A new study suggests that although both the EEG and behaviour of a rat indicate wakefulness, local populations of neurons in the cortex may be falling asleep, with negative consequences for performance.

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PARIS: The way sleep deprivation affects brain and cognitive function may be due to individual neurons falling asleep while the body is still awake, according a new study.

The results of an experiment using rats suggest that if a brain is tired, parts of it can fall asleep for a fraction of a second - or go ‘off-line’ - even though the organ is ostensibly awake at the time. The implications are far-reaching, especially for people doing tasks where lack of sleep could be dangerous, say its authors.

"Even before you feel fatigued, there are signs in the brain that you should stop certain activities that may require alertness," said Chiara Cirelli, a professor of psychiatry at the University of Wisconsin in Madison. "Specific groups of neurons may be falling asleep, with negative consequences on performance."

Summarising neuron activity

The investigation, published in Nature this week, challenges conventional belief that sleep deprivation affects the entire brain.

This theory is underpinned by evidence from electro-encephalograms, or EEGs, which show patterns of electrical activity by brain cells, also called neurons. EEGs, though, have limitations.

Their electrodes are placed on the scalp, which means they are better at picking up signals close to the skull rather than signals from deep within the cranium. And they essentially summarise activity by hundreds of millions of neurons and so cannot pinpoint individual cells.

Awake but off-line

Cirelli and colleagues inserted ultra-fine probes into the brains of 11 adult rats to monitor electrical activity in sub-groups of neurons in the motor cortex, which is in charge of semi-automatic 'motor' movements.

The rodents were then kept up four hours beyond their usual bedtime, thanks to novel objects that were introduced into their cage to keep them awake.

The monitors showed that even when the animals were to all outward appearances awake and active, neurons in these specific areas did not fire - in other words, parts of the brain were in 'local sleep' even when others were awake.

"Even when some neurons went off-line, the overall EEG measurements of the brain indicated wakefulness," said Cirelli.

Episodes of 'local sleep' affected the rats' behaviour, the scientists found.