We know that conscious experience can be entirely generated within the brain as in the case, for instance, when we dream and we are completely disconnected from the external world. However, we normally assess another individual's level of consciousness solely based on her/his ability to interact with the external environment. This discrepancy becomes particularly relevant in the case of patients who survive severe brain injuries, who may recover consciousness without recovering their ability to understand, move and communicate.
Thanks to a collaboration between the group of Marcello Massimini from the University of Milan, Italy, and the group of Steven Laureys from the University of Liège, Belgium, the work published in Brain shows that measuring the level of internal brain communication allows single-subject discrimination between vegetative state (VS) patients and patients who recover a minimal level of consciousness (MCS). Crucially, this can be obtained at the bedside and does not rely on the integrity of sensory and motor pathways nor on the subject's ability to comprehend or carry out instructions. This finding is relevant because distinguishing between VS and MCS patients can be extremely challenging, leading to a diagnostic error up to 40%1.
To achieve these results, the two research groups have employed a novel approach based on the combination of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) at the bedside of 17 brain-injured patients who evolved from coma into different clinical states. TMS/EEG allows measuring directly and non-invasively the internal brain communication, a theoretical requirement for consciousness to emerge. As demonstrated by previous experiments, this approach readily distinguishes between conditions in which consciousness is present (alert wakefulness, dreaming)2 and conditions in which consciousness is reduced, or lost (sleep and anesthesia)3-4.
In VS patients, behaviorally awake with their eyes open, but unresponsive, TMS/EEG revealed a breakdown of the internal brain communication, similar to the one previously observed in sleep or anesthesia; by contrast, in MCS patients, who recovered a minimal level of consciousness, TMS invariably revealed a preserved, effective communication between brain areas, irrespectively of the individual patient's ability to communicate.
These findings suggest that "knocking" directly on the brain (with TMS) to assess its capacity for internal communication (with EEG) may represent an effective way to track the neural correlates of recovery of consciousness in brain-injured, non-communicating patients.
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