The Role of Primary Visual Cortex in Perceptual Awareness

Under certain stimulus conditions a single interpretation of the external world cannot be unambiguously designated. When the brain is presented with such stimuli typically only one possible interpretation is perceived and after a few seconds the percept switches abruptly to another. Notably such perceptual alternations happen while the sensory input is kept constant, offering thus a clear dissociation of sensory stimulation and subjective awareness. A celebrated example of such a perceptual phenomenon is binocular rivalry (BR). It involves alternations of visual perception between two different images presented dichoptically at corresponding retinal locations. Based on many psychophysical studies over decades the primary visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression. However, the first neurophysiological evidence performed in monkeys did not corroborate this but instead found only a small percentage of neurons modulating their activity with the subjective awareness reported by the animals. On the contrary, studies using human functional magnetic resonance imaging (fMRI), have found V1 to be modulating to a large extent, creating an apparent controversy. Therefore, the role of primary visual cortex (V1) in subjective perception remains controversial. In this study, we studied the effects of perceptual suppression on neural activity in V1 of the macaque. We have used the binocular flash suppression (BFS) paradigm, a variant of BR which ensures excellent control over the subject’s perceptual state. We have recorded the spiking activity of a large number of well isolated single units (SUA) and acquired simultaneous local field potentials (LFPs) during the dichoptic presentation of orthogonal orientation gratings. Our design enabled us to determine a) which neurons and LFP bands are correlated with the percept and b) how this is related to their orientation and ocularity preferences. We find that only a small minority of about 20 of the single units modulate in consonance with the perceptual suppression. Furthermore, the magnitude of the perceptual effect was small (~15) in comparison to the sensory preference of the neurons. Results of the LFPs were very similar to the single units showing a similar percentage of sites modulating with perception. Analysis of the orientation and ocularity preference of neurons did not show a particular class of cells to be having a greater probability to show perceptual modulations