A 2-PROCESS ANALYSIS OF PATTERN MASKING

By comparing the masking effects of cosine gratings and uniform fields on spatially narrow-band test patterns, we obtain evidence that patte rn masking is mediated by two stages of visual processing: an early pr ocess of point-wise luminance adaptation and a late process of spatial -frequency and orientation-selective filtering. The early (presumably receptoral) component of masking is not affected by the polarity (incr emental or decremental) or spatial frequency of test patterns, and may either increase or decrease pattern sensitivity based on local light level. The late masking process is orientation and spatial-frequency d ependent, implying a cortical origin. For this cortical process, we fi nd competitive interaction between parallel ON and OFF visual mechanis ms: on a bright bar of a mask, threshold shift is greater for decremen tal than incremental tests, and the opposite is true on a dark bar of the mask. We suggest that ON-OFF interactions in pattern masking serve to normalize the gain of ON and OFF mechanisms simultaneously in orde r to preserve the relative contrast in the image.