As it seemed most improbable that improvement by training should continue much longer, I next gave additional tests with the 80 standard. Again a difference of one tenth was sufficient for accurate discrimination (18 per cent of errors). These series were followed immediately by further tests with the 5 standard. As the results indicated greater ease of discrimination with a difference of one tenth in the case of this standard than in the case of either of the others I was at first uncertain whether the results which I have tabulated under the dates June 3, 4, and 5 of the table should be interpreted in terms of Weber's law.

Up to this point the experiments had definitely established two facts: that the dancer's ability to discriminate by means of brightness differences improves with training for a much longer period and to a far greater extent than I had supposed it would; and that a difference of one tenth is sufficient to enable the animal to distinguish two lights in the case of the three standard values, 5, 20, and 80 hefners. The question remains, is this satisfactory evidence that Weber's law holds with respect to the brightness vision of the dancer, or do the results indicate rather, that this difference is more readily detected in the case of 5 as a standard (12 per cent error) than in the case of 20 as a standard (22 per cent error)?

For the purpose of settling this point I made tests for each of the three standards with a difference of only one fifteenth. In no instance did I obtain the least evidence of ability to discriminate. These final tests, in addition to establishing the fact that the limit of discrimination for No. 51, after she had been subjected to about two thousand tests, lay between one tenth and one fifteenth, proved to my satisfaction, when taken in connection with the results already discussed, that Weber's law does hold for the brightness vision of the dancer.

In concluding this discussion of the Weber's law experiment I wish to call attention to the chief facts which have been revealed, and to make a critical comment. In my opinion it is extremely important that the student of animal behavior should note the fact that the dancer with which I worked week after week in the Weber's law investigation gradually improved in her ability to discriminate on the basis of brightness differences until she was able to distinguish from one another two boxes whose difference in illumination was less than one tenth[1] that of the brighter box. At the beginning of the experiments a difference of one half did not enable her to choose as certainly as did a difference of one tenth after she had chosen several hundred times. Evidently we are p.r.o.ne to underestimate the educability of our animal subjects.

[Footnote 1: Under the conditions of the experiment I was unable to distinguish the electric-boxes when they differed by less than one twentieth.]

The reason that the experiments were carried out with only one mouse must now be apparent. It was a matter of time. The reader must not suppose that my study of this subject is completed. It is merely well begun, and I report it here in its unfinished state for the sake of the value of the method which I have worked out, rather than for the purpose of presenting the definite results which I obtained with No. 51.

The critical comment which I wish to make for the benefit of those who are working on similar problems is this. The phosphor bronze wires, on the bottom of the electric-boxes, by means of which an electric shock could be given to the mouse when it chose the wrong box, are needless sources of variability in the illumination of the boxes. They reflect the light into the eyes of the mouse too strongly, and unless they are kept perfectly clean and bright, serious inequalities of illumination appear. To avoid these undesirable conditions I propose hereafter to use a box within a box, so that the wires shall be hidden from the view of the animal as it attempts to discriminate.

A brief description of the behavior of the dancer in the brightness discrimination experiments which have been described may very appropriately form the closing section of this chapter. For the experimenter, the incessant activity and inexhaustible energy of the animal are a never-failing source of interest and surprise. When a dancer is inactive in the experiment box, it is a good indication either of indisposition or of too low a temperature in the room. In no animal with which I am familiar is activity so much an end in itself as in this odd species of mouse. With striking facility most of the mice learn to open the wire swing doors from either side. They push them open with their noses in the direction in which they were intended by the experimenter to work, and with almost equal ease they pull them open with their teeth in the direction in which they were not intended to work. In the rapidity with which this trick is learned, there are very noticeable individual differences. The pulling of these doors furnished an excellent opportunity for the study of the imitative tendency.

When confronted with the two entrances of the electric-boxes, the dancer manifested at first only the hesitation caused by being in a strange place. It did not seem much afraid, and usually did not hesitate long before entering one of the boxes. The first choice often determined the majority of the choices of the preference series. If the mouse happened to enter the left box, it kept on doing so until, having become so accustomed to its surroundings that it could take time from its strenuous running from _A_ by way of the left box to the alley and thence to _A_, to examine things in _B_ a little, it observed the other entrance and in a seemingly half-curious, half-venturesome way entered it. In the case of other individuals, the cardboards themselves seemed to determine the choices from the first.

The electric shock, as punishment for entering the wrong box, came as a surprise. At times an individual would persistently attempt to enter, or even enter and retreat from the wrong box repeatedly, in spite of the shock. This may have been due in some instances to the effects of fright, but in others it certainly was due to the strength of the tendency to follow the course which had been taken most often previously. The next effect of the shock was to cause the animal to hesitate before the entrances to the boxes, to run from one to the other, poking its head into each and peering about cautiously, touching the cardboards at the entrances, apparently smelling of them, and in every way attempting to determine which box could be entered safely. I have at times seen a mouse run from one entrance to the other twenty times before making its choice; now and then it would start to enter one and, when halfway in, draw back as if it had been shocked. Possibly merely touching the wires with its fore paws was responsible for this simulation of a reaction to the shock.

The gradual waning of this inhibition of the forward movement was one of the most interesting features of the experiment. Could we but discover what the psychical states and the physiological conditions of the animal were during this period of choosing, comparative psychology and physiology would advance by a bound.

If the conditions at the entrances of the two boxes were discriminable, the mouse usually learned within one hundred experiences to choose the right box without much hesitation. Three distinct methods of choice were exhibited by different individuals, and to a certain extent by the same individual from time to time. These methods, which I have designated _choice by affirmation_, _choice by negation_, and _choice by comparison_, are of peculiar interest to the psychologist and logician.

When an individual runs directly to the entrance of the right box, and, after stopping for an instant to examine it, enters, the choice may be described as recognition of the right box. I call it choice by affirmation because the act of the animal is equivalent to the judgment--"this is it."

If instead it runs directly to the wrong box, and, after examining it, turns to the other box and enters without pause for examination, its behavior may be described as recognition of the wrong box. This I call choice by negation because the act seems equivalent to the judgment--"this is not it." Further, it seems to imply the judgment--"therefore the other is it." In the light of this fact, this type of choice might appropriately be called choice by exclusion. Finally, when the mouse runs first to one box and then to the other, and repeats this anywhere from one to fifty times, the choice may be described as comparison of the boxes; therefore, I call it choice by comparison. Certain individuals choose first by comparison, and later almost uniformly by affirmation and negation.

Whenever the conditions are difficult to discriminate, choice by comparison occurs most frequently and persistently. If, however, the conditions happen to be absolutely indiscriminable, as was true, for example, in the case of the sound tests, in certain of the Weber's law tests, and in the plain electric-box tests, the period of hesitation rapidly increases during the first three or four series of tests, then the mouse seems to lessen its efforts to discriminate and more and more tends to rush into one of the boxes without hesitation or examination, and apparently with the expectation of a shock, but with the intention of getting it over as soon as possible. Now and then under such conditions there is a marked tendency to enter the same box each time.

Indiscriminable conditions are likely to render the animals fearful of the experiment; instead of going from _A_ to _A_ willingly, they fight against making the trip. They refuse to pa.s.s from _A_ to _B_; and when in _B_, they fight against being driven toward the entrances to the electric- boxes.

In marked contrast with this behavior on the part of the mouse under conditions which do not permit it to choose correctly is that of the animal which has learned what is expected of it. The latter, far from holding back or fighting against the conditions which urge it forward, is so eager to make the trip that it sometimes has to be forced to wait while the experimenter records the results of the tests. There is evidence of delight in the freedom of movement and in the variety of activity which the experiment furnishes. The thoroughly trained dancer runs into _B_ almost as soon as it has been placed in _A_ by the experimenter; it chooses the right entrance by one of the three methods described above, immediately, or after whirling about a few times in _B_; it runs through _E_ and back to _A_ as quickly as it can, and almost before the experimenter has had time to record the result of the choice it is again in _B_ ready for another choice.

CHAPTER IX

THE SENSE OF SIGHT: COLOR VISION

Is the dancing mouse able to discriminate colors as we do? Does it possess anything which may properly be called color vision? If so, what is the nature of its ability in this sense field? Early in my study of the mice I attempted to answer these and similar questions, for the fact that they are completely deaf during the whole or the greater part of their lives suggested to me the query, are they otherwise defective in sense equipment? In the following account of my study of color vision, I shall describe the evolution of my methods in addition to stating the results which were obtained and the conclusions to which they led me. For in this case the development of a method of research is quite as interesting as the facts which the method in its various stages of evolution revealed.

Observation of the behavior of the dancer under natural conditions caused me to suspect that it is either defective in color vision or possesses a sense which is very different from human vision. I therefore devised the following extremely simple method of testing the animal's ability to distinguish one color from another. In opposite corners of a wooden box 26 cm. long, 23 cm. wide, and 11 cm. deep, two tin boxes 5 cm. in diameter and 1.5 cm. deep were placed, as is shown in part I of Figure 18. One of these boxes was covered on the outside with blue paper (_B_ of Figure 18), and the other with orange[1] (_O_ of Figure 18). A small quant.i.ty of "force" was placed in the orange box. As the purpose of the test was to discover whether the animals could learn to go directly to the box which contained the food, the experiments were made each morning before the mice had been fed. The experimental procedure consisted in placing the individual to be tested in the end of the large wooden box opposite the color boxes, and then permitting it to run about exploring the box until it found the food in the orange box. While it was busily engaged in eating a piece of "force" which it had taken from the box and was holding in its fore paws, squirrel fashion, the color boxes were quickly and without disturbance shifted in the directions indicated by the arrows of Figure 18, I. Consequently, when the animal was ready for another piece of "force," the food-box was in the corresponding corner of the opposite end of the experiment box (position 2, 18, II). After the mouse had again succeeded in finding it, the orange box was shifted in position as is indicated by the arrows in Figure 18, II. Thus the tests were continued, the boxes being shifted after each success on the part of the animal in such a way that for no two successive tests was the position of the food- box the same; it occupied successively the positions 1, 2, 3, and 4 of the figure, and then returned to 1. Each series consisted of 20 tests.

[Footnote 1: These were the Milton Bradley blue and orange papers.]

[Ill.u.s.tration: FIGURE 18.--Food-box apparatus for color discrimination experiments. _O_, orange food-box; _B_, blue food-box; 1, 2, 3, 4, different positions of the food-boxes, _O_ and _B_; I, II, III, IV, figures in which the arrows indicate the direction in which the food-boxes were moved.]

[Ill.u.s.tration: FIGURE 19.--Food-box apparatus with movable part.i.tions.

_O_, orange food-box; _B_, blue food-box; _X_, starting point for mouse; _A_, point at which both food-boxes become visible to the mouse as it approaches them; 1, 2, two different positions of the food-boxes; _T_, _T_, movable part.i.tions. (After Doctor Waugh.)]

An improvement on this method, which was suggested by Doctor Karl Waugh, has been used by him in a study of the sense of vision in the common mouse. It consisted in the introduction, at the middle of the experiment box, of two wooden part.i.tions which were pivoted on their mid-vertical axes so that they could be placed in either of the positions indicated in Figure 19. Let us suppose that a mouse to be tested for color vision in this apparatus has been placed at _X_. In order to obtain food it must pa.s.s through _A_ and choose either the orange or the blue box. If it chooses the former, the test is recorded as correct; if it goes to the blue box first, and then to the orange, it is counted an error. While the animal is eating, the experimenter shifts the boxes to position 1 of Figure 19, and at the same time moves the part.i.tions so that they occupy the position indicated by the dotted lines. The chief advantage of this improvement in method is that the animal is forced to approach the color boxes from a point midway between them, instead of following the sides of the experiment box, as it is inclined to do, until it happens to come to the food-box. This renders the test fairer, for presumably the animal has an opportunity to see both boxes from _A_ and can make its choice at that point of vantage.

Two males, A and B, of whose age I am ignorant, were each given seventeen series of tests in the apparatus of Figure 18. A single series, consisting of twenty choices, was given daily. Whether the animal chose correctly or not, it was allowed to get food; that is, if it went first to the blue box, thus furnishing the condition for a record of error, it was permitted to pa.s.s on to the orange box and take a piece of "force." No attempt was made to increase the animal's desire for food by starving it. Usually it sought the food-box eagerly; when it would not do so, the series was abandoned and work postponed. "Force" proved a very convenient form of food in these tests. The mice are fond of it, and they quickly learned to take a flake out of the box instead of trying to get into the box and sit there eating, for when they attempted the latter they were promptly pushed to one side by the experimenter and the box, as well as the food, was removed to a new position.

The results of the tests appear in Table 15. No record of the choices in the first two of the 17 series was kept. The totals therefore include 15 series, or 300 tests, with each individual. Neither the daily records nor the totals of this table demonstrate choice on the basis of color discrimination. Either the dancers were not able to tell one box from the other, or they did not learn to go directly to the orange box. It might be urged with reason that there is no sufficiently strong motive for the avoidance of an incorrect choice. A mistake simply means a moment's delay in finding food, and this is not so serious a matter as stopping to discriminate. I am inclined, in the light of result of other experiments, to believe that there is a great deal in this objection to the method.

Reward for a correct choice should be supplemented by some form of punishment for a mistake. This conclusion was forced upon me by the results of these preliminary experiments on color vision and by my observation of the behavior of the animals in the apparatus. At the time the above tests were made I believed that I had demonstrated the inability of the dancer to distinguish orange from blue, but now, after two years'

additional work on the subject, I believe instead that the method was defective.

The next step in the evolution of a method of testing the dancer's color vision was the construction of the apparatus (Figures 14 and 15) which was described in Chapter VII. In connection with this experiment box the basis for a new motive was introduced, namely, the punishment of mistakes by an electric shock. Colored cardboards, instead of the white, black, or grays of the brightness tests, were placed in the electric-boxes.

TABLE 15

ORANGE-BLUE TESTS, WITH FOOD-BOX

MOUSE A MOUSE B SERIES DATE 1904 RIGHT WRONG RIGHT WRONG (ORANGE) (BLUE) (ORANGE) (BLUE)

1 Dec. 6 -- -- -- -- 2 7 -- -- -- -- 3 8 12 8 12 8 4 9 10 10 9 11 5 10 15 5 10 10 6 11 10 10 12 8 7 12 9 11 9 11 8 13 10 10 9 11 9 14 12 8 12 8 10 15 13 7 12 8 11 16 13 7 10 10 12 17 12 8 10 10 13 18 11 9 10 10 14 19 13 7 8 12 15 20 13 7 9 11 16 22 14 6 12 8 17 23 10 10 9 11

TOTALS 177 123 153 147

In preliminary tests, at the rate of four per day, the colored cardboards were placed only at the entrances to the boxes, not inside, and as was true also in the case of brightness tests under like conditions, no evidence of discrimination was obtained from ten days' training. This seemed to indicate that a considerable area of the colored surface should be exposed to the mouse's view, if discrimination were to be made reasonably easy.

This conclusion was supported by the results of other preliminary experiments in which rectangular pieces of colored papers[1] 6 by 3 cm., were placed on the floor at the entrances to the electric-boxes, instead of on the walls of the boxes. Mouse No. 2 was given five series of ten tests each with a yellow card to indicate the right box and a red card at the entrance to the wrong box. At first he chose the red almost uniformly, and at no time during these fifty tests did he exhibit ability to choose the right box by color discrimination. I present the results of these series in Table 16, because they indicate a fact to which I shall have to refer repeatedly later, namely, that the brightness values of different portions of the spectrum are not the same for the dancer as for us.

Previous to this yellow-red training, No. 2, as a result of ten days of white-black training (two tests per day), had partially learned to go to the brighter of the two electric-boxes. It is possible therefore that the choice of the box in the case of these color experiments was in reality the choice of what appeared to the mouse to be the brighter box. If this were not true, how are the results of Table 16 to be accounted for?

[Footnote 1: These were the only Hering papers used in my experiments.]

TABLE 16

YELLOW-RED TESTS

In Color Discrimination Box with 6 by 3 cm. Pieces of Hering Papers at Entrances to Boxes

No. 2

SERIES DATE RIGHT WRONG 1906 (Yellow) (Red) 1 Jan. 16 1 9 2 17 3 7 3 18 4 6 4 19 5 5 5 20 5 5

Without further mention of the many experiments which were necessary for the perfecting of this method of testing color vision, I may at once present the final results of the tests which were made with reflected light. These tests were made with the discrimination apparatus in essentially the same way as were the brightness discrimination tests of Chapter VII.

In all of the color experiments, unless otherwise stated, a series of ten tests each day was given, until satisfactory evidence of discrimination or proof of the lack of the ability to discriminate had been obtained. The difficulties of getting conclusive evidence in either direction will be considered in connection with the results themselves. For all of these tests with reflected light the Milton Bradley colored papers were used.

These colored papers were pasted on white cardboard carriers. I shall designate, in the Bradley nomenclature, the papers used in each experiment.