PART 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.