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.