The Dancing Mouse Part 15

Mouse No. 2 was given two series of tests, series 18 and 19, under these conditions, with the result that he showed absolutely no ability to tell the blue box from the green box. The opportunity was now taken to determine how quickly No. 2 would avail himself of any possibility of discriminating by means of brightness. With the blue at 21 candle meters, the green was increased to about 1800. Immediately discrimination appeared, and in the second series (22 of Table 27) there were only two mistakes.

The results of the blue-green experiments with light transmitted from in front of the animal and from above it are in entire agreement with those of the experiments in which reflected light was used. Since the range of intensities of illumination was sufficiently great to exclude the possibility of blinding and of under illumination, it is necessary to conclude that the dancer does not possess blue-green vision.

Again I must call attention to the fact that the behavior of the mice in these experiments is even more significant of their lack of discriminating ability than are the numerical results of the tables. After almost every series of tests, whether or not it came out numerically in favor of discrimination, I was forced to add the comment, "No satisfactory evidence of discrimination."

We have now examined the results of green-red, green-blue, and blue-green tests. One other important combination of the colors which were used in these experiments is possible, namely, blue-red. This is the most important of all the combinations in view of the results already described, for these colors represent the extremes of the visible spectrum, and might therefore be discriminable, even though those which are nearer together in the spectral series were not.

TABLE 28 BLUE-RED TESTS

No. 2 No. 205 SERIES DATE BRIGHTNESS VALUES RIGHT WRONG RIGHT WRONG (BLUE) (RED) (BLUE) (RED)

1 July 31 1800 c.m. on left 24 c.m. on right 5 5 6 4 2 Aug. 1 21 c.m. on left 1800 c.m. on right 6 4 6 4 3 2 1800 c.m. on left 21 c.m. on right 8 2 6 4 4 3 19 c.m. on left 1800 c.m. on right 9 1 6 4 5 4 1800 c.m. on left 7 c.m. on right 7 3 5 5 6 5 6 c.m. on left 1800 c.m. on right 10 0 7 3 7 6 18 c.m. on left 74 c.m. on right 10 0 9 1 8 7 1800 c.m. on left 7 c.m. on right 8 2 8 2 9 8 7 c.m. on left 1800 c.m. on right 7 3 8 2 10 9 Mixed values 6 to 1800 c.m. 8 2 9 1 11 10 Blue 3 c.m.

Red 1800 c.m. 7 3 6 4

Brightness tests were now made, without the use of colors.

11a 10 4 6 5 5

12 10 Blue 3 c.m.

Red 8 c.m. 4 6 6 4 13 11 Blue 3 c.m.

Red 7200 c.m. 8 2 5 5 14 13 Mixed values 3 to 7200 c.m. 7 3 7 3 15 13 Same 7 3 9 1 16 14 Blue 3 to 6 c.m.

Red 112 to 3650 c.m. 10 0 10 0

Series were now given to test the a.s.sumption that red appears dark to the dancer.

17 14 Darkness on one side Red 3 c.m. 5 5 7 3 18 14 Blue 3 to 3650 c.m.

Red 3 to 3650 c.m. 10 0 10 0 19 15 Darkness on one side Red 3 c.m. 5 5 4 6 20 15 Blue 3 to 3650 c.m.

Red 3 to 3650 c.m. 10 0 9 1 21 16 Darkess on one side Red 72 c.m. 5 5 7 3 22 16 Darkness on one side Red 1800 c.m. 6 4 10 0

As is shown by the results in Table 28, no combination of brightnesses rendered correct choice impossible in the case of the blue-red tests which are now to be described. Choice was extremely difficult at times, even more so perhaps than the table would lead one to suppose, and it is quite possible that color played no part in the discrimination. But that brightness difference in the colors was not responsible for whatever success these mice attained in selecting the right box is proved by the brightness-without-color series which follows series II of the table.

Neither No. 2 nor No. 205 showed preference for the lighter or the darker box. At the end of the sixteenth blue-red series, I was convinced that one of two conclusions must be drawn from the experiment: either the dancers possess a kind of blue-red vision, or red is of such a value for them that no brightness of visible green or blue precisely matches it.

The latter possibility was further tested by an experiment whose results appear in series 17 to 22 inclusive, of Table 28. The conditions of series 17 were a brightness value of 0 in one box (darkness) and in the other red of a brightness of 3 candle meters. Despite the fact that they had been perfectly trained in _blue-red tests_ to avoid the red, neither of the mice seemed able to discriminate the red from the darkness and to avoid it. This was followed by a series in which the brightness of both the blue and the red was varied between 3 and 3650 candle meters, with the striking result that neither mouse made any mistakes. In series 19 red was used with darkness as in series 17, and again there was a total lack of discrimination. Series 20 was a repet.i.tion of series 18, with practically the same result. I then attempted to find out, by increasing the brightness of the red, how great must be its value in order that the dancers should distinguish it readily from darkness. For the tests of series 21 it was made 72 candle meters, but discrimination did not clearly appear. At 1800 candle meters, as is shown in series 22, the red was sufficiently different in appearance from total darkness to enable No. 205 to discriminate perfectly between the two electric-boxes. For No. 2 discrimination was more difficult, but there was no doubt about his ability. It would appear from these tests that the dancers had not learned to avoid red. Therefore we are still confronted with the question, can they see colors?

TABLE 29

VISUAL CHECK TESTS With the Electric-boxes Precisely Alike Visually

No. 151 No. 152 SERIES DATE RIGHT WRONG RIGHT WRONG

1 Sept. 29 6 4 4 6 2 30 5 5 6 6 3 Oct. 1 3 7 4 6 4 2 5 5 3 7 5 3 3 7 5 5 6 4 6 4 5 5 7 5 5 5 5 5 8 6 -- -- 3 7 9 7 -- -- 6 4 10 8 -- -- 4 6

Averages 4.7 5.3 4.5 5.5

The account of my color vision experiments is finished. If it be objected that other than visual conditions may account for whatever measure of discriminating ability, apart from brightness discrimination, appears in some of the series, the results of the series of Table 29, in which all conceivable visual means of discrimination were purposely excluded, and those of the several check tests which have been described from time to time in the foregoing account, should furnish a satisfactory and definite answer. I am satisfied that whatever discrimination occurred was due to vision; whether we are justified in calling it color vision is quite another question.

I conclude from my experimental study of vision that although the dancer does not possess a color sense like ours, it probably discriminates the colors of the red end of the spectrum from those of other regions by difference in the stimulating value of light of different wave lengths, that such specific stimulating value is radically different in nature from the value of different wave lengths for the human eye, and that the red of the spectrum has a very low stimulating value for the dancer. In the light of these experiments we may safely conclude that many, if not most, of the tests of color vision in animals which have been made heretofore by other investigators have failed to touch the real problem because the possibility of brightness discrimination was not excluded.

Under the direction of Professor G. H. Parker, Doctor Karl Waugh has examined the structure of the retina of the dancing mouse for me, with the result that only a single type of retinal element was discovered.

Apparently the animals possess rod-like cells, but nothing closely similar to the cones of the typical mammalian retina. This is of peculiar interest and importance in connection with the results which I have reported in the foregoing pages, because the rods are supposed to have to do with brightness or luminosity vision and the cones with color vision. In fact, it is usually supposed that the absence of cones in the mammalian retina indicates the lack of color vision. That this inference of functional facts from structural conditions is correct I am by no means certain, but at any rate all of the experiments which I have made to determine the visual ability of the dancer go to show that color vision, if it exists at all, is extremely poor. It is gratifying indeed to learn, after such a study of behavior as has just been described, that the structural conditions, so far as we are able to judge at present, justify the conclusions which have been drawn.

CHAPTER XI

THE ROLE OF SIGHT IN THE DAILY LIFE OF THE DANCER

Darting hither and thither in its cage, whirling rapidly, now to the left, now to the right, running in circles, pa.s.sing through holes in the nest box quickly and neatly, the dancer, it would seem, must have excellent sight. But careful observation of its behavior modifies this inference.

For it appears that a pair of mice dancing together, or near one another, sometimes collide, and that it is only those holes with which the animal is familiar that are entered skillfully. In fact, the longer one observes the behavior of the dancer under natural conditions, the more he comes to believe in the importance of touch, and motor tendencies. Sight, which at first appears to be the chief guiding sense, comes to take a secondary place. In this chapter it is my purpose to show by means of simple experiments what part sight plays in the dancer's life of habit formation.

The evidence on this subject has been obtained from four sources: (1) observation of the behavior of dancers in their cages; (2) observation of their behavior when blinded; (3) observation of their behavior in a great variety of discrimination experiments, many of which have already been described; and (4) observation of their behavior in labyrinth experiments which were especially planned to exhibit the importance of the several kinds of vision which the dancer might be supposed to possess. The evidence from the first three of these sources may be presented summarily, for much of it has already appeared in earlier chapters. That from the fourth source will const.i.tute the bulk of the material of this chapter.

My observation of the behavior of the mice has furnished conclusive evidence of their ability to see moving objects. But that they do not see very distinctly, and that they do not have accurate perception of the form of objects, are conclusions which are supported by observations that I have made under both natural and experimental conditions. In Chapters VII, VIII, IX, and X, I have presented an abundance of evidence of brightness vision and, in addition, indications of a specific sensitiveness to wave length which may be said to correspond to our color vision. It is noteworthy, however, that all of the experimental proofs of visual ability were obtained as the result of long periods of training. Seldom, indeed, in my experience with them, have the dancers under natural conditions exhibited forms of activity which were unquestionably guided by vision.

It is claimed by those who have experimented with blinded dancers that the loss of sight decreases the amount and rapidity of movement, and the ability of the animals to avoid obstacles.

By means of the discrimination method previously used in the preliminary experiments on color vision, a full description of which may be found in Chapter IX, p. 133, the dancers' ability to perceive form was tested.

Immediately after the two males _A_ and _B_ had been given the "food-box"

tests, whose results appear in Table 15, they were tested in the same apparatus and by the same method for their ability to discriminate a rectangular food-box from a round one. In the case of the color discrimination tests, it will be remembered that the circular tin boxes 5 cm. in diameter by 1.5 cm. in depth, one of which was covered with blue paper, the other with orange, were used. For the form discrimination tests I used instead one of the circular boxes of the dimensions given above and a rectangular box 8.5 cm. long, 5.5 cm. wide and 2.5 cm. deep. "Force" was placed in the circular box. The tests were given, in series of 20, daily.

TABLE 30

VISUAL FORM TESTS

SERIES DATE MOUSE A MOUSE B RIGHT WRONG RIGHT WRONG (CIRCULAR (RECTANGU- (CIRCULAR (RECTANGU- BOX) LAR BOX) BOX) LAR BOX) 1 Jan. 5 10 10 9 11 2 7 12 8 13 7 3 10 6 14 10 10 4 11 7 13 10 10 5 12 9 11 10 10 6 13 11 9 11 9 7 14 13 7 9 11 8 15 10 10 11 9 9 16 10 10 11 9 10 17 11 9 9 11 11 18 11 9 12 8 12 19 12 8 10 10 13 20 10 10 12 8 14 21 10 10 8 12 15 22 10 10 10 10

Totals 152 148 155 145

The results of 15 series of these tests, as may be seen by the examination of Table 30, are about as definitely negative, so far as form discrimination is in question, as they possibly could be. From the first series to the last there is not one which justifies the inference that either of the dancers depended upon the form of the boxes in making its choice. In view of the general criticisms I have made concerning the use of hunger as a motive in experiments on animal behavior, and in view of the particular criticisms of this very method of testing the discriminating powers of the mouse, it may seem strange that s.p.a.ce should be given to a report of these tests. I sympathize with the feeling, if any one has it, but, at the same time, I wish to call attention to the fact that almost any mammal which is capable of profiting by experience, and which, under the same conditions, could distinguish the rectangular box from the circular one, would have chosen the right box with increasing accuracy as the result of such experience. The results are important in my opinion, not because they either prove or disprove the ability of the dancer to discriminate these particular forms, the discrimination of which might fairly be expected of any animal with an image-forming eye, but because they demonstrate an important characteristic of the dancing mouse, namely, its indifference to the straightforward or direct way of doing things.