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 orange1 (O of Figure 18). A small quantity 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.
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 partitions 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 pass 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 partitions 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 pass 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 papers1 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?
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.
With colored cardboards inside the electric-boxes as well as at their entrances (see Figure 14 for position of cardboards) blue-orange tests were given to Nos. 2 and 3 until they discriminated perfectly. The papers were Bradley’s blue tint No. 1 and orange. Number 2 was perfect in the twelfth series (Table 17), No 3 in the fourteenth and again in the sixteenth. They were then tested with a special brightness check series which was intended by the experimenter to reveal any dependence upon a possible brightness difference rather than upon the color difference of the boxes.
Table 17
Light Blue-Orange Tests in Color Discrimination Box
Series Date No. 2 No. 3 1906 Right Wrong Right Wrong (Light (Orange) (Light (Orange) Blue) Blue)
| 1 | Jan. 26 7 | 3 | 1 9 |
| 2 | 27 7 | 3 | 5 5 |
| 3 | 28 7 | 3 | 6 4 |
| 4 | 29 7 | 3 | 7 3 |
| 5 | 30 7 | 3 | 4 6 |
| 6 | 31 10 | 0 | 7 3 |
| 7 | Feb. 1 9 | 1 | 7 3 |
| 8 | 2 8 | 2 | 6 4 |
| 9 | 3 9 | 1 | 9 1 |
| 10 | 5 7 | 3 | 5 5 |
| 11 | 6 8 | 2 | 5 5 |
| 12 | 7 10 | 0 | 5 5 |
| Special | brightness check series (see | Table | 18) |
| 13 | 8 10 | 0 | 7 3 |
| Special | light blue-dark blue series | ||
| 14 | 9 8 | 2 | 10 0 |
| 15 | 10 9 | 1 | 9 1 |
| Special | light blue-dark blue series | ||
| 16 | 11 9 | 1 | 10 0 |
| Special brightness | |||
| check series | |||
| 17 | 12 10 | 0 | 9 1 |
Table 18
Light Blue-Orange
Brightness check series Mouse No. 2, Series 13
Feb. 8, 1906
Test Condition Right Wrong
1 Light blue on right
Orange on left Right ____
2 Light blue on left
Orange on right Right ____
3 Light blue on right
Red substituted for orange Right ____
4 Light blur on left
Red substituted for orange Right ____
5 Dark blue on right
Orange on left Right ____
6 Dark blue on left
Orange on right Right ____
7 Dark blue on left
Orange on right Right ____
8 Dark blue on right
Red substituted for orange Right ____
9 Dark blue on left
Red substituted for orange Right ____
10 Dark blue on left
Red substituted for orange Right ____
Totals 10
The nature of this brightness check series, as well as the results which No. 2 gave when tested by it, may be appreciated readily by reference to Table 18. Tint No. 1 of the blue, which is considerably brighter, in my judgment, than the Bradley blue, was replaced at intervals in this series by the latter. For it was thought that in case the mouse were choosing the blue of the series because it seemed brighter than the orange, this substitution might mislead it into choosing the orange. These blues are referred to in the table as light blue (tint No. 1) and dark blue (standard blue). Again a change in the opposite direction was made by substituting Bradley red for orange. As this was for the human eye the substitution of a color whose brightness was considerably less than that of the orange, it seemed possible that the mouse, if it had formed the habit of choosing the box which seemed the darker, might by this change be misled into choosing the red instead of the light blue. In a word, changes in the conditions of the experiments were made in such a way that now one color, now the other, appeared to be the brighter. But I did not attempt to exclude brightness discrimination on the part of the mouse by dependence upon the human judgment of brightness equality, for it is manifestly unsafe to assume that two colors which are of the same brightness for the human eye have a like relation for the eye of the dancer or of any other animal. My tests of color vision have been conducted without other reference to human standards of judgment or comparisons than was necessary for the description of the experimental conditions. In planning the experiments I assumed neither likeness nor difference between the human retinal processes and those of the dancer. It was my purpose to discover the nature of the mouse’s visual discriminative ability.
As is indicated in the tables, neither the substitution of dark blue for light blue, nor the replacement of the orange by red or dark blue rendered correct choice impossible, although certain of the combinations did render choice extremely difficult. In other words, despite all of the changes which were made in the brightness of the cardboards in connection with the light blue-orange tests, the mice continued to make almost perfect records. What are we to conclude from this? Either that the ability to discriminate certain colors is possessed by the dancer, or that for some reason the tests are unsatisfactory. If it be granted that the possibility of brightness discrimination was excluded in the check series, the first of these alternatives apparently is forced upon us. That such a possibility was not excluded, later experiments make perfectly clear. The fact was that not even in the check series was the brightness value of the orange as great as that of the blue. Consequently the mice may have chosen the brighter box each time while apparently choosing the blue.
Although conclusive proof of the truth of this statement is furnished only by later experiments, the results of the light blue-orange series, as given in Table 17, strongly suggest such a possibility. Mouse No. 3 had not been experimented with previous to these color discrimination tests. Her preference for the orange, which in the case of the first series was 9 to 1, consequently demands an explanation. If she had been trained previously to choose the white instead of the black, as was true of No. 2, it might be inferred that she went to the orange box because it appeared brighter than the blue. As this explanation is not available, we are driven back upon the results of the white-black preference tests in Chapter VII, which proved that many dancers prefer the black to the white. This may mean that they prefer the lower degree of brightness or illumination, and if so it might be argued, in turn, that the orange was chosen by No. 3 because it appeared darker than the blue. Since, as has already been stated, the orange was far brighter for me than the blue, this would also mean that the brightness values of different colors are not the same for man and mouse.
Practically the same kind of color tests as those described for Nos. 2 and 3 were given to Nos. 1000 and 5. The results appear in Table 19. These tests followed upon the formation of a habit to choose white instead of black (that is, the greater brightness). From the first both No. 1000 and No. 5 chose the light blue in preference to the orange or the red. It therefore seems probable that the former was considerably brighter than the latter. Number 1000, to be sure, was led into three erroneous choices by the brightness check series (series 7), but, on the other hand, No. 5 was not at all disturbed in her choices by similar check tests. It seems natural to conclude from these facts that both of these mice chose the blue at first because of its relatively greater brightness, and that they continued to do so for the same reason. In other words, their behavior indicates that the brightness check tests were valueless because not enough allowance had been made for the possible differences between the vision of mouse and man.
TABLE 19
LIGHT BLUE-ORANGE AND DARK BLUE-RED TESTS
No. 1000 No. 5
SERIES DATE Condition Right Wrong Right Wrong
(Light (Orange (Light (Orange
Blue or or Blue or
Dark Red) Dark Red)
Blue) Blue)
1 Jan. 25 Blue-red 8 2 10 0
2 26 Blue-red or
Light blue-orange 10 0 10 0
3 27 Light blue-orange 10 0 5 5
4 29 Light blue-orange 9 1 8 2
5 30 Light blue-orange 10 0 8 2
6 31 Light blue-orange 10 0 10 0
7 Feb. 1 Light blue-orange
or Dark blue-red 7 3 10 0
If only the final results of my experiments with the dancer and the conclusions to which they lead were of interest, all of this description of experiments which served merely to clear the ground and thus make possible crucial tests might be omitted. It has seemed to me, however, that the history of the investigation is valuable, and I am therefore presenting the evolution of my methods step by step. To be sure, not every detail of this process can be mentioned, and only a few of the individual results can be stated, but my purpose will have been fulfilled if I succeed in showing how one method of experimentation pointed the way to another, and how one set of results made possible the interpretation of others.
As the results of my color vision experiments seemed to indicate that the red end of the spectrum appears much darker to the dancer than to us, tests were now arranged with colors from adjacent regions of the spectrum, green and blue. The papers used were the Bradley green and tint No. 1 of the blue. They were not noticeably different in brightness for the human eye. Green marked the box to be chosen. Three of the individuals which had previously been used in the light blue-orange series, and which therefore had perfect habits of going to the light blue, were used for the green-light blue tests. Of these individuals, No. 1000 became inactive on the fifth day of the experiment, and the tests with him were discontinued. Twenty series were given to each of the other mice, with the results which appear in Table 20. To begin with, both No. 4 and No. 5 exhibited a preference for the light blue, as a result of the previous light blue-orange training. As this preference was gradually destroyed by the electric shock which was received each time the light blue box was entered, they seemed utterly at a loss to know which box to enter. Occasionally a record of six, seven, or even eight right choices would be made in a series, but in no case was this unquestionably due to color discrimination; usually it could be explained in the light of the order of the changes in the positions of the cardboards. For example, series 9, in which No. 5 made a record of 8 right and 2 wrong, had green on the right for the first three tests. The animal happened to choose correctly in the first test, and continued to do so three times in succession simply because there was no change in the position of the cardboards. I have occasionally observed a record of seven right choices result when it was perfectly evident to the observer that the mouse could not discriminate visually. It was to avoid unsafe conclusions and unfair comparisons, as the result of such misleading series, that three perfect series in succession were required as evidence of a perfectly formed habit of discrimination.
Table 20
Green-Light Blue Tests
Date No. 1000 No. 4 No. 5
SERIES 1906 RIGHT WRONG RIGHT WRONG RIGHT WRONG
(GREEN) (BLUE) (GREEN) (BLUE) (GREEN) (BLUE)
1 Feb.3 2 8 3 7 3 7
2 5 7 3 5 5 5 5
3 6 5 5 6 4 5 5
4 7 5 5 5 5 5 5
5 8 2 8 5 5 4 6
6 9 7 3 7 3
7 10 4 6 3 7
8 10 6 4 4 6
9 12 6 4 8 2
10 13 6 4 6 4
11 14 5 5 3 7
12 15 6 4 7 3
13 16 5 5 7 3
14 17 3 7 6 4
15 19 6 4 6 4
16 20 7 3 5 5
17 21 4 6 8 2
18 22 3 7 4 6
19 23 6 4 4 6
20 24 6 4 5 5
Twenty series, 200 tests for each of the individuals in the experiment, yielded no evidence whatever of the dancer’s ability to tell green from blue. As it has already been proved that they readily learn to choose the right box under discriminable conditions, it seems reasonable to conclude either that they lack green-blue vision, or that they have it in a relatively undeveloped state.
If it be objected that the number of training tests given was too small, and that the dancer probably would exhibit discrimination if it were given 1000 instead of 200 tests in such an experiment, I must reply that the behavior of the animal in the tests is even more satisfactory evidence of its inability to choose than are the results of Table 20. Had there been the least indication of improvement as the result of 200 tests, I should have continued the experiment; as a matter of fact, the mice each day hesitated more and more before choosing, and fought against being driven toward the entrance to the experiment box. That they were helpless was so evident that it would have been manifestly cruel to continue the experiment.
TABLE 21
VIOLET-RED TESTS
With Odor of All Cardboards the Same
| SERIES | DATE | NO. | 7 | NO. | 998 |
| RIGHT | WRONG | RIGHT | WRONG | ||
| (VIOLET) | (RED) | (VIOLET) | (RED) | ||
| A | MAR. 7 | 8 | 2 | 5 | 5 |
| B | 7 | 3 | 7 | 2 | 8 |
| 1 | 14 | 3 | 7 | 6 | 4 |
| 2 | 15 | 4 | 6 | 4 | 6 |
| 3 | 16 | 5 | 5 | 5 | 5 |
| 4 | 19 | 4 | 6 | 4 | 6 |
| 5 | 20 | 5 | 5 | 6 | 4 |
| 6 | 21 | 4 | 6 | 8 | 2 |
| 7 | 22 | 8 | 2 | 4 | 6 |
| 8 | 23 | 4 | 6 | 6 | 4 |
| 9 | 24 | 6 | 4 | 4 | 6 |
| 10 | 25 | 4 | 6 | 6 | 4 |
Further color tests with reflected light were made with violet and red. Two dancers, Nos. 998 and 7, neither of which had been in any experiment previously, were subjected to the ten series of tests whose results are to be found in Table 21. In this experiment the cardboards used had been coated with shellac to obviate discrimination by means of odor. It is therefore impossible to give a precise description of the color or brightness by referring to the Bradley papers.1 Both the violet and the red were rendered darker, and apparently less saturated, by the coating.
These violet-red tests were preceded by two series of preference tests (A and B), in which no shock was given and escape was possible through either electric-box. Although the results of these preference tests as they appear in Table 21 seem to indicate a preference for the red on the part of No. 998, examination of the record sheets reveals the fact that neither animal exhibited color preference, but that instead both chose by position. Number 998 chose the box on the right 15 times in 20, and No. 7 chose the box on the left 15 times in 20.
Ten series of tests with the violet-red cardboards failed to furnish the least indication of discrimination. The experiment was discontinued because the mice had ceased to try to discriminate and dashed into one or the other of the boxes on the chance of guessing correctly. When wrong they whirled about, rushed out of the red box and into the violet immediately. They had learned perfectly as much as they were able to learn of what the experiment required of them. Although we are not justified in concluding from this experiment that dancers cannot be taught to distinguish violet from red, there certainly is good ground for the statement that they do not readily discriminate between these colors.
The experiments on color vision which have been described and the records which have been presented will suffice to give the reader an accurate knowledge of the nature of the results, only a few of which could be printed, and of the methods by which they were obtained.
In brief, these results show that the dancer, under the conditions of the experiments, is not able to tell green from blue, or violet from red. The evidence of discrimination furnished by the light blue-orange tests is not satisfactory because the conditions of the experiment did not permit the use of a sufficiently wide range of brightnesses. It is obvious, therefore, that a method of experimentation should be devised in which the experimenter can more fully control the brightness of the colors which he is using. I shall now describe a method in which this was possible.
Notes
1 These were the Milton Bradley blue and orange papers.
1 These were the only Hering papers used in my experiments.
1 The violet was darker than Bradley’s shade No. 2, and the red was lighter than Bradley’s red.