Consciousness and unconsciousness: an EEG perspective.

At first glance, defining unconsciousness may seem easier than defining it's counterpart consciousness. Indeed, consciousness has many definitions 1 , while unconsciousness is " just " the absence of consciousness. However, the study of patients with epilepsy or severe disorders of consciousness (e.g., coma and vegetative states) or focal deficits of consciousness (e.g., blindsight) and studies on sleep and anesthesia have shown that consciousness and unconsciousness are graded. Measuring consciousness or its gradation is essential but there is little consensus on how it should be done. Different behavioral and neurophysiological measures of consciousness and theories of consciousness have been proposed (e.g., higher order thought theories, integration theories, and worldly discrimination theory 2). In clinical settings, the differentiation between consciousness and unconsciousness is limited to evaluating patients' motor responsiveness. This is extremely challenging because some patients are paralyzed (e.g., during anesthesia or following severe acute brain damage) or may be deprived of the capacity to make normal physical movements. Aphasia, apraxia, and cortical deafness or blindness are other possible confounders. Clinical studies have also shown how difficult it is to differentiate reflex from voluntary movements 3. Furthermore, besides the diagnostic problems (" Is the patient conscious? "); the problems of quantifying the level and content of consciousness, (" What is she/he conscious of? ") we need to tackle the prognostic challenges (" Will the patient ever recover consciousness?) ". In addition to standardized clinical evaluations, electroencephalography (EEG) recordings have been assessed as objective markers of consciousness 4. Routine clinical EEG allows the physician to monitor the brain's background electrical activity and possible seizures. To unravel physiological or pathological events, clinical EEG often involves provocative tests such as photic, auditory or painful stimulation. Reactivity of the brain to these external stimulations can be observed in unconscious brain damaged patients and has good prognosis value 5, 6. Small changes induced by sensory or cognitive activities may be observed with event related potentials (ERP). Short latency ERPs reflect activation in low-level sensory receptive structures of the brain. They have good prognostic value in comatose patients (for review see Vanhaudenhuyse et al., 2008 7). ERPs obtained after 100 ms of the presentation of a stimulus assess cognitive functions and are influenced by the level of attention and consciousness.