1 24 4 6 3 7
2 25 6 4 5 5
3 26 7 3 7 3
4 27 5 5 8 2
5 28 7 3 9 1
6 29 8 2 8 2
7 30 9 1 9 1
8 July 1 10 0 10 0
9 2 10 0 9 1
10 3 10 0 10 0
11 4 — — 10 0
12 5 — — 10 0
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).