Abstract
INTRODUCTION. In monocular rivalry, the observer experiences alternations between different perceptual representations of the same image, in which different components alternate in visibility, but without complete suppression as in binocular rivalry. Surprisingly, no previous fMRI studies have directly compared binocular and monocular rivalry.
METHODS. Here we used fMRI at 3T to image activity in visual cortex while subjects perceived either monocular or binocular rivalry. The stimulus patterns were colored gratings (left & right oblique orientations) or face/house composites. The stimulus components (red or green) were either presented dichoptically for binocular rivalry or as a (monoptic) composite image with both components shown to each eye. Linear polarizers were used for dichoptic presentation. Six subjects performed either a binocular or monocular rivalry report task with the face/house or grating stimuli by indicating alternating percepts with button presses to measure alternation rates. The luminance contrasts were 9%, 18% or 36%.
RESULTS. The cortical activation for monocular rivalry included occipital pole, ventral temporal, and superior parietal cortex, while the areas for binocular rivalry also prominently included lateral occipital regions including MT+ as well as inferior parietal cortex near the TPJ. Both binocular rivalry and monocular rivalry showed a U-shaped function of activation as a function of contrast, i.e. higher activity for most areas at 9% and 36%. The increase in activation at higher contrast can be explained by an increase in neuronal response gain reflected in faster alternation rates, while that at low contrast can be explained by disinhibition (Wilson, 2007).
CONCLUSIONS. Overall, our results call into question models that distinguish binocular from monocular rivalry solely on the basis of V1 interocular competition. Rather our results indicate that binocular rivalry invokes binocular competition and suppression in higher-tier levels, whereas competition in monocular rivalry is relatively focused in early visual areas, with less inhibition.
Supported by NSERC and NIH.