title: Quantifying cortical EEG responses to TMS in (un)consciousness.
abstract: We normally assess another individual's level of consciousness based on her or his ability to interact with the surrounding environment and communicate. Usually, if we observe purposeful behavior, appropriate responses to sensory inputs, and, above all, appropriate answers to questions, we can be reasonably sure that the person is conscious. However, we know that consciousness can be entirely within the brain, even in the absence of any interaction with the external world; this happens almost every night, while we dream. Yet, to this day, we lack an objective, dependable measure of the level of consciousness that is independent of processing sensory inputs and producing appropriate motor outputs. Theoretically, consciousness is thought to require the joint presence of functional integration and functional differentiation, otherwise defined as brain complexity. Here we review a series of recent studies in which Transcranial Magnetic Stimulation combined with electroencephalography (TMS/EEG) has been employed to quantify brain complexity in wakefulness and during physiological (sleep), pharmacological (anesthesia) and pathological (brain injury) loss of consciousness. These studies invariably show that the complexity of the cortical response to TMS collapses when consciousness is lost during deep sleep, anesthesia and vegetative state following severe brain injury, while it recovers when consciousness resurges in wakefulness, during dreaming, in the minimally conscious state or locked-in syndrome. The present paper will also focus on how this approach may contribute to unveiling the pathophysiology of disorders of consciousness affecting brain-injured patients. Finally, we will underline some crucial methodological aspects concerning TMS/EEG measurements of brain complexity.
authors.affiliation: Department of Biomedical and Clinical Sciences "Luigi Sacco," University of Milan, Milan, Italy.
authors.forename: Simone
authors.initials: S
authors.lastname: Sarasso
authors.forename: Mario
authors.initials: M
authors.lastname: Rosanova
authors.forename: Adenauer G
authors.initials: AG
authors.lastname: Casali
authors.forename: Silvia
authors.initials: S
authors.lastname: Casarotto
authors.forename: Matteo
authors.initials: M
authors.lastname: Fecchio
authors.forename: Melanie
authors.initials: M
authors.lastname: Boly
authors.forename: Olivia
authors.initials: O
authors.lastname: Gosseries
authors.forename: Giulio
authors.initials: G
authors.lastname: Tononi
authors.forename: Steven
authors.initials: S
authors.lastname: Laureys
authors.forename: Marcello
authors.initials: M
authors.lastname: Massimini
mesh_terms: D003243:Consciousness / Q000502:physiology*; D003244:Consciousness Disorders / Q000503:physiopathology*; D004325:Dreams / Q000502:physiology; D004569:Electroencephalography; D006801:Humans; D006993:Hypnotics and Sedatives / Q000494:pharmacology; D008874:Midazolam / Q000494:pharmacology; D020127:Recovery of Function; D012890:Sleep / Q000502:physiology; D050781:Transcranial Magnetic Stimulation; D014851:Wakefulness / Q000502:physiology
chemical_list: D006993:Hypnotics and Sedatives; D008874:Midazolam
keywords: TMS/EEG; anesthesia; bistability; brain complexity; coma; sleep