Probing consciousness with event-related potentials in the vegetative state

Abstract

Objective: Probing consciousness in noncommunicating patients is a major medical and neuroscientific challenge. While standardized and expert behavioral assessment of patients constitutes a mandatory step, this clinical evaluation stage is often difficult and doubtful, and calls for complementary measures which may overcome its inherent limitations. Several functional brain imaging methods are currently being developed within this perspective, including fMRI and cognitive event-related potentials (ERPs). We recently designed an original rule extraction ERP test that is positive only in subjects who are conscious of the long-term regularity of auditory stimuli.

Methods: In the present work, we report the results of this test in a population of 22 patients who met clinical criteria for vegetative state.

Results: We identified 2 patients showing this neural signature of consciousness. Interestingly, these 2 patients showed unequivocal clinical signs of consciousness within the 3 to 4 days following ERP recording.

Conclusions: Taken together, these results strengthen the relevance of bedside neurophysiological tools to improve diagnosis of consciousness in noncommunicating patients.

Figure. Test design and illustration of bedside recording in intensive care unit (ICU)

(A) Bedside recording in ICU. Photography of the recording setting in a patient in the ICU (with the patient's permission). Installation of the net and EEG calibration requires about 15 minutes. Earphones are then applied, task instruction delivered, and EEG recording starts. (B) Auditory paradigm. On each trial 5 sounds were presented. Each block started with 20–30 frequent series of sounds to establish the long-term regularity before delivering the first infrequent rule deviant stimulus. (C) Three representative results. Global field power of rule standard (green) and rule deviant (red) trials are plotted for one conscious control subject (C.a), for a patient with a rule violation effect (C.b), and for a patient without rule violation effect (C.c). Early peaks to each of the 5 sounds (S1 to S5) are indicated for the control subject. Statistical significance of event-related potential (ERP) differences within the time window of the rule violation effect is indicated by a color code on the X axis. Voltage topography maps averaged across time windows of significant ERP effects are displayed on the right. Panel C is reprinted from Bekinschtein et al.