Watching Speech Comprehension Fade As The Brain Falls Asleep, UK

Main Category: Neurology / Neuroscience
Also Included In: MRI / PET / Ultrasound
Article Date: 11 Oct 2007 - 1:00 PDT

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Scientists in Cambridge have used brain imaging to show that sedation reduces activity in the brain regions critical for forming memories and understanding the meaning of sentences. Although the brain can process sounds of speech when heavily sedated, it appears not to fully comprehend or remember it. This discovery has implications for the care of patients undergoing general anaesthesia or emerging from a coma, and for understanding the relationship between speech comprehension and awareness in the healthy brain.

The finding comes from collaborative research conducted in Cambridge by the Medical Research Council Cognition and Brain Sciences Unit (MRC CBSU), the Impaired Consciousness Research Group, and the Division of Anaesthesia at the University of Cambridge. These studies were part of the research programme of the British Oxygen Professorship, funded by the Royal College of Anaesthetists. The full results are published in the Proceedings of the National Academy of Sciences. Researchers from UCL (University College London) and Queen's University, Ontario also contributed.

Dr Matt Davis, a cognitive neuroscientist at the MRC CBSU, and lead author on the paper, said:

''Everyone knows that if you have a conversation with someone as they are falling asleep they might not answer or have any memory of what you said. However, it's hard to know whether this is because drowsy people don't understand speech or just aren't awake enough to respond or remember. It's a similar situation when people are sedated or unconscious. We asked the question: do people have some hidden understanding of speech when they are at a low level of awareness?''

Using a scanning technique called functional magnetic resonance imaging (fMRI) that maps brain activity, Dr Davis and his colleagues mapped speech-related brain activity in volunteers at varying levels of sedation. Their aim was to show how the brain's response to speech changes as people become more sedated and whether understanding of speech might continue, even while consciousness and memory is impaired. The scanning took place at the Wolfson Brain Imaging Centre in Cambridge.

The 12 volunteers, all anaesthetists, were scanned before and during sedation with propofol, a medicine commonly used to anaesthetise patients for surgery. Responses to sentences and to non-speech noises were assessed at three levels of awareness: fully awake, lightly sedated and heavily sedated. The heavy state didn't go as far as general anaesthesia and all volunteers showed some response to the sound of speech, for instance, waking up when their name was called.

Dr Davis and colleagues found that the more sedated the volunteers became, the less active their brains were in response to speech. Even at light levels of sedation, they saw no activity in brain areas that are normally used to comprehend sentences containing words with more than one meaning (ambiguous words like bark, or rain/reign). This suggested that despite being able to respond to simple questions, volunteers could no longer fully understand speech. When volunteers are heavily sedated, brain areas involved in conscious awareness of speech and storing sentences in memory became inactive.

Overall the fMRI scans revealed that although the brain can process speech even when heavily sedated, it may not fully comprehend or remember it. Dr Davis explained:

''The difference in brain activity shown on the fMRI scans demonstrates that higher level functions, like interpreting a sentence containing ambiguous words or forming a memory of a sentence, rely on conscious awareness. Lower level functions involved in hearing speech sounds remain intact but later steps, critical for fully understanding and remembering what was said, don't happen. The brain simply doesn't react in the same way under sedation.''

Professor David Menon, British Oxygen Professor and Professor of Anaesthesia at the University of Cambridge, pointed out that the research has important parallels in two clinical situations:

"First, 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.''

''Overall our research shows that a high level of awareness is required to comprehend sentences containing ambiguous words like rain or reign. Brain-injured patients that show an additional response to these sentences are therefore likely to be at a high-level of awareness,'' he continued.

This research into awareness and understanding is one branch of numerous studies using fMRI to analyse how the brain works not only at different levels of consciousness but in healthy people too. As such the results will contribute to interpretation of other research studies involving people who are in a vegetative state or who have suffered an injury that has left them unable to communicate.

Original research paper: Dissociating speech perception and comprehension at reduced levels of awareness is published in the Proceedings of the National Academy of Sciences.

Medical Research Council