Measuring Consciousness | The Scientist Magazine® - 0 views
www.the-scientist.com/?articles.view/articleNo/35140/title/Measuring-Consciousness/
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General anesthesia has transformed surgery from a ghastly ordeal to a procedure in which the patient feels no pain.
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“integrated-information theory,” which holds that consciousness relies on communication between different brain areas, and fades as that communication breaks down.
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neural markers of consciousness—or more precisely, the loss of consciousness—a group led by Patrick Purdon
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The purpose of the surgery was to remove electrodes that had previously been implanted in the patients’ brains to monitor seizures. But before they were taken out, the electrodes enabled the researchers to study the activity of individual neurons in the cortex, in addition to large-scale brain activity from EEG recordings.
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importance of communication between discrete groups of neurons, both within the cortex and across brain regions, is analogous to a band playing music, said George Mashour, a neuroscientist and anesthesiologist at the University of Michigan, Ann Arbor. “You need musical information to come together either in time or space to really make sense,”
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“Consciousness and cognitive activity may be similar. If different areas of the brain aren’t in synch or if a critical area that normally integrates cognitive activity isn’t functioning, you could be rendered unconscious.”
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, Purdon and colleagues were able to discern a more detailed neural signature of loss of unconsciousness, this time by using EEG alone. Monitoring brain activity in healthy patients for 2 hours as they underwent propofol-induced anesthesia, they observed that as responsiveness fades, high-frequency brain waves (12–35 hertz) rippling across the cortex and the thalamus were replaced by two different brain waves superimposed on top on one another: a low-frequency (<1 hertz) wave and an alpha frequency (8–12 hertz) wave. “These two waves pretty much come at loss of consciousness,”
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o measure the gradual breakdown of connectivity between neural networks during natural REM sleep and anesthesia, as well as in brain-injured, unresponsive patients. Using an electromagnetic coil to activate neurons in a small patch of the human cortex, then recording EEG output to track the propagation of those signals to other neuronal groups, the researchers can measure the connectivity between collections of neurons in the cortex and other brain regions.
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minimally conscious patients, the magnetically stimulated signals propagated fairly far and wide, occasionally reaching distant cortical areas, much like activations seen in locked-in but conscious patients. In patients in a persistent vegetative state, on the other hand, propagation was severely limited—a breakdown of connectivity similar to that observed in previous tests of anesthetized patients. What’s more, in three vegetative patients that later recovered consciousness, the test picked up signs of increased connectivity before clinical signs of improvement became evident.
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“I think understanding consciousness itself is going to help us find successful [measurement] approaches that are universally applicable,” said Pearce.