Abstract
Recent studies combining TMS and EEG during Vegetative State (VS) show that a cortical perturbation produces a positive–negative wave that closely resembles spontaneous NREM sleep slow waves. Sleep slow waves reflect intracellular oscillations between periods of depolarization (intense firing – upstate) and hyperpolarization (neuronal silence – downstate) revealing intrinsic bistability in thalamo-cortical networks during NREM sleep. Interestingly, a downstate is detected – at scalp level – with a high frequency power decrease concomitant with the negative deflection of the EEG voltage. To show that slow waves evoked by TMS during pathological loss of consciousness (LOC) reflect a cortical downstate characterized by EEG high-frequency suppression, we recorded TMS/EEG responses in 10 brain-injured VS patients, and we compared these responses to those of 2 locked-in patients (LIS) and 3 healthy awake subjects. Unlike conscious subjects, TMS/EEG responses in VS are characterized by large slow waves, with significant suppression of high frequency EEG power (>13 Hz) concomitant with the negative deflection, thus reflecting a cortical downstate. These data suggest that pathological LOC is characterized by a cortical downstate, which could possibly disrupt fast long-range interactions between different thalamocortical modules, a key requirement for consciousness.
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