Research into the brain's response to speech when under sedation has revealed reduced activity in areas critical for memory and understanding language.
Sedated volunteers' scans showed they could not process speech
Cambridge University scientists used brain imaging to find evidence which may influence the amount of anaesthetic given to patients undergoing surgery.
It may also affect attitudes to patients in a coma or vegetative state.
Researchers said: "The brain processes speech when sedated but it appears not to fully comprehend or remember it."
Using a scanning technique called functional magnetic resonance imaging (fMRI) that registers brain activity, Dr Matt Davis, a cognitive neuroscientist at Cambridge University, and his colleagues mapped speech-related brain activity in volunteers at varying levels of sedation.
'Return of consciousness'
Their aim was to show how the brain's response to speech changed as people became more sedated and whether understanding of speech might continue even while consciousness and memory were impaired.
Professor David Menon, professor of anaesthesia at the University of Cambridge, said the research has important parallels in two clinical situations.
"A small proportion of anaesthetised patients report memories of events that occurred in the operating theatre, implying an inadvertent return of consciousness.
"It is possible that even more patients may have some awareness of events during anaesthesia, but this may fail to be detected because patients have no memories of the event afterwards.
"This implies that these patients, although unable to respond, are not sufficiently anaesthetised. There are good clinical reasons to use only as much anaesthetic as is needed, since this increases patient safety.
"However, this needs to be balanced against the risk of inadvertent return of consciousness during general anaesthesia.
"Our research will help develop techniques to measure how deep anaesthesia needs to be to prevent awareness.
"Secondly, there is an emerging consensus that some severely brain-injured patients in a coma or vegetative state might understand but not be able to respond to speech."