Brain physiology and unconscious states
The distinction between different kinds of unconscious states such as anaesthesia, coma, vegetative state, minimally conscious state and locked-in syndrome are hard to draw. One of the methods used as tools for distinguishing between these states, or at least grossly between conscious and unconscious states, is the EEG. Some studies have shown that the EEG […]
The distinction between different kinds of unconscious states such as anaesthesia, coma, vegetative state, minimally conscious state and locked-in syndrome are hard to draw. One of the methods used as tools for distinguishing between these states, or at least grossly between conscious and unconscious states, is the EEG. Some studies have shown that the EEG might be a viable option in our search for proper distinction between these states.
However, as Kotchoubey writes in an article in Progress in Brain Research, the use of EEG is confounded by several variables, and the tool should be used with great caution.
Event-related potential measures of consciousness: two equations with three unknowns
by Kotchoubey B in Prog Brain Res. 2005; 150: 427-44
This is a brief review of event-related brain potentials (ERPs) as indices of cortical information processing in conditions in which conscious perception of stimuli is supposed to be absent: sleep, coma, vegetative state, general anesthesia, neglect as well as presentation of subliminal or masked stimuli. Exogenous ERP components such as N1 and P2 are much more likely to remain in all these conditions than endogenous components. Further, all varieties of the late posterior positive ERP waves (e.g., P3b, P600, late positive complex) are most difficult to be elicited in such conditions, indicating that the cortical activity underlying the late posterior positivity may have a particularly close relationship to brain mechanisms of conscious perception. Contrary to what might be expected, reliable ERP effects indicating complex analysis of semantic stimulus features (i.e., meaning) can be recorded without conscious awareness, generally, as easy as (in some conditions, even easier than) ERP components related to rather simple physical stimulus features. It should be emphasized, however, that we never should overestimate our confidence about the degree of subjects’ unawareness. Particularly in the conditions in which no behavioral response can be obtained (e.g., sleep, coma, anesthesia), residual conscious processing, at least in some subjects and on some trials, cannot be ruled out.
