A two-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 pattern masking is mediated by two stages of visual processing: an early process 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 (incremental 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 dependent, implying a cortical origin. For this cortical process, we find competitive interaction between parallel ON and OFF visual mechanisms: on a bright bar of a mask, threshold shift is greater for decremental 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 order to preserve the relative contrast in the image.