The dynamics of cross-orientation masking at monocular and interocular sites

We investigated the temporal properties of monocular and dichoptic cross-orientation masking (XOM) mediating suppressive or facilitatory cross-channel interactions between the neural detectors for the test and orthogonal mask stimuli. We measured the evolution of masking as a function of the duration of the test and mask stimuli to determine its time constant, and determined its dependence on stimulus onset asynchrony (SOA), for three contrast combinations: color-only (red-green color test and mask), luminance-only (luminance test and mask) and color-luminance (color test and luminance mask). Results show that the temporal properties of monocular and dichoptic masking differ markedly from each other and across contrast type. For the color-only condition, the dichoptic suppressive interaction is significantly longer than for the monocular one and both are largely independent of SOA. For the luminance-only condition, the suppressive interactions in both presentations are faster than for color, have similar time constants, but have different dependencies on SOA. For the color-luminance condition under the monocular conditions, cross-orientation facilitation (XOF) occurs with the luminance mask speeding up the processing of the color test with greatest XOF when the luminance mask precedes the color test by around 22 ms. No significant effects are observed for the dichoptic condition. Effects are invariant across spatial frequency. These strongly differential dynamic effects suggest that there is separate encoding of color contrast, luminance contrast, and their combination at the relatively early within-eye stage of processing, which is distinct from the dichoptic site.