| How can we objectively estimate a brain's ability to sustain conscious experience if the subject does not manifest intentional behavior and is unable to respond? According to some recent ideas in theoretical neuroscience, what really matters for consciousness in the brain are not neuronal firing rates, permeability to sensory inputs, activity levels or neural synchronization per se, but rather the ability of different modules of the thalamocortical system to interact causally with each other to form an integrated whole. In particular, the Information Integration Theory of Consciousness (Tononi, 2004) argues that consciousness is integrated information and that the brain should be able to generate consciousness to the extent that it has a large repertoire of available states (information), yet it cannot be decomposed into a collection of causally independent subsystems (integration). Thus, in order to evaluate a brain's capacity of consciousness, one should measure its ability to integrate information among distributed cortical regions. To this aim, it is not sufficient to observe the brain in action. Instead, one must employ a perturbational approach and examine to what extent different regions of the thalamocortical system can interact causally (integration) and produce specific responses (information). A recently developed technique, which combines transcranial magnetic stimulation and high-density electroencephalography (TMS/hd-EEG), allows approximating in the human brain these theorethical measures. Indeed, TMS/hd-EEG allows, for the first time, recording the immediate reaction of the human thalamocortical system to controlled perturbations of its different subparts. We have used sleep and anesthesia as models of unconsciousness. First, we show that TMS/hd-EEG can detect clear-cut changes in the ability of the thalamocortical system to integrate information when the level consciousness fluctuates across the sleep-wake cycle. Second, while during wakefulness the EEG responses to TMS are highly differentiated and strongly depend on the site of stimulation, during sleep and anesthesia the site-specificity is lost, as if the thalamocortical system is composed of homogeneous modules. Based on these results, we discuss the potential applications of this novel technique to evaluate objectively the brain's capacity for consciousness at the bedside of brain injured patients. |
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