The spectral exponent of spontaneous EEG is linked to the presence/absence of consciousness across multiple anesthetics, in sleep stages and in disorders of consciousness.

Michele Colombo
Università degli Studi di Milano, Dipartimento di Scienze Biomediche e Cliniche "L.Sacco"


Despite the absence of responsiveness, a person can retain conscious experience, during sleep, anesthesia and the recovery from coma. the presence of consciousness can usually be inferred from behavior, but fail in the cases where sensorimotor connectedness to the environment is lost. It is desirable to develop an objective marker for the presence of conscious experience, that does not depend on behavior. However, reliable, easily acquirable and interpretable neurophysiological markers of the presence of consciousness in unresponsive states are still missing.

A promising marker is based on the decay-rate of the power spectral density (PSD) of the resting EEG. In a recent study, the spectral exponent of the resting EEG discriminated states in which consciousness was present (wakefulness, ketamine) from states where consciousness was reduced or abolished (xenon, propofol). Loss of consciousness substantially decreased the (negative) broad-band spectral exponent in each subject undergoing xenon or propofol anesthesia—indexing an overall steeper PSD decay. Consistent preliminary results in the spectral exponent were obtained assessing sleep depth and in patients with disorders of consciousness.
The spectral exponent has been linked to the ratio between excitation and inhibition in synaptic signaling, and could potentially signal the presence of critical dynamics in the brain. In this seminar, we review the evidence, collected by the author and others, that the loss of critical dynamics in the brain is accompanied by the loss of consciousness.