A few observations on metacontrast stimuli

Metacontrast is a form of visual masking in which the target and mask are non-overlapping. In metacontrast, the masking effect is typically largest when the mask is presented some time after the target. This is known as Type-B masking. The present report examines to what extent Type-B metacontrast masking can be explained based on the stimuli involved. The assumption is made that the visibility of the target is, at least in part, determined by the correlation between the amplitude spectrum of the target-and-mask combination and that of the target alone. It is found that the correlation is higher when the stimuli are presented at the same time relative to when they are presented at different times. This relationship follows from the stimuli alone. Thus, one would expect the masking to be weakest when the two stimuli are simultaneous. Type-B correlation functions, in which the largest reductions occur only when the mask is presented after the target, can be obtained by further assuming a temporal integration window with a rapid onset and a shallow decline. In agreement with psychophysical masking studies; the analyses yield functions that are most similar to Type-B masking for moderate mask intensities and become less Type-B like at higher mask intensities. The effects of dark adaptation and spatial separation of target and mask are also modeled.