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They suspect the same holds true for normal, non-drug-induced sleep. Researchers led by Matt Davis of the Medical Research Council Cognition and Brain Sciences Unit in Cambridge, U.K., studied 12 volunteers under the influence of varying amounts of an anaesthetic called propofol, which induced varying levels of drowsiness.

They played them recordings of speech or other sounds, and monitored their brains using a technique called functional magnetic resonance imaging.

The volunteers’ brains were more active in response to speech than to generic noise, suggesting that they still recognized spoken words. But the part of the brain involved with the more subtle job of untangling words that can have alternative meanings depending on context or spelling (such as ‘bark,’ or ‘pear’/ ‘pair’) showed no activity in the drowsiest volunteers.

Neither did the part involved with forming memories of speech. This suggests that the brain simply shuts down higher-level aspects of speech recognition as sleep starts to set in, making it hard to remember or understand what was said in the moments before sleep.

The results appear in the journal Proceedings of the National Academy of Sciences.

The results also show that speech comprehension can suffer even when only lightly sedated, or when slightly sleepy, says anaesthetist David Menon of the University of Cambridge, who also worked on the study.

Menon and his colleagues hope that this type of work might one day help them to discover more about the degree of awareness experienced by patients in operating theatres.

Data on how many people are aware of, or have memories of their operations, are sketchy. But the overall figure is estimated at as much as 0.2 per cent of patients.

That incidence may be even higher in certain procedures in which anaesthetists err on the side of caution and administer less anaesthetic, such as Caesarean sections, heart surgeries and operations on elderly people. So researchers are keen for improved methods for gauging the level of awareness among people under anaesthetic.

“We don’t want to overdose but we want to provide a measure of how much is ‘enough’ anaesthetic,” Menon says.

Menon admits that it is difficult to know, simply by scanning the brain, exactly what the patient is experiencing.

But he hopes that more studies of lightly sedated healthy volunteers will yield accurate descriptions of their cognitive experience to go with their brain readings.

“This has to be seen as a first step, where we try and calibrate brain responses,” Menon says.

He notes that because memory seems to be impaired before other functions relating to awareness, patients may be aware of their operations but have no explicit memories of them afterward.

This could potentially lead to post-traumatic stress disorder or a worse recovery without the patient knowing why or being able to tell doctors what had happened. “They might be aware of ongoing events but we’d never know about it,” Menon says. Another area in which such work could be useful is in measuring the cognitive experience of people in vegetative or minimally conscious states.

Although these conditions feature little or no outward signs of consciousness, previous research has shown that patients can potentially achieve remarkable feats, such as mentally walking around a house or even imagining themselves playing tennis.

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