The role of primary visual cortex (V1) in visual awareness

In the search for the neural correlate of visual awareness, much controversy exists about the role of primary visual cortex. Here, the neurophysiological data from V1 recordings in awake monkeys are examined in light of two general classes of models of visual awareness. In the first model type, visual awareness is seen as being mediated either by a particular set of areas or pathways, or alternatively by a specific set of neurons. In these models, the role of V1 seems rather limited, as the mere activity of V1 cells seems insufficient to mediate awareness. In the second model type, awareness is hypothesized to be mediated by a global mechanism, i.e. a specific kind of activity not linked to a particular area or cell type. Two separate versions of global models are discussed, synchronous oscillations and spike rate modulations. It is shown that V1 synchrony does not reflect perception but rather the horizontal connections between neurons, indicating that V1 synchrony cannot be a direct neural correlate of conscious percepts. However, the rate of spike discharges of V1 neurons is strongly modulated by perceptual context, and these modulations correlate very well with aspects of perceptual organization, visual awareness, and attention. If these modulations serve as a neural correlate of visual awareness, then V1 contributes to that neural correlate. Whether V1 plays a role in the neural correlate of visual awareness thus strongly depends on the way visual awareness is hypothesized to be implemented in the brain.