Effect of masking by light on the apparent contrast of flashed gratings.

Abstract

Two observers judged the apparent contrast of briefly flashed gratings with spatial frequencies of 0.25, 1.0, or 4.0 cycles per deg. The stimuli had equal physical contrasts and were either preceded or followed by an overlapping white masking flash. When the masking flash preceded the stimulus flash (forward masking), there was a linear increase of apparent contrast with spatial frequency, indicating the presence of lateral-inhibitory activity in the visual system. When the masking flash followed the stimulus flash (backward masking), there was no increase of apparent contrast with spatial frequency, indicating the absence of lateral-inhibitory activity. To account for these results, a model of the visual system was formulated in which the neural response to a stimulus is presumed to consist of an early excitatory and a lagging inhibitory component. According to this model, a forward mask disrupts the excitatory-response component to a relatively greater extent than the inhibitory component, whereas the opposite is true for a backward mask.