Perceived spatial frequency of flickering gratings: a local spatial distortion

Abstract

Perceived spatial frequency of a low-frequency grating increases if the grating contrast is temporally modulated at about 4 to 16 Hz. The amount of increase can be up to 50% as estimated by a grating-match technique. It is shown here that the illusion is not a simple spatial-frequency shift phenomenon but also involves local spatial distortions. This was discovered by using the method of distance match, in which the perceived length of a spatial period or its multiples was estimated in a flickering grating. The results indicate that the illusory increase in perceived spatial frequency does not predict the perceived period length of a grating in a simple reciprocal manner. The decrease in the apparent period length is restricted to distances covering only one or two spatial cycles. This points out that care should be taken in interpreting various illusory changes in spatial frequency simply as shifts of frequency. The results are interpreted as demonstrating the operation of a size-analyzing mechanism which has a limited spatial bandwidth and which is sensitive to flicker-induced disturbances.