In this paper we present the respective EEG markers of unconscious and conscious processing of two classes of auditory stimuli (sounds and words).
We show that in both cases, conscious access to the corresponding representation (e.g.: auditory regularity and verbal semantic content) shares a similar neural signature (P3b and P600/LPC) that can be distinguished from unconscious processing occurring during an earlier stage (MMN and N400).
We propose a two-stage serial model of processing and discuss how unconscious and conscious signatures can be measured at bedside providing relevant informations for both diagnosis and prognosis of consciousness recovery.
Link toward the online version on Revue Neurologique.
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Figure 1. Schematic representation of Disorders of Consciousness according to Wakefulness and Awareness dimensions. Acute alteration of consciousness, previously described using imprecise terms such as stupor, lethargy, etc. (in the ‘‘cloud’’) are better described using dedicated scale such as the Glasgow Coma Scale or the FOUR-score. Coma is commonly described by a state of unresponsiveness in which the patient lies with eyes closed and cannot be aroused to respond appropriately to stimuli even with vigorous stimulation. Vegetative State (VS) and Minimally Conscious State (MCS) are mostly used to describe chronic or subacute disorders of consciousness adapted from (adapted form Laureys, S. Trends Cogn Sci 2005).
Figure 2. ERPs paradigms and their respective neurophysiological responses. Schematic representation of the ‘‘Odd-ball’’ (a), the ‘‘Local Global’’ (b) and the ‘‘Semantic priming’’ (c) paradigms. Each paradigm elicits specific event-related potentials (ERPs) displayed across the time form one electrode (d) and as scalp topography of the subtraction (red curve–blue curve) at a specific time (e, the black dot represents the position of the electrode). The ‘‘Local Global’’ paradigm has been designed to disentangle the 2 components of the P300 classically elicited by the ‘‘Odd-ball’’ paradigm: (P3a & P3b). Note the topography and the temporal relation similarity between MMN/P300 sequence elicited by the ‘‘Odd-ball’’ and ‘‘Local Global’’ paradigms and the N400/LPC sequence elicited by the semantic paradigm. LS: local standard trial; LD: local deviant trial; GS: global standard trial; GD: global deviant trial. MMN: Mismatch Negativity; LPC Late positive component.
Reference: Rohaut B & Naccache L. Disentangling conscious from unconscious cognitive processing with event-related EEG potentials. Revue Neurologique. 2017 Aug;173:521-28. (postprint).