Surgically severing part of the brain can send the area into a deep sleep.Credit: Peter Cade/Getty
Slow, sleep-like brain waves persist in part of the brain that has been surgically disconnected from the rest of the organ even though the person is awake. The findings1, published in PLoS Biology, add to researchers’ understanding of what conscious and unconscious brain states look like.
Children with severe epilepsy who do not respond to medication can undergo a surgical procedure called a hemispherotomy. During surgery, clinicians disconnect the part of the brain in which seizures originate from the rest of the brain, stopping them from spreading. The disconnected tissue is left in the skull and has an intact blood supply.
The team wanted to find out whether the disconnected part has some form of awareness — or was capable of exhibiting consciousness, says co-author Marcello Massimini, a neurophysiology researcher at the University of Milan in Italy. “The question arises because we have no access” to the disconnected region, he says, adding that it was unclear what happens once part of the brain is isolated.
Studies investigating consciousness are difficult because there is no consensus on what conscious and unconscious states in the brain look like, says Ariel Zeleznikow-Johnston, a neuroscientist at Monash University in Melbourne, Australia. “There’s no generally accepted definitive signatures of consciousness in terms of electrical readings or brain activity,” he adds.
Even defining unconsciousness is challenging, because activities associated with consciousness, such as remembering dreams, can occur during states associated with unconsciousness, such as sleep or anaesthesia, Massimini says.
Asleep or awake
Researchers in Italy, Australia and the United Kingdom examined electroencephalograms (EEGs) — a test that measures electrical activity in the brain — from ten awake children before they had a hemispherotomy and between six months and three years after the procedure. They compared the recordings with a database of EEGs from children with typical brains that were taken while they were awake or asleep.
They found that the electrical activity slowed in the disconnected region after surgery, whereas activity in the intact brain did not change. The EEG of the intact region was also similar to EEGs from control children who were awake. After surgery, the disconnected region became dominated by slow rhythms called delta waves and was similar to EEGs taken while control children were in deep sleep.
The slow pattern persists for months or years after surgery. Massimini says this finding raises questions about its function. The pattern has also been observed in injured parts of the brain among awake people who have had a stroke or traumatic brain injury. It could be protective, involved in maintaining a stable environment in the brain or helping the brain adapt to injury, he says.
