Illusion decrement as a function of duration of inspection and figure type

Illusion decrement for the Muller-Lyer and Horizontal-Vertical illu sions was examined. The experiment consisted of an initial adjustment of an illusion followed by 20 test trials, each with an intervening 60-sec. intertrial interval during which a comparator line and a standard line set to equality were visually inspected for 0, 20, 40, or 60 sec. After each intertrial interval the length of the comparator line was reset by the experimenter to either 0 or 90 cm, and subjects then adjusted its length to perceived equality with the standard line (42 cm). Illusion decrement was inversely related to the duration of inspection for each illusion, with significant reductions in magnitude of illusion observed for all groups. These results support prior demonstrations that perceptual learning mechanisms are operative during brief periods of visual inspection, especially when these periods are followed by the opportunity to make repeated adjustments.