A luminance decrement selectively masks the achromatic system when presented in the same eye as the stimulus. The purpose of this study was to determine whether the masking effect can be elicited under dichoptic conditions. The achromatic and color-opponent systems were identified with monocularly presented 10- and 200-ms spectral increments. A luminance decrement mask was presented to the fellow eye. This mask was 50-ms in duration and either preceded the onset of the increment by 25 ms (forward masking) or followed the increment by 25 ms (backward masking). The spectral sensitivity functions obtained with 10-ms increments, either preceded or followed by the dichoptic mask, were profoundly different from those obtained with isolated 10-ms increments: the broadband achromatic function was replaced by a color-opponent function of lesser sensitivity. In comparison, the masks did not have a substantial effect on the form or sensitivity of the color-opponent functions obtained with 200-ms increments. These data reveal that a luminance decrement mask, presented under dichoptic viewing conditions, suppresses the achromatic system but not the color-opponent system. Therefore selective masking of the achromatic system by a luminance decrement can be mediated at a site after the signals from the two eyes are combined—the striate cortex or beyond. Implications for models of binocular, blink, and saccadic suppression are discussed.
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