Abstract
Two monkeys experienced with delayed matching to sample were given a 30-day baseline training period during which the delay interval was illuminated. Both subjects showed an increase in matching accuracy when shifted to dark delay intervals, and accuracy declined when the illuminated delay interval was reinstituted. These results, as well as earlier reports of facilitation of delayed matching behavior by dark delay intervals, support the view that the absolute level of delay-interval illumination can importantly affect visual retention in monkeys and may be indicative of significant differences in the retention mechanisms employed by monkeys and birds.