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FIGURE 13.—Multiple-choice apparatus, showing observer's bench and writing stand. FIGURE 14.—Apparatus as seen from observer's bench. FIGURE 15.—Entrances to multiple-choice boxes as seen from the response-compartment. FIGURE 16.—Apparatus as seen from the rear, showing exit doors, food receptacles, and covers for same.

The apparatus was built in room A (figure 12), this room having been especially planned for it with respect to lighting as well as dimensions and approaches. It was unfortunately impossible to obtain photographs showing the whole of the apparatus, but it is hoped that the four partial views of plate IV may aid the reader who is unfamiliar with previously described similar devices to grasp readily the chief points of construction. In this plate, figure 13 shows the front of the complete apparatus, with the alleyway and door by way of which the experimenter could enter. The investigator's observation-bench and record-table also appear in this figure, together with weighted cords used to operate the various doors and the vertically placed levers by means of which each pair of doors could be locked. Figure 14 is the view presented to the observer as he stood on the bench or observation stand of figure 13 and looked over the entire apparatus. Three of the entrance doors are shown at the right of this figure as raised, whereas the remainder of the nine entrance doors of the apparatus are closed. Figure 15 is a view of the entrance doors from below the wire roof of the apparatus. Again, two of the doors are shown as raised, and three additional ones as closed. The rear of the apparatus appears in figure 16, in which some of the exit doors are closed and others open. In the latter case, the food receptacles appear, and on the lower part of the raised doors of the corresponding boxes may be seen metal covers for the food receptacles projecting at right angles to the doors, while on the lower edge of each door is an iron staple used to receive a sliding bar which could be operated from the observer's bench as a means of locking the doors after they had been closed. The space beyond the exit doors was used as an alleyway for the return of the animals to the starting point.

It will be necessary at various points in later descriptions to refer to these several figures. But further description of them will be more readily appreciated after a careful examination of the ground plan of the apparatus presented as figure 17 In accordance with the labelling of this figure, the experimenter enters the apparatus room through doorway 16, passes thence through doorways 17 and 10 to the large cage Z, from which he has direct access to the animals and can bring them into the apparatus. The multiple-choice mechanism proper, consisting of nine similar boxes (nine were used instead of twelve as a matter of convenience of construction, not because this smaller number is otherwise preferable) is labelled F. These boxes are numbered 1 to 9, beginning at the left. This numbering was adhered to in the recording of results throughout the investigation. The other important portions of the apparatus are the runway D, from which the subject at the experimenter's pleasure could be admitted through doorway 12 to the large response-chamber E; the alleyways G, H, and I, by way of which return to the starting point was possible; the observation bench C, with its approach step 13; and the observer's writing table A.

In the construction of this large apparatus, it was necessary to make provision for the extremely destructive tendencies of monkeys and anthropoid apes—hence the apparent cumbersomeness of certain portions. It was equally necessary to provide for the protection of the observer and the prevention of escape of the subjects by completely covering the apparatus and alleyways with a heavy wire netting.

Each of the eighteen doors of the multiple-choice boxes, and in addition doors 11, 12, and 15 of the runway D, were operated by the observer from his bench C by means of weighted window cords which were carried by pulleys appropriately placed above the apparatus. Each weight was so chosen as to be just sufficient to hold its door in position after the experimenter had raised it. For the convenience of the experimenter in the rapid operation of the twenty-one doors, the weights for the doors of runway D were painted gray, those for the entrance doors, white, and those for the exit doors, black.

In each entrance door, as is shown in figure 15 of plate IV, a window was cut so that the experimenter might watch the animal after it had entered a given box, and especially note when it left the box after having received its reward. This window was covered with wire netting. No such windows were necessary in the exit doors, but to them were attached heavy galvanized iron flanges which served to cover the food receptacles. One of these flanges is labelled o in figure 17. The food receptacles were provided by boring holes in a 2 by 4 inch timber securely nailed to the floor immediately outside of the exit doors. Into these holes aluminum cups fitted snugly, and the iron flanges, when the doors were closed, fitted so closely over the cups that it was impossible for the animals to obtain food from them.

[Illustration: FIGURE 17.—Ground plan of multiple-choice apparatus in experiment room A. Scale ⅙0

A, record stand; C, bench for observer; B, step as approach to C; D, alleyway leading to E, response-compartment; F, one of the nine (1–9) similar multiple-choice boxes; G, H, alleyways leading from boxes to starting point at D; I, alleyway used by experimenter as approach to rear of apparatus; W, W, windows; P, alleyway; Z, large cage; 16, entrance to room A; 17, entrance to apparatus and thence via 10 to cages; 18, entrance to alleyway 1; 11, 15, entrances to D; 12, entrance to E; 13, entrance door of box 5; 14, exit door of box 5; o, cover for food receptacle.]

As originally constructed, no provision was made in the apparatus for locking the entrance and exit doors of the several boxes when they were closed. But as two of the subjects after a time learned to open the doors from either outside or inside the boxes, it became necessary to introduce locking devices which could be operated by the experimenter from the observation bench. This was readily accomplished by cutting holes in the floor, which permitted an iron staple, screwed to the lower edge of each door, to project through the floor. Through these staples by means of a lever for each of the nine boxes, the observer was able to slide a wooden bar, placed beneath the floor of the room, thus locking or unlocking either the entrance door, the exit door, or both, in the case of any one of the nine boxes.

Since figure 17 is drawn to scale, it will be needless to give more than a few of the dimensions of the apparatus. Each of the boxes was 42 inches long, 18 inches wide, and 72 inches deep, inside measurements. The alleys D, I, and H were 24 inches, and G 30 inches wide, by 6 feet deep. The doors of the several boxes were 18 inches wide, by 5 feet high, while those in the alleyways were 24 inches wide by 6 feet high. The response-compartment E of figure 17 was 14 feet 4 inches, by 8 feet, by 6 feet in depth. In order that the apparatus might be used with adult human subjects conveniently, if such use should prove desirable, the depth throughout was made 6 feet, and it was therefore possible for the experimenter to walk about erect in it.

The experimental procedure was briefly as follows: A small quantity of food having been placed in each of the food cups and covered by the metal flanges on the exit doors, the experimenter raised door 11 of figure 17 and then opened door 10 and the door of the cage in which the desired subject was confined. After the latter, in search of food, had entered the runway D, the experimenter lowered door 11 to keep it in this runway, and immediately proceeded to set the reaction-mechanisms for an experiment (trial). Let us suppose that the first setting to be tried involved all of the nine boxes. Each of the entrance doors would therefore be raised. Let us further suppose that the right door is defined as the middle one of the group. With the apparatus properly set, the experimenter next raises door 12, thus admitting the animal to the response-compartment E. Any one of the nine boxes may now be entered by it. But if any except number 5, the middle member of the group, be entered, the entrance door is immediately lowered and both the exit and entrance doors locked in position so that the animal is forced to remain in the box for a stated period, say thirty seconds. At the expiration of this time the entrance door is raised and the animal allowed to retrace its steps and make another choice. When the middle box is chosen, the entrance door is lowered and the exit door immediately raised, thus uncovering the food, which the animal eats. As a rule, by my monkeys and ape the reward was eaten in the alleyway G instead of in the multiple-choice box. As soon as the food has been eaten, the exit door is lowered by the experimenter, and the animal returns by way of G and H to runway D, where it awaits its next trial.

As rewards, bananas and peanuts were found very satisfactory, and although occasionally other foods were supplied in small quantities, they were on the whole less constantly desired than the former.

Four problems which had previously been presented to other organisms were in precisely the same form presented to the three primates. These problems may be described, briefly, by definition of the right reaction mechanism, thus: problem 1, the first mechanism at the subject's left; problem 2, the second mechanism at the subject's right (that is, from the end of the series at the subject's right); problem 3, alternately, the first mechanism at the subject's left and the first at its right; problem 4, the middle mechanism of the group.

It was my intention to present these four problems, in order, to each of the three animals, proceeding with them as rapidly as they were solved. But as it happened, only one of the three subjects got as far as the fourth problem. When observations had to be discontinued, Sobke was well along with the last, or fourth problem; Skirrl was at work at the third problem; and Julius had failed to solve the second problem.

For each of the problems, a series of ten different settings of the doors was determined upon in advance. These settings differ from those employed in a similar investigation with the pig only in that the numbering of the doors is reversed. In the present apparatus, the boxes as viewed from the front (entrance) are numbered from the left to the right end, whereas those of the pig apparatus were numbered from the right end to the left end.

Below are presented for each of the several problems (1) the numbers of the settings presented in series; (2) the numbers of the doors open; (3) the number of doors open in each setting and for the series of ten settings; and (4) the number of the right door.

PROBLEM 1. First mechanism at left of group

Doors No. of No. of

Settings open doors open right door

1 … … … … … … 1.2.3 … … … … … … . … 3 … … … … … … 1

2 … … … … … … 8.9 … … … … … … … … 2 … … … … … … 8

3 … … … … … … 3.4.5.6.7 … … … … … … 5 … … … … … … 3

4 … … … … … … 7.8.9 … … … … … … . … 3 … … … … … … 7

5 … … … … … … 2.3.4.5.6 … … … … … … 5 … … … … … … 2

6 … … … … … … 6.7.8 … … … … … … . … 3 … … … … … … 6

7 … … … … … … 5.6.7 … … … … … … . … 3 … … … … … … 5

8 … … … … … … 4.5.6.7.8 … … … … … … 5 … … … … … … 4

9 … … … … … … 7.8.9 … … … … … … . … 3 … … … … … … 7

10 … … … … … … 1.2.3 … … … … … … . … 3 … … … … … … 1

—

Total 35

PROBLEM 2. Second mechanism from the right end of group

Doors No. of No. of

Settings open doors open right door

1 … … … … … … 7.8.9 … … … … … … . … 3 … … … … … … 8

2 … … … … … … 1.2.3.4 … … … … … . … .4 … … … … … … 3

3 … … … … … … 2.3.4.5.6.7 … … … … . … 6 … … … … … … 6

4 … … … … … … 1.2.3.4.5.6 … … … … . … 6 … … … … … … 5

5 … … … … … … 4.5.6.7.8 … … … … … … 5 … … … … … … 7

6 … … … … … … 1.2.3 … … … … … … . … 3 … … … … … … 2

7 … … … … … … 2.3.4.5 … … … … … . … .4 … … … … … … 4

8 … … … … … … 1.2.3.4.5.6.7.8.9 … … . … 9 … … … … … … 8

9 … … … … … … 1.2.3.4 … … … … … . … .4 … … … … … … 3

10 … … … … … … 3.4.5.6.7.8 … … … … . … 6 … … … … … … 7

—

Total 50

PROBLEM 3. Alternately the first mechanism at the left and the

first at the right end of the group

Doors No. of No. of

Settings open doors open right door

1 … … … … … … 5.6.7 … … … … … … . … 3 … … … … … … 5

2 … … … … … … 5.6.7 … … … … … … . … 3 … … … … … … 7

3 … … … … … … 1.2.3.4.5.6 … … … … . … 6 … … … … … … 1

4 … … … … … … 1.2.3.4.5.6 … … … … . … 6 … … … … … … 6

5 … … … … … … 4.5.6.7.8 … … … … … … 5 … … … … … … 4

6 … … … … … … 4.5.6.7.8 … … … … … … 5 … … … … … … 8

7 … … … … … … 2.3.4.5 … … … … … . … .4 … … … … … … 2

8 … … … … … … 2.3.4.5 … … … … … . … .4 … … … … … … 5

9 … … … … … … 3.4.5.6.7.8.9 … … … . … .7 … … … … … … 3

10 … … … … … … 3.4.5.6.7.8.9 … … … . … .7 … … … … … … 9

—

Total 50

PROBLEM 4. Middle mechanism of the group

Doors No. of No. of

Settings open doors open right door

1 … … … … … … 2.3.4 … … … … … … . … 3 … … … … … … 3

2 … … … … … … 5.6.7.8.9 … … … … … … 5 … … … … … … 7

3 … … … … … … 1.2.3.4.5.6.7 … … … . … .7 … … … … … … 4

4 … … … … … … 7.8.9 … … … … … … . … 3 … … … … … … 8

5 … … … … … … 4.5.6.7.8 … … … … … … 5 … … … … … … 6

6 … … … … … … 1.2.3.4.5.6.7.8.9 … … . … 9 … … … … … … 5

7 … … … … … … 1.2.3 … … … … … … . … 3 … … … … … … 2

8 … … … … … … 2.3.4.5.6 … … … … … … 5 … … … … … … 4

9 … … … … … … 3.4.5.6.7.8.9 … … … . … .7 … … … … … … 6

10 … … … … … … 6.7.8 … … … … … … . … 3 … … … … … … 7

—

Total 50

It was found desirable after a problem had been solved to present a new and radically different series of settings in order to determine to what extent the subject had learned to choose the correct door by memorizing each particular setting. These supplementary observations may be known as control experiments, and the settings as supplementary settings. In case of these, as for the original settings, the essential facts are presented in tabular arrangement.

Settings for Control Experiments

PROBLEM 1. First at left end

Doors No. of No. of

Settings open doors open right door

1 … … … … … … 2.3.4 … … … … … … . … 3 … … … … … … 2

2 … … … … … … 6.7.8.9 … … … … … . … .4 … … … … … … 6

3 … … … … … … 3.4.5 … … … … … … . … 3 … … … … … … 3

4 … … … … … … 4.5.6.7.8.9 … … … … . … 6 … … … … … … 4

5 … … … … … … 6.7.8.9 … … … … … . … .4 … … … … … … 6

6 … … … … … … 1.2.3.4.5 … … … … … … 5 … … … … … … 1

7 … … … … … … 2.3.4.5.6.7.8 … … … . … .7 … … … … … … 2

8 … … … … … … 3.4.5.6.7.8 … … … … . … 6 … … … … … … 3

9 … … … … … … 5.6.7 … … … … … … . … 3 … … … … … … 5

10 … … … … … … 1.2.3.4.5.6.7.8.9 … … . … 9 … … … … … … 1

PROBLEM 2. Second from right end

Doors No. of No. of

Settings open doors open right door

1 … … … … … … 5.6.7.8 … … … … … . … .4 … … … … … … 7

2 … … … … … … 2.3.4.5.6 … … … … … … 5 … … … … … … 5

3 … … … … … … 1.2.3.4.5.6.7.8.9 … … . … 9 … … … … … … 8

4 … … … … … … 5.6.7 … … … … … … . … 3 … … … … … … 6

5 … … … … … … 1.2.3.4 … … … … … . … .4 … … … … … … 3

6 … … … … … … 4.5.6 … … … … … … . … 3 … … … … … … 5

7 … … … … … … 2.3.4.5 … … … … … . … .4 … … … … … … 4

8 … … … … … … 1.2.3 … … … … … … . … 3 … … … … … … 2

9 … … … … … … 1.2.3.4.5.6.7 … … … . … .7 … … … … … … 6

10 … … … … … … 2.3.4.5.6.7.8.9 … … … … 8 … … … … … … 8

PROBLEM 3. Alternate left and right ends

Doors No. of No. of

Settings open doors open right door

1 … … … … … … 5.6 … … … … … … … … 2 … … … … … … 5

2 … … … … … … 5.6 … … … … … … … … 2 … … … … … … 6

3 … … … … … … 4.5.6.7.8.9 … … … … . … 6 … … … … … … 4

4 … … … … … … 4.5.6.7.8.9 … … … … . … 6 … … … … … … 9

5 … … … … … … 1.2.3.4.5 … … … … … … 5 … … … … … … 1

6 … … … … … … 1.2.3.4.5 … … … … … … 5 … … … … … … 5

7 … … … … … … 2.3.4.5.6.7 … … … … . … 6 … … … … … … 2

8 … … … … … … 2.3.4.5.6.7 … … … … . … 6 … … … … … … 7

9 … … … … … … 3.4.5.6.7.8 … … … … . … 6 … … … … … … 3

10 … … … … … … 3.4.5.6.7.8 … … … … . … 6 … … … … … … 8

PROBLEM 4. Middle

Doors No. of No. of

Settings open doors open right door

1 … … … … … … 4.5.6.7.8 … … … … … … 5 … … … … … … 6

2 … … … … … … 1.2.3 … … … … … … . … 3 … … … … … … 2

3 … … … … … … 1.2.3.4.5.6.7.8.9 … … . … 9 … … … … … … 5

4 … … … … … … 2.3.4.5.6 … … … … … … 5 … … … … … … 4

5 … … … … … … 6.7.8 … … … … … … . … 3 … … … … … … 7

6 … … … … … … 3.4.5.6.7.8.9 … … … . … .7 … … … … … … 6

7 … … … … … … 7.8.9 … … … … … … . … 3 … … … … … … 8

8 … … … … … … 1.2.3.4.5.6.7 … … … . … .7 … … … … … … 4

9 … … … … … … 2.3.4 … … … … … … . … 3 … … … … … … 3

10 … … … … … … 3.4.5.6.7 … … … … … … 5 … … … … … … 5

It was my aim so far as possible to present to a given subject each day the ten settings under a given problem in order, without interruption. If for any reason the series of observations had to be interrupted, it was resumed at the same point subsequently. Occasionally it was found desirable or necessary to present only five of the series of ten settings in succession and then to interrupt observations for an interval of a few minutes or even several hours. But as a rule it was possible to present the series of ten settings. All things being considered, it proved more satisfactory to give only ten trials a day to each subject. Frequently twenty and rarely thirty trials were given on the same day. In such cases the series of settings was simply repeated. The only pause between trials was that necessary for resetting the entrance doors and replenishing the food which served as a reward for success.