In the descriptions of the various visual experiments of this and the following chapters, the first word of the couplet which describes the condition of the experiment, for example, white-black, always designates the visual condition which the animal was to choose, the second that which it was to avoid on penalty of an electric shock. In the case of Tables 6 and 7, for example, white cardboard was placed in one box, black in the other, and the animal was required to enter the white box. In the tables the first column at the left gives the number of the test, the second the positions of the cardboards, and the third and fourth the result of the choice. The first test of Table 6 was made with the white cardboard on the box which stood at the left of the mouse as it approached from A, and, consequently, with the black cardboard on the right. As is indicated by the record in the "wrong" column, the mouse chose the black instead of the white. The result of this first series was choice of the white box four times as compared with choice of the black box six times. On the eleventh day, that is, after No. 5 had been given 100 tests in this brightness vision experiment, she made no mistakes in a series of ten trials (Table 7).
TABLE 6
BRIGHTNESS DISCRIMINATION
White-Black, Series 1
Experimented on No. 5 January 15, 1906
POSITION OF
TEST CARDBOARDS RIGHT WRONG
1 White left — Wrong 2 White right — Wrong 3 White left — Wrong 4 White right — Wrong 5 White left Right — 6 White right Right — 7 White left — Wrong 8 White right Right — 9 White left — Wrong 10 White right Right —
Totals 4 6
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 pass through to the nest-box without receiving a shock and without having its way blocked by the glass plate. The conditions of these preference tests may be referred to hereafter briefly as "No shock, open passages." 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.