Peaks and troughs: a brain gets busy (Image: Hank Morgan/SPL)

Parts of the brain may still be alive after a person’s brain activity is said to have flatlined.

When someone is in a deep coma, their brain activity can go silent. An electroencephalogram measuring this activity may eventually show a flatline, usually taken as a sign of brain death. However, while monitoring a patient who had been placed in a deep coma to prevent seizures following a cardiac arrest, Bogdan Florea, a physician at the Regina Maria Medical Centre in Cluj-Napoca, Romania, noticed a strange thing – some tiny intermittent bursts of activity were interrupting an otherwise flatline signal, each lasting a few seconds.

He asked Florin Amzica of the University of Montreal in Canada and his colleagues to investigate what might be happening. To imitate what happened in the patient, Amzica’s team put cats into a deep coma using a high dose of anaesthesia. While EEG recordings taken from the surface of the brain – the cortex – showed a flatline, recordings from deep-brain electrodes revealed tiny bursts of activity originating in the hippocampus, responsible for memory and learning, which spread within minutes to the cortex.


“These ripples build up a synchrony that rises in a crescendo to reach a threshold where they can spread beyond the hippocampus and trigger activity in the cortex,” says Amzica.

He says these tiny ripples of activity – which he calls nu-complexes – are unlikely to show up on an EEG but may help keep the brain ticking over when someone is in a deep coma.

Florea’s patient was revived after being taken off anti-seizure medication but died a month later from heart complications.

Brain dead

The findings challenge the notion that an EEG flatline is the ultimate sign of a brain death. Legal criteria vary, but a diagnosis of brain death generally requires two examinations 24 hours apart that show irreversible and complete cessation of brain activity. “We should abandon the idea that a flat EEG proves zero neuronal activity,” says Steven Laureys who researches consciousness at the University of Liege in Belgium.

The findings do not suggest that a person who has been deemed brain-dead would be likely to wake up. But Amzica speculates that it might be beneficial to place someone with severe brain damage in this unusually deep coma state to keep the brain silent but alive while working on treating the problem.

“The results offer us a better understanding of the residual activity of the brain under coma or anaesthetic,” says Laureys. “These ripples are known to be important in sleep, but their role in coma is ill-understood, so I think this is something to be investigated further.”

Journal reference: PLoS ONE, DOI: 10.1371/journal.pone.0075257