A new study reveals what happens inside the brain as patients lose consciousness during anesthesia. By monitoring brain activity as patients were given a common anesthetic, the researchers were able to identify a distinctive brain activity pattern that marked the loss of consciousness. This pattern, characterized by very slow oscillation, corresponds to a breakdown of communication between different brain regions, each of which experiences short bursts of activity interrupted by longer silences.
“Within a small area, things can look pretty normal, but because of this periodic silencing, everything gets interrupted every few hundred milliseconds, and that prevents any communication,” said Laura Lewis, one of the lead authors of a paper describing the findings.
For this study, the researchers used propofol, one of the most common anesthesia drugs. Propofol activates receptors found on neurons that are likely to make the neurons less active. The researchers studied epileptic patients, who had electrodes implanted in their brains to monitor their seizures and were undergoing surgery to have the electrodes removed. Loss of consciousness occurred within 40 seconds of propofol administration, and was defined by the moment when patients stopped responding to sounds that were played every four seconds.
Using two different-sized electrodes, the researchers were able to obtain two different readings of brain activity. The larger electrodes were spaced about a centimeter apart and recorded the overall EEG brain-wave pattern. Smaller electrodes, in an array 4 millimeters wide, were clustered in different regions and recorded from individual neurons. From the large electrodes the researchers observed that within a couple of seconds of losing consciousness, the brain EEG abruptly took on a pattern of low-frequency oscillation, about one cycle per second. At the same time, the electrodes recording from individual neurons revealed that within localized brain regions, neurons were active for a few hundred milliseconds, then shut off again for a few hundred milliseconds. This flickering of activity created the slow oscillation seen in the EEG. “When one area was active, it was likely that another brain area that it was trying to communicate with was not active. Even when the neurons were on, they still couldn’t send information to other brain regions,” Lewis said.
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