TABLE 10 WHITE-BLACK TESTS
No. 14 No. 13
AGE, 32 DAYS AGE, 32 DAYS
SERIES DATE RIGHT WRONG RIGHT WRONG
(WHITE) (BLACK) (WHITE) (BLACK)
1 May 13[1] 5 5 7 3
2 14 8 2 6 4
3 15 7 3 9 1
4 16 9 1 9 1
5 17 10 0 10 0
6 18 10 0 9 1
7 19 10 0 10 0
8 20 — — 10 0
9 21 — — 10 0
[Footnote 1: No preference tests were given.]
It is now necessary to justify the interpretation of these results as evidence of brightness discrimination by proving that all other conditions for choice except brightness difference may be excluded without interfering with the animal's ability to select the right box. We shall consider in order the possibility of discrimination by position, by odor, and by texture and form of the cardboards.
The tendency which the dancer has in common with many, if not all, animals to perform the same movement or follow the same path under uniform conditions is an important source of error in many habit-formation experiments. This tendency is evident even from casual observation of the behavior of the dancer. The ease with which the habit of choosing the box on the left or the box on the right is formed in comparison with that of choosing the white box or the black box is strikingly shown by the following experiment. Five mice were given one series of ten trials each in the discrimination box of Figure 14 without the presence of cardboards or of other means of visual discrimination. The electric shock was given whenever the box on the left was entered. Thus without other guidance than that of direction, for the boxes themselves were interchanged in position, and, as was proved by additional tests, the animals were utterly unable to tell one from the other, the mouse was required to choose the box on its right. Only one of the five animals went to the box on the left after once experiencing the electric shock. The results of the series are given in Table 11.
TABLE 11
CHOICE BY POSITION
Choices of Choices of
Box on Right Box on Left
First mouse 9 1
Second mouse 8 2
Third mouse 9 1
Fourth mouse 9 1
Fifth mouse 9 1
This conclusively proves that the habit of turning in a certain direction or of choosing by position can be formed more readily than a habit which depends upon visual discrimination. A rough comparison justifies the statement that it takes from six to ten times as long for the dancer to learn to choose the white box as it does to learn to choose the box on the right. Since this is true, it is exceedingly important that the possibility of choice by position or direction of movement be excluded in the case of tests of brightness discrimination. To indicate how this was effectively accomplished in the experiments, the changes in the position of the cardboards made in the case of a standard set of white-black series are shown in Table 12. The number of the series, beginning at the top of the table with the two lettered preference series, is given in the first column at the left, the number of the tests at the top of the table, and the position of the white cardboard, left or right, is indicated below by the letters l (left) and r (right).
TABLE 12
POSITION OF WHITE CARDBOARDS FOR A SET OF 150 TESTS
SERIES 1 2 3 4 5 6 7 8 9 10
Preference
A l r l r l r l r l r
B r l r l r l r l r l
1 r l r l r l r l r l
2 l l r r l r l l r r
3 r r l r l l r l r l
4 l l l r r r l r r l
5 r l r l r l r l r l
6 l l r l r r l r l r
7 r l l l r r r l r l
8 r r l l r l r l r l
9 r r r l l l r l r l
10 l l l l r r r r l r
11 r l r r r l l l r l
12 r l r l r r l l r l
13 r l r l l l r r r l
14 l l l l r r r r l r
15 r l r r r l l l r l
It is to be noted that in the case of each series of ten tests the white cardboard was on the left five times and on the right five times. Thus the establishment of a tendency in favor of one side was avoided. The irregularity of the changes in position rendered it impossible for the mouse to depend upon position in its choice. It is an interesting fact that the dancer quickly learns to choose correctly by position if the cardboards are alternately on the left box and on the right.
The prevalent, although ill-founded, impression that mice have an exceedingly keen sense of smell might lead a critic of these experiments to claim that discrimination in all probability was olfactory rather than visual. As precautions against this possibility the cardboards were renewed frequently, so that no odor from the body of the mouse itself should serve as a guiding condition, different kinds of cardboard were used from time to time, and, as a final test, the cardboards were coated with shellac so that whatever characteristic odor they may have had for the dancer was disguised if not totally destroyed. Despite all these precautions the discrimination of the boxes continued. A still more conclusive proof that we have to do with brightness discrimination, and that alone, in these experiments is furnished by the results of white- black tests made with an apparatus which was so arranged that light was transmitted into the two electric-boxes through a ground glass plate in the end of each box. No cardboards were used. The illumination of each box was controlled by changes in the position of the sources of light. Under these conditions, so far as could be ascertained by critical examination of the results, in addition to careful observation of the behavior of the animals as they made their choices, there was no other guiding factor than brightness difference. Nevertheless the mice discriminated the white from the black perfectly. This renders unnecessary any discussion of the possibility of discrimination by the texture or form of the cardboards.
Since a variety of precautionary tests failed to reveal the presence, in these experiments, of any condition other than brightness difference by which the mice were enabled to choose correctly, and since evidence of ability to discriminate brightness differences was obtained by the use of both reflected light (cardboards) and transmitted light (lamps behind ground glass), it is necessary to conclude that the dancer possesses brightness vision.