Differences in the magnitude of the Delboeuf Illusion and Usnadze Effect during adulthood.

The Delboeuf illusion (Type I) and Usnadze Effect (Type II) were used to investigate changes in perceptual processes in 72 female subjects, aged 20-79 years. Unlike other studies, small figures (less than 2 degrees) were presented for brief periods under fixation conditions. The magnitude of the Delboeuf illusion increased in old age, while the magnitude of the Usnadze Effect showed no change.