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When dissimilar images are presented to the two eyes, perception starts alternating spontaneously between each monocular view, a phenomenon called binocular rivalry (Leopold and Logothetis, 1999). Several imaging studies in humans have shown the involvement of a frontoparietal network of cortical areas in perceptual transitions during binocular rivalry (Lumer et al., 1998). Here we investigate the possible role of parietal visual areas in perceptual alternations during rivalry in the rhesus macaque. Neural activity in the lateral intraparietal area (LIP) was recorded extracellularly while the subject was presented dichoptically and asynchronously with two rivalrous patterns, resulting in flash suppression (Keliris et al., 2010). The paradigm ensures excellent control over the subject’s perceptual state.
Preliminary results confirm the transient change of brain activity around perceptual reversals at the single cell level. The recorded cells typically showed an initial burst of activity after the onset of a stimulus as well as at stimulus/perceptual changes, followed by a sustained response (Bisley, 2004). The transient response of recorded neurons has a short latency, lasts a few hundred milliseconds and is always positive while the sustained response is suppressive in some cells and excitatory in others. We speculate that these responses may reflect two separate underlying processes. The short latency response may reflect a fast sensory signal conveying the information in a bottom-up manner, while the sustained activity may represent top-down influences originating from higher areas in the prefrontal cortex. The functional magnetic resonance imaging (fMRI) studies performed previously could not dissociate these two tightly overlapping signals because of the poor temporal resolution of the technique.
Analysis of the firing rates of single and multi-units indicate that the transient part of the response predicts well the change in perception while the sustained activity does not show a significant correlation with perceptual state. This might be explained by the little selectivity of the sustained response of parietal neurons towards particular stimuli (Lehky and Sereno, 2007). It is believed that LIP neurons provide a representational map of saliency, integrating bottom-up and top-down information to guide the allocation of spatial attention (Bisley et al., 2011). We argue that the transient response of LIP neurons after perceptual switches is an indication for a role of this region in providing a change signal to higher areas. It is possible, that the intraparietal activation observed in humans around perceptual transitions may simply reflect the elevation of neural activity as a result of a novel percept rather than a causal role of the region in driving the switches. We are therefore planning to extend the binocular flash suppression paradigm to normal binocular rivalry and monitor the activity around spontaneous perceptual alternations in order to delineate what happens without any concomitant physical change in the stimulus.
Furthermore, local field potentials, temporal dynamics of single unit activity and synchronization between neurons might provide a better understanding of the top-down influences of prefrontal cortex especially during the sustained response. This analysis is currently in progress.
