shows that the frog is able to a.s.sociate 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 a.s.sociations. The animals discriminate colors to some extent.

5. Perfectly formed habits are hard to change.

6. Fear interferes with the formation of a.s.sociations.

7. a.s.sociations persist for at least a month.

PART 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 mult.i.tude 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.