Apparent spatial frequency and contrast of gratings: separate effects of contrast and duration

Variations in apparent spatial frequency of sinusoidal gratings were assessed using a matching method. Perceived spatial frequency increased as contrast decreased and as duration of presentation decreased (from 320 to 20 msec). The effect of duration was not produced via changes in apparent contrast (a) because the changes in perceived spatial frequency were much the same when subjective contrasts were matched as when physical contrasts were matched and (b) because reducing duration to 20 msec produced very little change in perceived contrast, despite a factor of four reduction in threshold contrast sensitivity. The effects of duration and contrast on apparent spatial frequency are therefore dissociable, but both might exert their effects via changes in the balance of centre-surround antagonism within receptive fields; the "selectivity shift" theory of these size perception phenomena is discussed.