Severe brain injuries can result in Disorders of Consciousness (DoC), whereby consciousness may be retained, despite widely different limitations of the behavioural repertoire. Objective, mechanistically interpretable neurophysiological markers of consciousness are thus needed to help overcome the limitations of behavioural assessments of DoC following severe brain injuries. Severe brain injuries, by removing thalamocortical afferent connectivity, may reduce excitatory inputs to cortical regions spared from the injury, thereby inducing slower activity. Broad-band slowing, measured by the spectral exponent, has been previously linked to lower excitation/inhibition ratio (E/I) of cortical activity. We thus hypothesized that the EEG spectral exponent may index the presence of consciousness, following severe focal/multifocal brain injury.
We validate our spontaneous-EEG findings considering behavioural responsiveness (when present) and, during unresponsiveness, an independently-established consciousness marker, based on the complexity of EEG activity evoked Transcranial Magnetic Stimulation (TMS), the Perturbational Complexity Index (PCI). We further investigate the relationship between these two spontaneous and evoked markers.
The findings obtained in DoC patients (n=59) of traumatic and vascular aetiology, were moreover validated in a Reference dataset (n=65), of known neurotypical, neurological and anesthesia conditions.
The spectral exponent robustly stratified patients and discriminated un/consciousness among traumatic and vascular DoC across un/responsive conditions, and optimally generalized to the reference dataset. Further, it predicted PCI values, suggesting that a single mechanism may underlie broad-band slowing, lower complexity of TMS-evoked cortical activity, and loss of consciousness. Lower E/I may constitute such mechanism, by favoring highly integrated yet differentiated activity, a theoretical requirement for achieving both high PCI and the emergence consciousness. More generally, our empirical results support theories that predict the emergence of consciousness near a critical transition between activity propagation and extinction.