The nightmare scenario for patients undergoing surgery is that they become aware but remain paralysed as they are sliced open. Anaesthesia almost always prevents this, but we don’t know how it works, nor how it affects our consciousness. What’s more, because general anaesthetic also induces amnesia it’s been difficult to investigate its effect by asking people to recount their experience.
To get around this, researchers have now infused volunteers with anaesthetics, and roused them at different points during the process to ask them what they had been experiencing. It turns out that consciousness is not completely lost during anaesthesia. Many subjects recalled having been dreaming, despite the received understanding that proper dreaming only occurs in REM sleep.
“Anaesthesia could resemble normal sleep more than we have previously thought,” says Katja Valli, a neuroscientist at the University of Skövde in Sweden.
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Moreover, words presented to people under general anaesthetic were found to have been processed by the brain, although the subjects had no memory of them afterwards. It means anaesthesia does not require complete unconsciousness to work.
Dream state
Valli and colleagues dosed 23 student volunteers with dexmedetomidine and a further 24 with propofol (the anaesthetic ill-fatedly used by Michael Jackson) until they were unresponsive. The scientists then roused the students and asked them a structured series of questions about their experiences whilst under the influence of the drug. The dose was then upped by 50 per cent. When the volunteers came round, they were asked again about their experiences.
Most of the students roused from anaesthesia reported having dreams. “I remember digging into a hole,” said one student of his dream. “At the same time I was dreaming, I had the feeling I was lying on a bed, preparing for the experiment.”
The levels of anaesthetic used in the study were lower than those used in surgery, but the experiment shows that patients anaesthetised to the point of behavioural unresponsiveness may be dreaming and susceptible to waking. The boundary between wakefulness and unresponsiveness is more fluid than we appreciated.
Disconnected consciousness
Valli says the results carry an important message. “People should not be afraid to undergo anaesthesia,” she says. This is because the dreams the volunteers had were internally generated – or “disconnected” – conscious experiences, rather than being “connected” experiences that arise as a result of interaction with the environment.
“General anaesthesia causes profound disconnectedness, meaning that the patient does not have awareness of self or the environment or the operation that is going on,” says Valli. “It means the person has no sensations or perceptions of his or her body or the external world. Thus no pain is experienced, nor can patients hear what is happening in the operating room.”
“Consciousness is not really a binary phenomenon,” says Jamie Sleigh, an anaesthesiologist at the University of Auckland, New Zealand. “There is increasing evidence that general anaesthetic drugs can preferentially disrupt very specific brain mechanisms required for perception, memory, and responsiveness.”
Since this can leave the patient in an odd dissociated state that is simultaneously both awake and asleep, Sleigh suggests we should understand the effects of anaesthesia as progressive loss of selfhood, rather than loss of consciousness.
Nick Franks, professor of biophysics and anaesthetics at Imperial College London, says the study is fascinating and provides the clearest evidence yet that the brain can process thoughts and images in a dreamlike way when we are sedated. “What makes this study different is that it shows convincingly that these internal workings of the brain most likely occurred during the anaesthetic state itself, and not during the recovery period after anaesthesia.”
British Journal of Anaesthesia
