The sense of sight in the dancer has received little attention hitherto. In the literature there are a few casual statements to the effect that it is of importance. Zoth, for example (31 p. 149), remarks that it seems to be keenly developed; and other writers, on the basis of their observation of the animal's behavior, hazard similar statements. The descriptions of the behavior of blinded mice, as given by Cyon, Alexander and Kreidl, and Kishi (p.47), apparently indicate that the sense is of some value; they do not, however, furnish definite information concerning its nature and its role in the daily life of the animal.
The experimental study of this subject which is now to be described was undertaken, after careful and long-continued observation of the general behavior of the dancer, in order that our knowledge of the nature and value of the sense of sight in this representative of the Mammalia might be increased in scope and definiteness. The results of this study naturally fall into three groups: (1) those which concern brightness vision, (2) those which concern color vision, and (3) those which indicate the role of sight in the life of the dancer.
Too frequently investigators, in their work on vision in animals, have assumed that brightness vision and color vision are inseparable; or, if not making this assumption, they have failed to realize that the same wave-length probably has markedly different effects upon the retinal elements of the eyes of unlike organisms. In a study of the sense of sight it is extremely important to discover whether difference in the quality, as well as in the intensity, of a visual stimulus influences the organism; in other words, whether color sensitiveness, as well as brightness sensitiveness, is present. If the dancer perceives only brightness or luminosity, and not color, it is evident that its visual world is strikingly different from that of the normal human being. The experiments now to be described were planned to show what the facts really are.
[Illustration: Figure 14.—Discrimination box. W, electric-box with white cardboards; B, electric-box with black cardboards. Drawn by Mr. C.H. Toll.]
As a means of testing the ability of the dancer to distinguish differences in brightness, the experiment box represented by Figures 14 and 15 was devised. Figure 14 is the box as seen from the position of the experimenter during the tests. Figure 15 is its ground plan. This box, which was made of wood, was 98 cm. long, 38 cm. wide, and 17 cm. deep, as measured on the outside. The plan of construction and its significance in connection with these experiments on vision will be clear from the following account of the experimental procedure. A mouse whose brightness vision was to be tested was placed in the nest-box, A (Figure 15). Thence by pushing open the swinging door at I, it could pass into the entrance chamber, B. Having entered B it could return to A only by passing through one of the electric-boxes, marked W, and following the alley to O, where by pushing open the swing door it could enter the nest-box. The door at I swung inward, toward B, only; those at O, right and left, swung outward, toward A, only. It was therefore impossible for the mouse to follow any other course than A-I-B-L-W-E-O or A-I-B-R-W-E-O. The doors at I and O were pieces of wire netting of 1/2 cm. mesh, hinged at the top so that a mouse could readily open them, in one direction, by pushing with its nose at any point along the bottom. On the floor of each of the electric-boxes, W, was an oak board 1 cm. in thickness, which carried electric wires by means of which the mouse could be shocked in W when the tests demanded it. The interrupted circuit constituted by the wires in the two electric-boxes, in connection with the induction apparatus, IC, the dry battery, C, and the hand key, K, was made by taking two pieces of No. 20 American standard gauge copper wire and winding them around the oak board which was to be placed on the floor of each electric-box. The wires, which ran parallel with one another, 1/2 cm. apart, fitted into shallow grooves in the edges of the board, and thus, as well as by being drawn taut, they were held firmly in position. The coils of the two pieces of wire alternated, forming an interrupted circuit which, when the key K was closed, was completed if the feet of a mouse rested on points of both pieces of wire. Since copper wire stretches easily and becomes loose on the wooden base, it is better to use phosphor bronze wire of about the same size, if the surface covered by the interrupted circuit is more than three or four inches in width. The phosphor bronze wire is more difficult to wind satisfactorily, for it is harder to bend than the copper wire, and it has the further disadvantage of being more brittle. But when once placed properly, it forms a far more lasting and satisfactory interrupted circuit for such experiments as those to be described than does copper wire. In the case of the electric-boxes under consideration, the oak boards which carried the interrupted circuits were separate, and the two circuits were joined by the union of the wires between the boxes. The free ends of the two pieces of wire which constituted the interrupted circuit were connected with the secondary coil of a Porter inductorium whose primary coil was in circuit with a No. 6 Columbia dry battery. In the light of preliminary experiments, made in preparation for the tests of vision, the strength of the induced current received by the mouse was so regulated, by changing the position of the secondary coil with reference to the primary, that it was disagreeable but not injurious to the animal. What part the disagreeable shock played in the test of brightness vision will now be explained.
[Illustration: FIGURE 15.—Ground plan of discrimination box. A, nest- box; B, entrance chamber; W,W, electric-boxes; L, doorway of left electric-box; R, doorway of right electric-box; E, exit from electric- box to alley; I, swinging door between A and B; O, swinging door between alley and A; IC, induction apparatus; C, electric cell; K, key in circuit.]
An opportunity for visual discrimination by brightness difference was provided by placing dead black cardboard at the entrance and on the inside of one of the electric-boxes, as shown in Figure 14, B, and white cardboard similarly in the other box. These cardboards were movable and could be changed from one box to the other at the will of the experimenter. The test consisted in requiring the mouse to choose a certain brightness, for example, the white cardboard side, in order to return to the nest-box without receiving an electric shock. The question which the experimenter asked in connection with this test really is, Can a dancer learn to go to the white box and thus avoid discomfort? If we assume its ability to profit by experience within the limits of the number of experiences which it was given, such a modification of behavior would indicate discrimination of brightness. Can the dancer distinguish white from black; light gray from dark gray; two grays which are almost of the same brightness? The results which make up the remainder of this and the following chapter furnish a definite answer to these questions.
To return to the experimental procedure, the mouse which is being tested is placed by the experimenter in the nest-box, where frequently in the early tests food and a comfortable nest were attractions. If it does not of its own accord, as a result of its abundant random activity, pass through I into B within a few seconds, it is directed to the doorway and urged through. A choice is now demanded of the animal; to return to the nest-box it must enter either the white electric-box or the black one. Should it choose the white box, it is permitted to return directly to A by way of the doorway E, the alley, and the swinging door at O, and it thus gets the satisfaction of unobstructed activity, freedom to whirl, to feed, and to retreat for a time to the nest. Should it choose to attempt to enter the black box, as it touches the wires of the interrupted circuit it receives a shock as a result of the closing of the key in the circuit by the experimenter, and further, if it continues its forward course instead of retreating from the "stinging" black box, its passage through E is blocked by a barrier of glass temporarily placed there by the experimenter, and the only way of escape to the nest-box is an indirect route by way of B and the white box. Ordinarily the shock was given only when the mouse entered the wrong box, not when it retreated from it; it was never given when the right box was chosen. The box to be chosen, whether it was white, gray, or black, will be called the right box. The electric shock served as a means of forcing the animal to use its discriminating ability. But the question of motives in the tests is not so simple as might appear from this statement.
The reader will wonder why the mouse should have any tendency to enter B, and why after so doing, it should trouble to go further, knowing, as it does from previous experiences, that entering one of the electric-boxes may result in discomfort. The fact is, a dancer has no very constant tendency to go from A to B at the beginning of the tests, but after it has become accustomed to the box and has learned what the situation demands, it shows eagerness to make the trip from A to B, and thence by way of either the right or the left route to A. That the mouse should be willing to enter either of the electric-boxes, after it has experienced the shock, is even more surprising than its eagerness to run from A to B. When first tested for brightness discrimination in this apparatus, a dancer usually hesitated at the entrance to the electric-boxes, and this hesitation increased rapidly unless it were able to discriminate the boxes by their difference in brightness and thus to choose the right one. During the period of increasing hesitancy in making the choice, the experimenter, by carefully moving from I toward the entrances to the electric-boxes a piece of cardboard which extended all the way across B, greatly increased the mouse's desire to enter one of the boxes by depriving it of dancing space in B. If an individual which did not know which entrance to choose were permitted to run about in B, it would often do so for minutes at a time without approaching the entrance to the boxes; but the same individual, when confined to a dancing space 4 or 5 cm. wide in front of the entrances, would enter one of the electric-boxes almost immediately. This facilitation of choice by decrease in the amount of space for whirling was not to any considerable extent the result of fear, for all the dancers experimented with were tame, and instead of forcing them to rush into one of the boxes blindly and without attempt at discrimination, the narrowing of the space simply increased their efforts to discriminate. The common mouse when subjected to similar experimental conditions is likely to be frightened by being forced to approach the entrances to the boxes, and fails to choose; it rushes into one box directly, and in consequence it is as often wrong as right. The dancer always chooses, but its eagerness to choose is markedly increased by the restriction of its movements to a narrow space in front of the entrances between which it is required to discriminate. It is evident that the animal is uncomfortable in a space which is too narrow for it to whirl in freely. It must have room to dance. This furnished a sufficiently strong motive for the entering of the electric-boxes. It must avoid disagreeable and unfavorable stimuli. This is a basis for attempts to choose, by visual discrimination, the electric-box in which the shock is not given. It may safely be said that the success of the majority of the experiments of this book depended upon three facts: (1) the dancer's tendency to avoid disagreeable external conditions, (2) its escape-from-confinement- impelling need of space in which to dance freely, and (3) its abundant and incessant activity.
Of these three conditions of success in the experiments, the second and third made possible the advantageous use of the first. For the avoidance of a disagreeable stimulus could be made use of effectively in the tests just because the mice are so restless and so active. In fact their eagerness to do things is so great that the experimenter, instead of having to wait for them to perform the desired act, often is forced to make them wait while he completes his observation and record. In this respect they are unlike most other animals.