Before tests, such as have been described, can be presented as conclusive proof of discrimination, it must be shown that the mouse has no preference for the particular brightness which the arrangement of the test requires it to select. That any preference which the mouse to be tested might have for white, rather than black, or for a light gray rather than a dark gray, might be discovered, what may be called preference test series were given before the discrimination tests were begun. These series, two of which were given usually, consisted of ten tests each, with the white alternately on the left and on the right. The mouse was permitted to enter either the white or the black box, as it chose, and to pa.s.s through to the nest-box without receiving a shock and without having its way blocked by the gla.s.s plate. The conditions of these preference tests may be referred to hereafter briefly as "No shock, open pa.s.sages." The preference tests, which of course would be valueless as such unless they preceded the training tests, were given as preliminary experiments, in order that the experimenter might know how to plan his discrimination tests, and how to interpret his results.

TABLE 7

BRIGHTNESS DISCRIMINATION

White-Black, Series II

Experimented on No. 5 February 2, 1906

POSITION TEST OF CARDBOARDS RIGHT WRONG

1 White left Right -- 2 White left Right -- 3 White right Right -- 4 White right Right -- 5 White right Right -- 6 White left Right -- 7 White left Right -- 8 White left Right -- 9 White right Right -- 10 White right Right --

Totals 10 0

The results given in the white-black preference tests by ten males and ten females are presented in Table 8. Three facts which bear upon the brightness discrimination tests appear from this table: (1) black is preferred by both males and females, (2) this preference is more marked in the first series of tests than in the second, and (3) it is slightly stronger for the first series in the case of females than in the case of males.

TABLE 8

WHITE-BLACK PREFERENCE TESTS

MALES FIRST SERIES SECOND SERIES WHITE BLACK WHITE BLACK

No. 10 3 7 3 7 18 5 5 5 5 20 2 8 4 6 152 4 6 6 4 210 4 6 4 6 214 6 4 3 7 220 5 5 3 7 230 4 6 2 8 410 4 6 5 5 420 4 6 9 1

Averages 4.1 5.9 4.4 5.6

FEMALES FIRST SERIES SECOND SERIES WHITE BLACK WHITE BLACK

No. 11 5 5 4 6 151 6 4 5 5 215 2 8 2 8 213 2 8 5 5 225 4 6 2 8 227 4 6 6 4 235 6 4 4 6 415 2 8 4 6 425 5 5 8 2 229 2 8 5 5

Averages 3.8 6.2 4.5 5.5

That the dancers should prefer to enter the dark rather than the light box is not surprising in view of the fact that the nests in which they were kept were ordinarily rather dark. But whatever the basis of the preference, it is clear that it must be taken account of in the visual discrimination experiments, for an individual which strongly preferred black might choose correctly, to all appearances, in its first black-white series. Such a result would demonstrate preference, and therefore one kind of discrimination, but not the formation of a habit of choice by discrimination. The preference for black is less marked in the second series of tests because the mouse as it becomes more accustomed to the experiment box tends more and more to be influenced by other conditions than those of brightness. The record sheets for both series almost invariably indicate a strong tendency to continue to go to the left or the right entrance according to the way by which the animal escaped the first time. This cannot properly be described as visual choice, for the mouse apparently followed the previous course without regard to the conditions of illumination. We have here an expression of the tendency to the repet.i.tion of an act. It is only after an animal acquires considerable familiarity with a situation that it begins to vary its behavior in accordance with relatively unimportant factors in the situation. It is this fact, ill.u.s.trations of which may be seen in human life, as well as throughout the realm of animal behavior, that renders it imperative that an animal be thoroughly acquainted with the apparatus for experimentation and with the experimenter before regular experiments are begun. Any animal will do things under most experimental conditions, but to discover the nature and scope of its ability it is necessary to make it thoroughly at home in the experimental situation. As the dancer began to feel at home in the visual discrimination apparatus it began to exercise its discriminating ability, the first form of which was choice according to position.

Since there appears to be a slight preference on the part of most dancers'

for the black box in comparison with the white box, white-black training tests were given to fifty mice, and black-white to only four. The tests with each individual were continued until it had chosen correctly in all of the tests of three successive series (thirty tests). As the reproduction of all the record sheets of these experiments would fill hundreds of pages and would provide most readers with little more information than is obtainable from a simple statement of the number of right and wrong choices, only the brightness discrimination records of Tables 6 and 7 are given in full.

As a basis for the comparison of the results of the white-black tests with those of the black-white tests, two representative sets of data for each of these conditions of brightness discrimination are presented (Tables 9 and 10). In these tables only the number of right and wrong choices for each series of ten tests appears.

Tables 9 and 10 indicate--if we grant that the precautionary tests to be described later exclude the possibility of other forms of discrimination-- that the dancer is able to tell white from black; that it is somewhat easier, as the preference tests might lead us to expect, for it to learn to go to the black than to the white, and that the male forms the habit of choosing on the basis of brightness discrimination more quickly than the female.

TABLE 9 WHITE-BLACK TESTS

No. 210 No. 215 AGE, 28 DAYS AGE, 28 DAYS SERIES DATE RIGHT WRONG RIGHT WRONG (WHITE) (BLACK) (WHITE) (BLACK) A June 22 4 6 2 8 B 23 4 6 2 8

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).

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.