Undetected Brain Waves?: A brain dead individual produces a "flat line" on an electroencephalogram, or EEG. However, recent research shows the potential for previously unseen brain activity in flat line and coma cases, although scientists differ on the significance of these findings. Image: Markus Spring/Flickr
What does it mean to be “brain dead”? For years one of the hallmarks has been a lack of electrical activity in the brain, which shows up on an electroencephalogram, or EEG, as a flat line. But what if inside the brain of a person who had already flat lined, there was still something going on—some murmur of electrical signals? Could the person still be considered brain dead?
These questions were thrown into relief when researchers investigated a heart-attack patient in a hospital in Romania in August 2011. The patient had lapsed into a coma, and was having seizures. Doctors placed the man on antiepileptic medication and an anesthetic. When they took an EEG recording they observed a pattern of activity they did not recognize: a series of small v-shaped signals. When they took the man off the antiepileptic after six days, his EEG readings briefly showed a flat line, also known as an isoelectric line, before returning to the pattern of activity characteristic of the coma state immediately preceding a flat line, a period called burst-suppression. Unable to interpret this phenomenon, the doctors asked a team of researchers at the Universite de Montreal for help.
To replicate the previously-unidentified v-shaped signals the Universite de Montreal researchers put 26 cats into comas by administering large doses of the anesthetic isoflurane. Once the cats registered flat EEG readings, the researchers upped the dose of isoflurane, placing the cats in deeper comas. At this point, they saw a “re-vitalization” of brain activity, characterized by sharp, low frequency EEG waves—the same activity the Romanian doctors had observed in their heart-attack patient, they say. The Montreal researchers called their discovery “v-complex” waves, partially because the waves resemble the Greek letter.
The v-complex electrical impulses most likely originated in the hippocampus, an ancient structure located in the middle of the brain thought to be involved in memory storage and consolidation. The pulses generated by the hippocampus were reverberating and rippling out to other brain structures, the researchers wrote, eventually reaching the outer cortex, which is responsible for higher-order cognitive processes like thought and language.
In a conscious, healthy brain, the cortex sends signals down to the older brain regions that govern our baser urges. The researchers hypothesize that by placing the cats in such a deep coma, they may have shut off all brain activity in the cortex, at which point the hippocampus took over, and electrical signals began to be transmitted from the bottom up rather than top down.
“Everybody thinks that the flat line is the ultimate frontier of living brain,” says Florin Amzica, a neurophysiologist at Universite de Montreal and coauthor on the paper. He suspects the waves have not been discovered before simply because no one bothered to look.
Amzica is quick to note that this brain activity does not mean that the patient is capable of cognition, however. “During a flat line, consciousness is abolished,” he says, adding that it does not make sense that consciousness would return in an even deeper coma state.
Furthermore, coma patients with significant brain damage are unlikely to move past a flat line to v-complex activity, which appears to occur only when the brain has not experienced extensive cellular death, according to Amzica. That includes most coma patients who have become unresponsive through some sort of traumatic event. For them, Amzica says, a flat line on the EEG probably represents true brain death from which they cannot return.
Amzica says he is not yet sure why the hippocampus continues to send signals to the rest of the brain, but for coma patients who have not experienced cell death, he thinks the pulses may be functioning as some sort of “neuroprotection.” Like any organ, when the brain is not used, the connections between the nerve cells atrophy. According to Amzica, v-complex pulses may serve to preserve a minimum level of brain function, mitigating the deterioration in a coma patient whose brain has gone through a prolonged period of disuse.