The potential of nTMS/EEG: Measuring consciousness

Abstract

Measuring electroencephalographic responses to nTMS/EEG harbors a huge potential to investigate cortico-cortical connectivity. In the present chapter, we address one major application of nTMS/EEG in neurosurgery: the problem of objectively estimating the brain's capacity for consciousness in severely brain-injured patients who show little or no interaction with the outer world. Recently, a theory-inspired measure (perturbational complexity index-PCI) computed from nTMS/EEG has been developed to gauge the ability of thalamocortical circuits to integrate information, irrespectively of the integrity of sensory processing, motor behavior, and subject participation. The validation of PCI has implied its computation on a large benchmark population in order to derive an empirical cutoff (PCI*) able to distinguish between consciousness and unconsciousness as assessed through subjective reports. Notably, this benchmark population deliberately included conscious brain-injured patients and healthy subjects who were unresponsive during the recordings but retrospectively reported having had vivid conscious experiences upon awakening from dreaming and ketamine anesthesia (disconnected consciousness). Then, PCI has been demonstrated to accurately detect minimally conscious state (MCS) patients who may show fluctuating but unequivocal behavioral signs of consciousness. The extremely high sensitivity of PCI prompts its application to further stratify vegetative state (VS) patients, who are seemingly characterized by behavioral unresponsiveness but might actually show different potentiality for consciousness as measured from the capacity to integrate information within their brain. In order to demonstrate the use of nTMS/EEG in neurosurgery, the present chapter therefore addresses the detection of consciousness in severely brain-injured patients by nTMS/EEG as one of its major application in neurosurgery.