Part 16 (1/2)

Most mammals which have been experimentally studied have proved their eagerness and ability to learn the shortest, quickest, and simplest route to food without the additional spur of punishment for wandering. With the dancer it is different. It is content to be moving; whether the movement carries it directly towards the food is of secondary importance. On its way to the food-box, no matter whether the box be slightly or strikingly different from its companion box, the dancer may go by way of the wrong box, may take a few turns, cut some figure-eights, or even spin like a top for seconds almost within vibrissa-reach of the food-box, and all this even though it be very hungry. Activity is pre-eminently important in the dancer's life.

In pa.s.sing I may emphasize the importance of the fact that at no time did the brightness or color discrimination tests furnish evidence of attempts on the part of the dancers to choose by means of slight differences in the form of the cardboards or the cardboard carriers. Several times form differences, which were easily perceivable by the human subject, were introduced in order to discover whether the mice would detect them and learn to discriminate thereby instead of by the visual conditions of brightness or color. As these experiments failed to furnish evidences of form discrimination, the following special test in the discrimination box was devised.

[Ill.u.s.tration: FIGURE 22.--Cards used for tests of form discrimination.]

The color discrimination box of Chapter X was arranged so that the light at the entrance to each electric-box had a value of 20 candle meters, less the diminution caused by a piece of ground gla.s.s which was placed over the end of the electric-boxes to diffuse the light. The windows through which the light entered the electric-boxes were covered with pieces of black cardboard; in one of these cardboards I had cut a circular opening 4 cm.

in diameter, and in the other an opening of the same area but markedly different shape. These openings are shown in Figure 22. As the mouse approached the entrance to the electric-boxes, it was confronted by these two equally illuminated areas, whose chief difference was one of form.

Difference in the amount of light within the boxes was excluded so far as possible. The question which I asked was, can the dancer discriminate by means of this difference in visual form?

For the purpose of settling this point and of gaining additional knowledge of the role of vision, two individuals were tested in the discrimination box under the conditions which have just been described. During the first ten days of the experiment each of these mice, Nos. 420 and 425, was given a series of ten tests daily. At the end of this period experimentation with No. 425 had to be discontinued, and the number of daily tests given to No. 420 was increased to twenty.

Instead of taking s.p.a.ce for the presentation of the daily records, I may state the general results of the tests. Neither of the mice learned to choose the right box by means of form discrimination. In fact, there was absolutely no sign of discrimination at any time during the tests. This result is as surprising as it is interesting. I could not at first believe that the mice were unable to perceive the difference in the lighted areas, but a.s.sumed that they were prevented from getting the outlines of the areas by the blinding effect of the light. However, decreasing the intensity of the illumination did not alter the result. According to the indications of this experiment, the dancer's ability to perceive visual form is extremely poor.

Thus far the purpose of our experiments has been to ascertain what the dancer is enabled to do by sight. Suppose we now approach the problem of the role of this sense by trying to find out what it can do without sight.

[Ill.u.s.tration: FIGURE 23.--Labyrinth B. _I_, entrance; _O_, exit; 1, 2, 3, doorways between alleys.]

For the investigation of this matter the labyrinth method seemed eminently suitable. The first form of labyrinth which was used in these visual tests appears in ground plan in Figure 23. It was made of 1-1/2 cm. boards. The length was 52 cm., the width 17 cm., the depth 10 cm. Each of the doorways, _I_ (the entrance), 1, 2, 3, and _O_ (the exit), was 5 by 5 cm.

The alleys were 2-1/2 cm. wide. For this width the necessity is obvious from what has already been said of the animal's propensity to whirl on all occasions. As the mice almost never tried to climb up the walls, no cover for the labyrinth was needed. The direct route is indicated by the symbols _I_-1-2-3-_O_. If an error be defined as a choice of the wrong path as the animal progressed toward the exit, five mistakes were possible in the forward course: the first by turning to the left at the entrance; the second by failing to pa.s.s through doorway 1; the third by turning to the right after pa.s.sing through doorway 1; the fourth by failing to pa.s.s through doorway 3, and the fifth by turning to the left after pa.s.sing through 3. In case the mouse retraced its course, any mistakes made as it again progressed towards _O_ were counted, as at first, no matter how many times it went over the same ground. Thus an individual might make the same mistake several times in the course of a single test in the labyrinth.

With this labyrinth Nos. 7, 998, 15, 16, 151, and 152 were tested. At first a record was kept of the time which elapsed from the instant the animal entered _I_ to the instant it emerged at _O_, of the path which it followed, and of the number of errors which it made; but later only the number of errors was recorded.

TABLE 31

THE ROLE OF SIGHT

Labyrinth-B Experiments

NO. 7 NO. 998

TEST DATE TIME ERRORS TIME ERRORS 1 June 16 66” 8 127” 19 2 16 11 0 94 12 3 16 15 2 18 3 4 16 7 0 13 2 5 16 5 0 10 1 6 18 61 15 12 3 7 18 13 3 14 4 8 18 14 5 8 1 9 18 24 9 16 2 10 18 10 1 9 1 11 19 36 13 80 17 12 19 8 3 10 1 13 19 6 1 7 1 14 19 9 1 8 0 15 19 12 2 7 0 16 20 14 1 25 0 17 20 28 3 18 20 No efforts No efforts to escape to escape

TABLE 32

LABYRINTH-B EXPERIMENTS

with

Electric Shock given as Punishment for Mistakes

No. 7 No. 998 TEST DATE CONDITION ERRORS CONDITION ERRORS

1 June 29 Light 4 Light 9 2 29 Light 1 Light 3 3 29 Light 1 Light 2 4 29 Light 0 Light 0 5 29 Light 0 Light 0 6 29 Light 0 Light 0 7 29 Light 1 Light 0 8 29 Light 0 Light 0 9 29 Light 1 Darkness 0 10 29 Light 1 Light 0 11 29 Light 1 Darkness 0 12 29 Light 0 Light 0 13 29 Light 0 Light 0 14 29 Light 0 Light 0 15 29 Light 0 Light 0 16 29 Light 0 Light 0 17 29 Darkness 2 Darkness 0 18 29 Light 2 Light 0 with paper 19 29 Light 0 Light 0 20 29 Darkness 0 Light 0 with paper 21 29 Light 0 Light 0 22 29 Light 0 Darkness 0 23 29 Light 0 Odorless 0 24 June 29 Light 0 Darkness 0 25 29 Light 0 26 29 Darkness 4 27 29 Light with paper 1 28 29 Light 0 29 29 Light with paper 1 30 29 Darkness 0 31 29 Odorless 2 32 29 Darkness 4

As the results in Table 31 show, the time and number of errors rapidly diminished. Number 7, for example, made no errors in the second test. The chiefly significant fact which appeared in these preliminary experiments, however, was that the mice soon ceased to care whether they got out of the labyrinth or not. After they knew the path perfectly, they would enter the wrong pa.s.sages repeatedly and wander about indefinitely. It was obvious, therefore, that the labyrinth could not be used to reveal the role of sight unless some sufficiently strong motive for continuous effort to escape from it could be discovered. Naturally I looked to the electric shock for aid.