Selective masking of the achromatic system can be mediated cortically

Purpose. Our earlier work demonstrated that a luminance decrement selectively masks the achromatic system when presented in the same eye as the stimulus (Schwartz and Godwin, Vision Res., in press). The aim of this study was to determine if the effect can be elicited under dichoptic viewing conditions. Methods. A three channel Maxwellian view system was used to present spectral increments (420 to 620 nm) to the left eye and a luminance decrement mask to the right eye. The spectral incs were presented on a 340 td, 3.27 deg, spatially coincident, white background. 10 ms incs were used to study the achromatic system, while 200 msec incs were used to study the color-opponent system. The spectral incs were presented in isolation or in conjunction with a 50 ms, suprathreshold, dichoptic, luminance decrement mask (24 td, 3.27 deg). The mask terminated 25 ms prior to the onset of the inc (forward masking) or was initiated 25 ms following the termination of the inc (backward masking). At least two subjects participated in each phase of the experiment. Results. The spectral sensitivity functions obtained with 10 ms incs, either preceded or followed by a dichoptic mask, were profoundly different than those obtained with isolated 10 ms incs: the broad-band achromatic function was replaced by a color-opponent function of reduced 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 incs. Conclusions. A luminance decrement mask, presented under dichoptic viewing conditions, suppresses the achromatic system, but not the color-opponent system. Therefore, we conclude that selective masking of the achromatic system can be mediated cortically. This result is consistent with models of saccadic suppression that posit a masking mechanism.