Social stimuli as incentives for delayed response performance by infant pigtailed macaques (Macaca nemestrina)

Six pigtailed macaque monkeys (Macaca nemestrina), ranging in age from 4.5 to 13.5 months, were studied longitudinally on a delayed response procedure in a spatial choice apparatus. In each trial the subjects were exposed to two stimulus animals, an unfamiliar adult female and a familiar age-mate. Prior work showed that such nursery-reared infants overwhelmingly prefer a familiar peer over an unfamiliar adult female. Therefore, the peer was considered to be a positive incentive and choosing the peer was defined as the “correct” response. After the infants were given visual exposure to the stimulus animals, an opaque door was lowered to block them from the subject's view. Then a single delay period of 0, 5, 15, 60, 120, 240, or 480 sec was introduced during 20-trial sessions. The delays were increased over sessions, with about two weeks between sessions. The subjects reliably chose the familiar peer after delays of up to 60 sec, with one subject maintaining correct choices even after 8-min delays. These results revealed that the delayed response performance of young macaques with social incentives was as good as, or better than, maximum performance levels reported for macaques with food incentives. Cues such as odors, sounds, and visual-postural orientation sets could not explain the performance levels at long delays found in this study.