Preserved gain control for luminance contrast during binocular rivalry suppression

Binocular rivalry elevates contrast increment thresholds for the detection of a transient stimulus presented to the suppressed eye, while thresholds measured during dominance are identical to those during monocular viewing (e.g. [Wales, R., & Fox, R. (1970). Increment detection thresholds during binocular rivalry suppression. Perception and Psychophysics, 8, 90-94]). It is well established that contrast increment thresholds depend on reference (pedestal) contrast. With high contrasts, increment thresholds increase with pedestal contrast, reflecting a gain control with sigmoidal non-linearity. We examined how this gain control mechanism operates during binocular rivalry (i.e., with and without perception of a pedestal mask). Subjects viewed a horizontal sine-wave grating (steady pedestal) and a radial checkerboard dichoptically. When the grating achieved a pre-specified phenomenal state (dominance or suppressed), subjects initiated the transient presentation (500-ms Gaussian pulse) of a contrast increment of the same spatial frequency. The pulse appeared in either the upper or lower half of the pedestal. Subjects indicated which half of the pedestal contained the pulse. Contrast increment thresholds were measured using a staircase method with various pedestal contrasts, which yielded threshold versus contrast (TvC) functions during dominance and suppression. The measured thresholds were reliably higher during suppression, but the rising slopes of TvC functions did not differ significantly between dominance and suppression (i.e., constant upward shift of TvC function). A control experiment demonstrated that the TvC function during dominance was identical to that during non-rivalry, monocular viewing. Evidently, the contrast gain control for transient luminance increment does not require the perception of pedestal contrast.