Abstract
The neural correlates of binocular rivalry have been actively debated in recent years, and are of considerable interest as they may shed light on mechanisms of conscious awareness. In a related phenomenon, monocular rivalry, a composite image is shown to both eyes. The subject experiences perceptual alternations in which the two stimulus components alternate in clarity or salience. The experience is similar to perceptual alternations in binocular rivalry, although the reduction in visibility of the suppressed component is greater for binocular rivalry, especially at higher stimulus contrasts. We used fMRI at 3T to image activity in visual cortex while subjects perceived either monocular or binocular rivalry, or a matched non-rivalrous control condition. The stimulus patterns were left/right oblique gratings with the luminance contrast set at 9%, 18% or 36%. Compared to a blank screen, both binocular and monocular rivalry showed a U-shaped function of activation as a function of stimulus contrast, i.e. higher activity for most areas at 9% and 36%. The sites of cortical activation for monocular rivalry included occipital pole (V1, V2, V3), ventral temporal, and superior parietal cortex. The additional areas for binocular rivalry included lateral occipital regions, as well as inferior parietal cortex close to the temporoparietal junction (TPJ). In particular, higher-tier areas MT+ and V3A were more active for binocular than monocular rivalry for all contrasts. In comparison, activation in V2 and V3 was reduced for binocular compared to monocular rivalry at the higher contrasts that evoked stronger binocular perceptual suppression, indicating that the effects of suppression are not limited to interocular suppression in V1.
Highlights
Multistable images comprise important examples of conscious visual perceptual changes without any change in the stimulus being viewed
There was a slight tendency for alternations in monocular rivalry to be slower than those for binocular rivalry, but both had approximately the same dependence on contrast, increasing as contrast was increased, and reaching a plateau at high contrasts
The alternations in binocular rivalry were accompanied by much greater suppression than monocular rivalry, associated with a much greater reduction in visibility of the suppressed pattern
Summary
Multistable images comprise important examples of conscious visual perceptual changes without any change in the stimulus being viewed. Multistability can be induced by either using an ambiguous figure with more than one perceptual interpretation such as the Necker cube [1] or Rubin’s vase/face [2] or by showing different images to the left and right eye, as in binocular rivalry [3]. Most previous functional neuroimaging studies of binocular rivalry reported activation in early visual areas (V1, V2, V3) [3,12,13], and further studies indicate that eyespecific dominance and suppression are reflected at an even earlier stage of visual processing, in the lateral geniculate nucleus [14,15]. Neuronal asynchrony in V1 might produce rivalrous response suppression at later stages in the visual pathway [16]. Other cortical regions are implicated, such as occipito-parietal areas (V3a, V4d-topo, V7) [17], ventral temporal areas [18,19], superior parietal lobe and caudal intraparietal sulcus [17,19,20], as well as frontal areas [19,20]
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