A theoretically based index of consciousness independent of sensory processing and behavior.
One challenging aspect of the clinical assessment of brain-injured, unresponsive patients is the lack of an objective measure of consciousness that is independent of the subject's ability to interact with the external environment. Theoretical considerations suggest that consciousness depends on the brain's ability to support complex activity patterns that are, at once, distributed among interacting cortical areas (integrated) and differentiated in space and time (information-rich). We introduce and test a theory-driven index of the level of consciousness called the perturbational complexity index (PCI). PCI is calculated by (i) perturbing the cortex with transcranial magnetic stimulation (TMS) to engage distributed interactions in the brain (integration) and (ii) compressing the spatiotemporal pattern of these electrocortical responses to measure their algorithmic complexity (information). We test PCI on a large data set of TMS-evoked potentials recorded in healthy subjects during wakefulness, dreaming, nonrapid eye movement sleep, and different levels of sedation induced by anesthetic agents (midazolam, xenon, and propofol), as well as in patients who had emerged from coma (vegetative state, minimally conscious state, and locked-in syndrome). PCI reliably discriminated the level of consciousness in single individuals during wakefulness, sleep, and anesthesia, as well as in patients who had emerged from coma and recovered a minimal level of consciousness. PCI can potentially be used for objective determination of the level of consciousness at the bedside.
Department of Biomedical and Clinical Sciences 'Luigi Sacco', University of Milan, 20157 Milan, Italy.
Adenauer G
AG
Casali
Olivia
O
Gosseries
Mario
M
Rosanova
Mélanie
M
Boly
Simone
S
Sarasso
Karina R
KR
Casali
Silvia
S
Casarotto
Marie-Aurélie
MA
Bruno
Steven
S
Laureys
Giulio
G
Tononi
Marcello
M
Massimini
D000777:Anesthetics / Q000494:pharmacology; D001921:Brain / Q000187:drug effects / Q000502:physiology*; D003243:Consciousness / Q000187:drug effects / Q000502:physiology*; D005071:Evoked Potentials / Q000187:drug effects; D005260:Female; D006801:Humans; D008297:Male; D008874:Midazolam / Q000494:pharmacology; D015742:Propofol / Q000494:pharmacology; D050781:Transcranial Magnetic Stimulation; D014978:Xenon / Q000494:pharmacology
D000777:Anesthetics; D014978:Xenon; D008874:Midazolam; D015742:Propofol