An experimental study of representative factors in monkeys.

Abstract

Two monkeys, a [male] Macacus rhesus and a [female] M. cynomologus, gave 68-100 % correct choices in delayed reaction tests in which the delay ranged from 15 sec. to 20 hrs. The subject saw food placed in 1 of 2 similar containers arranged on the floor 8 ft. away from and in front of him. A board screen was then erected between the containers and the subject. After the delay the subject was permitted to make 1 choice between the 2 containers. During the long delays the subject was removed from the situation and returned when required to make the choice. Under these conditions the maintenance of bodily orientation toward the correct container was impossible. The monkeys tended to survey the situation after a delay and appeared to reorient themselves prior to choosing. Distractions during the delays had no effect upon the correctness of the choices. By modifying the experimental situation evidence was secured to show that the [male] responded to the direction of the correct container on the basis of whether it was to his right or left, while the [female] responded to the position of the correct container without relation to herself. There was nothing in the sum total of this evidence or of that secured by other investigators to show that the subjects were not merely responding to the position of the correct container. To ascertain whether the monkeys also had representations of the nature of the food, the "Substitution Method" was developed. With the same experimental situation as before, a preferred food such as banana was placed under one of the containers. The board screen was erected and under cover of it the investigator secretly removed the banana and substituted less relished food such as lettuce. When the subject responded, it rushed around the board screen and picked up the proper container in the usual manner. But instead of seizing the substituted food, although the monkey was hungry for it, he hesitated, searched in the container and around the floor as though looking for the banana. The subject then usually walked away angrily, leaving the food on the floor where it was found. The "surprise," "disappointment," hesitation and searching behavior which appeared only in these cases of substitution are considered evidence of representative factors standing for qualitative aspects of the food. When a monkey saw 2 pieces of banana placed under 1 of the containers and found but 1 when he responded, he seized the one piece and searched for the other, demonstrating in the same manner representations of quantity. The substitution test was used with 2 5-yr.-old boys, with 2 kinds of candy as bait. The boys'' behavior was very similar to that of the monkeys.