The Dancing Mouse Part 13

Accepting provisionally the conclusion that the dancers cannot tell green from blue except by brightness differences, we may proceed to inquire whether they can discriminate other colors. Are green and red distinguishable?

Green-red discrimination now was tested by a method which it was hoped might from the first prevent dependence upon brightness. The light in the light-box on the left was so placed that it had a value of 18 candle meters, that in the light-box on the right so that it had a value of 1800 candle meters. Neither light was moved during the first four series of the green-red tests which were given to Nos. 151 and 152.

TABLE 23

GREEN-BLUE TESTS

Brightnesses Different for Human Eye

Green 18 candle meters Blue 18 candle meters

No. 5 No. 12 DATE SERIES 1906 RIGHT WRONG RIGHT WRONG (GREEN) (BLUE) (GREEN) (BLUE)

1 April 10 6 4 5 5 2 11 5 5 7 3 3 12 6 4 7 3 4 13 4 6 7 3 5 14 7 3 5 5 6 15 4 6 6 4 7 16 6 4 8 2 8 17 5 5 4 6

As it was now evident that the intensity difference was not sufficient to render discrimination easy, the blue was reduced to 0 and the green left at 18.

9 17 7 3 8 2

Now the brightnesses were made, green 64, blue 18, just the reverse of those of series of Table 22.

10 17 8 2 8 2

Each of these series consisted of 20 tests instead of 10. As a result of the arrangement of the lights just mentioned, the green appeared to me very much brighter than the red when it was on the right and very much darker when it was on the left. If this were true for the mouse also, it is difficult to see how it could successfully depend upon brightness for guidance in its choices. Such dependence would cause it to choose now the green, now the red.

The first four series of green-red tests so clearly demonstrated discrimination, of some sort, that it was at once necessary to alter the conditions of the experiment. The only criticism of the above method of excluding brightness discrimination, of which I could think, was that the red at no time had been brighter than the green. In other words, that despite a value of 1800 candle meters for the red and only 18 candle meters for the green, the latter still appeared the brighter to the mouse.

To meet this objection, I made the extreme brightness values 1 and 1800 candle meters in some of the later series, of which the results appear in Table 24. From day to day different degrees of brightness were used, as is indicated in the second column of the table. Instead of having first one color and then the other the brighter, after the fourth series I changed the position of the lights each time the position of the filters was changed; hence, the table states a certain brightness value for each color instead of for each electric-box.

Series 5 to 14 so clearly indicated discrimination, that it seemed necessary to devise some other means than that of changing the brightnesses of the colored lights themselves to test the a.s.sumption that the animals were choosing the brighter light. I therefore removed the light filters so that the colors which had been present as conditions of discrimination were lacking, and arranged the apparatus so that first one box, then the other, was illuminated the more brightly. The purpose of this was to discover whether as the result of their green-red training the mice had acquired the habit of choosing uniformly either the lighter or the darker box. One series was given under the conditions of illumination specified in Table 24 with the result that the brighter box was chosen eight times in ten by No. 151 and every time by No. 152. Since neither of these individuals had previously been trained by white-black tests to go to the white, and since, furthermore, the dancers usually manifest a slight preference for the lower instead of the higher illumination, this result may be interpreted as indicative of dependence upon brightness in the previous color tests. It looks very much indeed as if the green had been chosen, not because of its greenness, but on account of its relatively greater brightness.

This test of brightness preference was followed by two series, 16 and 17, under conditions similar to those of the first four series of the table.

For series 16 the value of the light in the left box was 1 candle meter, that of the light in the right box 1800 candle meters. Discrimination was perfect. For series 17 the value for the left remained at 1 candle meter, but that of the right box was decreased to 0. In this series No. 152 was entirely at a loss to know which box to choose. Of course this was an entirely new set of conditions for choice, namely, a colored box, the green or the red as the case might be, beside a dark box, the one which was not illuminated. If the mice really had been choosing correctly because of a habit of avoiding the red or of seeking the green, this method should bring out the fact, for the red box, since with it the disagreeable electric shock had always been a.s.sociated, should be a box to be avoided. For No. 151 this seemed to be the case.

Series 23 to 27 of Table 24 were given as final and crucial tests of the relation of brightness discrimination to color discrimination. As it is not possible to express in a simple formula the conditions of the tests, a sample series which indicates the brightness of the colors in each of the twenty tests of a series, and in addition the results given by No. 151 in the first of these final series, is reproduced in Table 25. For an animal which had presumably learned perfectly to choose green in preference to red, the record of 8 mistakes in 20 choices as a result of changes in relative brightness is rather bad, and it renders doubtful the existence of color discrimination in any of these experiments. No. 152 showed no ability whatever to choose the green in the first of the series (series 23 of Table 24) of which that of Table 25 is a sample. His record, 10 mistakes in 20 choices, was even poorer than that of No. 151. That both of these mice learned to choose fairly accurately in these final tests is shown by the results of series 24, 25, 26, and 27. I must admit, however, that these records indicate little ability on the part of the animals to discriminate colors.

TABLE 24

GREEN-RED TESTS

Brightnesses Extremely Different for Human Eye Intensities are given in candle meters (c.m.)

NO. 151 NO. 152 SERIES DATE CONDITIONS RIGHT WRONG RIGHT WRONG (GREEN) (RED) (GREEN) (RED)

1 April 26 18 c.m. on left 1800 c.m. on right 11 9 7 13 2 27 Same 16 4 16 4 3 28 Same 20 0 17 3 4 29 Same 19 1 19 1 5 30 Green 18 c.m.

Red 18 c.m. 9 1 10 0 6 30 Green 64 c.m.

Red 18 c.m. 9 1 8 2 7 May 1 Green 6 c.m.

Red 1500 c.m. 7 3 9 1 8 1 Green 4 c.m.

Red 1500 c.m. 8 2 7 3 9 2 Both varied from 4 to 1500 c.m. 18 2 18 2 10 3 Green 2 c.m.

Red 1800 c.m. 6 4 7 3 11 3 Same 10 0 10 0 12 4 Same 7 3 8 2 13 4 Same 8 2 6 4 14 5 Green 1 c.m.

Red 1800 c.m. 19 1 19 1

Filters were now removed. An illumination of 15 c.m. was established on one side and an illumination of 0 on the other side, in order to ascertain whether the mice would choose the brighter box. This was done to test the a.s.sumption that the green in the previous tests had always appeared brighter to the mice than did the red, and that in consequence they had chosen the brighter box instead of the green box.

TABLE 24--CONTINUED

No. 151 No. 152

SERIES DATE CONDITIONS RIGHT WRONG RIGHT WRONG (GREEN) (RED) (GREEN) (RED)

15 May 5 Brighter 15 c.m. 8[1] 2[2] 10[1] 0[2]

Darker 0 c.m.

16 5 1 c.m. on left 1800 c.m. on right 10 0 10 0 17 5 1 c.m. on left 0 c.m. on right 9 1 4 6 18 5 Green 18 c.m.

Red 18 c.m. 19 1 17 3 19 9 Same 9 1 9 1 20 9 Same 10 0 10 0 21 10 Same 10 0 10 0 22 11 Same 10 0 10 0 23 June 1 Both varied from 1 to 1800 c.m. 12 8 10 10 24 2 Same 18 2 14 6 25 June 3 Both varied from 2 to 1800 c.m. 19 1 17 3 26 4 Same 17 3 17 3 27 5 Same 18 2 18 2

[Footnote 1: Brighter.]

[Footnote 2: Darker.]

These long-continued and varied tests with Nos. 151 and 152 revealed three facts: that the mice depend chiefly upon brightness differences in visual discrimination; that they probably have something which corresponds to our red-green vision, although their color experience may be totally unlike ours; and that the red end of the spectrum seems much darker to them than to us, or, in other words, that the least refrangible rays are of lower stimulating value for them than for us.

TABLE 25

GREEN-RED TESTS

June 1, 1906 No. 151

BRIGHTNESS VALUE IN CANDLE RIGHT WRONG TEST POSITION METERS (GREEN) (RED) 1 Green on left Green 4, Red 448 Right -- 2 Green on right Green 448, Red 4 Right -- 3 Green on right Green 4, Red 448 Right -- 4 Green on left Green 448, Red 4 Right -- 5 Green on left Green 3, Red 1800 -- Wrong 6 Green on right Green 1800, Red 3 -- Wrong 7 Green on right Green 3, Red 1800 -- Wrong 8 Green on left Green 1800, Red 3 Right -- 9 Green on right Green 5, Red 34 Right -- 10 Green on left Green 34, Red 5 Right -- 11 Green on right Green 6, Red 74 Right -- 12 Green on left Green 74, Red 6 Right -- 13 Green on left Green 4, Red 448 -- Wrong 14 Green on right Green 448, Red 4 Right -- 15 Green on right Green 4, Red 448 -- Wrong 16 Green on left Green 448, Red 4 Right -- 17 Green on right Green 3, Red 1800 -- Wrong 18 Green on left Green 1800, Red 3 -- Wrong 19 Green on right Green 1800, Red 3 -- Wrong 20 Green on left Green 3, Red 1800 Right --

Totals 12 8