Abstract
Introduction: During binocular rivalry suppression an ordinarily visible stimulus is erased from awareness, but how is the sensory representation of that stimulus affected? While it is well established that contrast sensitivity is attenuated during suppression, the influence of suppression on stimulus tuning profiles remains unknown. Here, we examine the influence of interocular suppression on the gain and bandwidth of orientation tuning curves inferred by noise masking.
Methods: Two competing stimuli were viewed dichoptically: a bullseye and a random noise patch whose orientation content was varied. To control rivalry state, we used the flash suppression technique. Following flash-induced dominance/suppression, a small, vertically oriented Gabor patch briefly appeared within the noise patch, above or below fixation, and observers reported the probe's location (2AFC). To obtain perceptual tuning functions, we used an orientation-bandpass noise masking procedure in which the noise and probe ranged from being identical in orientation, to the noise orientations being orthogonal to the Gabor. An adaptive procedure produced estimates of contrast thresholds for the probe embedded within varying orientation bandpass noise, yielding tuning curves for conditions when the noise and probe were presented during suppression and during dominance.
Results: Besides the expected overall increase in contrast thresholds for suppressed probe stimuli, we discovered significant broadening of the tuning curve bandwidth under suppression. To rule out the possibility that this broadening was attributable simply to a decrease in the noise patch's contrast representation under suppression, we decreased the contrast of the noise patch and measured tuning under monocular nonrivalry, but this led to no change in tuning bandwidth. These results indicate that the discriminability of a suppressed stimulus under rivalry is degraded not only by a general decrease in signal strength, but also by degradation of neural events promoting tuning sharpness which effectively increases the noisiness of the suppressed stimulus representation.