Abstract
In visual cognitive neuroscience the debate on consciousness is focused on two major topics: the search for the neural correlates of the different properties of visual awareness and the controversy on the graded versus dichotomous nature of visual conscious experience. The aim of this study is to search for the possible neural correlates of different grades of visual awareness investigating the Event Related Potentials to reduced contrast visual stimuli whose perceptual clarity was rated on the four-point Perceptual Awareness Scale. Results revealed a left centro-parietal negative deflection (Visual Awareness Negativity; VAN) peaking at 280–320 ms from stimulus onset, related to the perceptual content of the stimulus, followed by a bilateral positive deflection (Late Positivity; LP) peaking at 510–550 ms over almost all electrodes, reflecting post-perceptual processes performed on such content. Interestingly, the amplitude of both deflections gradually increased as a function of visual awareness. Moreover, the intracranial generators of the phenomenal content (VAN) were found to be located in the left temporal lobe. The present data thus seem to suggest (1) that visual conscious experience is characterized by a gradual increase of perceived clarity at both behavioral and neural level and (2) that the actual content of perceptual experiences emerges from early local activation in temporal areas, without the need of later widespread frontal engagement.
Highlights
Consciousness refers to the fact that, when we are awake, we have experiences
Visual inspection of the grand average event-related brain potentials (ERPs) of each category of the Perceptual Awareness Scale (PAS) confirmed the presence of an early negative deflection (VAN) peaking at 300 ms at left channels followed by a later bilateral positive deflection (LP) starting at ∼400 ms (Figure 1C)
Visual Awareness Negativity In the VAN time range (∼280–300 ms) the first consciousunconscious pairwise comparison performed on PAS = 1 versus PAS = 0 conditions demonstrated that the ERP amplitudes differed significantly at electrodes T7 and Cp5 in the left hemisphere, contralateral to stimulus presentation (Figure 2A)
Summary
Consciousness (or awareness) refers to the fact that, when we are awake, we have experiences. Recent ERP studies have found that conscious perception consistently correlates with an early component called Visual Awareness Negativity (VAN; Koivisto and Revonsuo, 2003), that is a negative amplitude difference wave between aware and unaware trials peaking at about 200 ms after stimulus onset in occipitotemporal sites (Koivisto et al, 2008), and observed at central, fronto-polar (Wilenius-Emet et al, 2004) and occipital-parietal (Pitts et al, 2014) electrodes. The latency of this component is prolonged (up to 200 ms later) when the contrast of the stimuli is lowered (Ojanen et al, 2003). Weaker evidence has been found for an enhancement of P1 amplitude in response to aware trials at around 100–130 ms in the occipital sites (Pins and Ffytche, 2003), even if this early positive component might better reflect attention-related processes (Hillyard et al, 1998)
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