Abstract
Binocular rivalry elevates contrast increment thresholds for detection of a transient stimulus presented to the suppressed eye; thresholds measured during dominance are identical to those during monocular viewing. 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 compressive nonlinearity. We examined how this gain-control mechanism operates during binocular rivalry (i.e. with and without perception of a pedestal mask). Observers viewed a horizontal sine-wave grating (steady pedestal) and a radial checkerboard dichoptically. When the grating achieved a prespecified phenomenal state (dominant or suppressed), observers 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. Observers indicated which half contained a pulse. Contrast increment thresholds were measured using a staircase method with various pedestal contrasts, yielding threshold versus contrast (TvC) functions during dominant and suppression phases. The measured thresholds were reliably higher during suppression phases, but the rising slopes of TvC functions did not differ significantly between dominant and suppression phases (i.e. constant upward shift of TvC function during suppression). Thus, the amount of transient contrast increment required for the task increased as a function of pedestal contrast with a constant Weber ratio. A control experiment revealed that the TvC function during dominant phase 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. The suppression during binocular rivalry can be considered a multiplicative noise infusion to the signal pulse that follows the gain control.
Supported by NIH grant EY13358 and AIST Japan