Spatial frequency and duration effects on the tilt illusion and orientation acuity.

The simultaneous tilt illusion and the decline in variance of orientation judgements (Andrews effect) were measured as a function of exposure duration and spatial frequency. The illusions increased in size (to more than 10 deg) with exposure times up to 30-100 msec, then declined. The Andrews effect was largest at the shortest exposure and asymptoted (for a particular spatial frequency) at about the same exposure duration at which the illusion peaked. The exposure duration at which the illusion peaked was longer if the subject was more dark adapted. When the subjects' rating of the perceptual clarity of the gratings was plotted against the size of the Andrews effect (for the same duration and spatial frequency), the data fell on a single function, whether the spatial frequency was 2, 5, or 10 c/deg. The functional significance of these effects is discussed.