Effect of delay-interval illumination on matching behavior in the capuchin monkey.

Experiment 1 demonstrated that delayed matching-to-sample in the capuchin monkey was superior when the delay interval was spent in darkness rather than in moderate illumination. In contrast with previous studies in which the delayed-matching ability of primates appeared limited to 60 sec or less, in the dark condition all subjects showed above-chance matching at a 120-sec delay interval. Experiment 2 verified that darkness during the delay interval can facilitate delayed matching and provided evidence that the effective variable was the illumination level of the delay interval rather than change in illumination, which in Exp. 1 was confounded with illumination level.