Physiological Correlates of Sensory Consciousness: Evidence for a Perceptual Awareness Negativity

A critical review of EEG and MEG markers of the neural correlates of consciousness (NCC) for visual, auditory, and tactile stimuli concludes that two prominent signals—the early, modality-specific visual and auditory awareness negativities (VAN and AAN) and the late, modality-independent P3b—are not equally valid NCC. More than twelve studies found the P3b absent despite conscious perception, ruling it out. No convincing evidence dissociates the awareness negativities from consciousness. Evidence for a tactile equivalent, the somatosensory awareness negativity (SAN), supports a generalized perceptual awareness negativity (PAN) correlated with perceptual consciousness, arising 120–200 ms after stimulus onset from sensory cortices. This PAN points to new research avenues.

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Abstract

We critically review the recent literature on six EEG and MEG markers of the neural correlates of consciousness (NCC) for visual, auditory and tactile stimuli in neurotypical volunteers and neurological patients. After ruling out four of these as candidate NCC, we focus on two prominent evoked signals – an early, modality-specific negativity, termed the visual or auditory awareness negativity (VAN and AAN, respectively) and a late, modality-independent positivity termed the P3b. More than twelve diverse experimental studies found that the P3b is absent despite consciously seeing, hearing, or feeling stimuli, ruling out the P3b as a true NCC. In contrast, no convincing evidence for a dissociation between the awareness negativities and consciousness has been reported thus far. Furthermore, there is evidence for an equivalent signal in the tactile domain, which we term the somatosensory awareness negativity (SAN). These three neural signals are usually maximal on the side of the scalp contralateral to the evoking stimulus, above the associated sensory cortices. We conclude that the data from these three modalities is consistent with a generalized perceptual awareness negativity (PAN) correlated with perceptual consciousness that arises 120-200 ms following stimulus onset and originates from the underlying sensory cortices. The identification of this PAN points towards new, promising avenues for future research and raises an array of concrete questions that can be empirically investigated.