Electrophysiological correlates of visual awareness and its different properties

Two ERP components, an early negative deflection (Visual Awareness Negativity; VAN), and a later positive deflection (Late Positivity; LP) are thought to reflect different properties of consciousness: the phenomenal content of a perception and access to it, respectively. The first experiment investigated the graded vs. dichotomous nature of consciousness. The aim was to search for the neural correlates of different grades of visual awareness analyzing the ERPs to reduced contrast stimuli, whose clarity was rated on the 4-point Perceptual Awareness Scale (PAS). Results revealed a left centro-parietal negative deflection (VAN; peak at ~280-320ms), followed by a bilateral positive deflection (LP; ~510-550ms) over almost all electrodes. Interestingly, the amplitude of both deflections gradually increased along with visual awareness and the intracranial generators of the phenomenal content (VAN) were located in the left temporal lobe. Data thus suggest that visual awareness is characterized by a gradual increase of perceived clarity at behavioral and neural level, and that the perceptual content emerges from early local activation in temporal areas. The aim of the second experiment was to use an integrative approach as a diagnostic tool to discriminate between blindsight or degraded conscious vision. Blindsight is the ability of some patients with a visual field defect (hemianopia) to exhibit visually guided behavior also in their blind field, despite reporting no awareness of stimuli. Patients with degraded conscious vision, differently from blindsight patients, should thus show the same ERP components (VAN and LP) and the same visual awareness modulation observed in healthy subjects. To this end, a hemianopic patient was presented with stimuli in her blind visual field, asked to discriminate and then rate them on the PAS. At behavioral level, her discrimination accuracy depended on the level of awareness, thus suggesting degraded conscious vision. Electrophysiological data revealed the presence of the early VAN (peak at ~200ms) and the late LP (from ~300ms), and, crucially, the amplitude of both components was modulated by the level of awareness. Electrophysiological signatures can thus be a fine-grained diagnostic tool when assessing hemianopic patients. The last experiment aimed at better characterizing the cognitive processes reflected in the LP. Not only it has been associated with conscious access to a perceptual content, but also with accumulation of sensory evidence leading to decision-making. To disentangle between the two, stimuli at different contrast levels were presented, asking 6 participants to perform a discrimination task and then rate the quality of their perception on the PAS. Results showed that the LP was modulated only by the subjective ratings of awareness, and not by the different levels of sensory stimulation. Data suggest that the component can be considered an intermediate stage between merely sensory input and decision, reflecting the level of access to internal representation, partly regardless of the physical information. What thus appears to be accumulated is not only sensory evidence, but also stimulus-independent neural noise produced within the brain itself. Overall, phenomenal and access consciousness were confirmed to be distinctly reflected in the VAN and LP. Moreover, since conscious visual perception occurs outside the primary visual cortex, V1 appears not to be necessary for the emergence of awareness.