The third state of consciousness under the knife
Having a general anaesthetic is a routine part of many operations. But what happens if you are still conscious after you close your eyes?
You are in the operating theatre. Anaesthetic has been administered and the countdown to unconsciousness begins.
For most people, the next thing they will remember is being roused from a deep sleep.
But in a small minority of cases, complete unconsciousness never comes.
Being awake during surgery and not being able to do anything about it is every patient's worst nightmare.
But according to one of the country's leading anaesthetists, it may not be as simple as saying that someone is either asleep or awake.
Prof Jaideep Pandit, consultant anaesthetist at the Oxford University Hospitals, believes there is an alternative "third state" of consciousness somewhere between sleeping and waking that patients under general anaesthetic can and do experience.
"I call it dysanaesthesia," he explains. "A type of awareness where the patient is aware of the surgery but is neither conscious nor unconscious."
A third dimension of consciousness
Despite being used around the world on a daily basis, no-one is exactly sure how a general anaesthetic acts upon the body.
Anaesthesia is a complicated business requiring many years of training and a proper understanding of which drugs to give, and how much.
However in general, a patient can be expected to receive an anaesthetic to make them unconscious and a painkiller to numb the pain.
A specialist doctor will then monitor a whole host of measurements throughout the operation to make sure that the patient really is out for the count.
Even so, in an estimated 1 in 15,000 surgeries patients experience what is known as accidental awareness where they remember aspects of their surgery after waking. Around a third of those will feel pain.
The question whether someone is conscious or not is mind-bogglingly complex.
But Professor Pandit is investigating something that goes one step further - a twilight state that lies between conscious and unconscious states - and one that his experiments show happens much more frequently than accidental awareness.
He has been able to show this through the use of something known as the Isolated Forearm Technique.
Before the advent of carefully calibrated monitoring equipment, anaesthetists were able to use this technique during surgery to check whether someone was fully unconscious.
The method uses a tourniquet to prevent any muscle relaxants from paralysing the forearm. This means that if the patient becomes conscious during surgery, they can move their arm to alert the surgeon.
Professor Pandit has used the technique to show some fascinating aspects to human awareness.
In repeated tests, one third of patients who are seemingly unconscious are able to squeeze the experimenter's fingers on command using their non-paralysed forearm. But none of them will move spontaneously to show that they are awake or in pain during surgery.
"To all extents and purposes, these patients are unconscious," says Prof Pandit. "But they are clearly in a state where they can respond to some stimuli like verbal commands but not to others like the surgery, possibly because they are not distressed by it."
According to Prof Pandit, this suggests there is a third level of consciousness.
But analysing the concept remains extremely difficult.
"Mainly when patients come round they will have no recall of the event or if they do they do it's just vague memories. They're unable to remember anything with clarity," he told the BBC.
Still a mystery
One way doctors frequently explain general anaesthesia to patients is to compare it to being asleep. But that is technically inaccurate says Dr Emory Brown, a professor of anaesthesia at Harvard Medical School.
"What we need to do to safely and humanely operate on you - to perform a procedure which is indeed, very invasive and certainly traumatic - is to put you in a state which is a coma that we can readily reverse," he told the BBC.
This coma-like condition potentially protects the patient from pain and memory of the surgery. But it also helps the surgeons to operate by keeping the patient still and maintaining bodily functions at a stable level.
However, what's happening in the brain as it slips from conscious into unconsciousness is still uncertain.
In 2011, a research team at Manchester University were able to watch for the first time the effects on a human brain as it lost consciousness under anaesthetic.
Using a novel method of brain imaging, they were able to construct real-time 3D images of the brain - rather than the 2D slices of standard brain scans.
It enabled them to see how electrical activity in the brain changed as the patients went under.
Interestingly, activity in the brain appeared to hot up as the patient became unconscious. This suggests that rather than switching off, the brain works hard to dampen down or inhibit consciousness while they are under the influence of anaesthetic.
But scientists are still a long way off from understanding the impact on the brain as unconsciousness occurs.
Pain and Trauma
"We are still unable at this time to define what human consciousness is, so trying to find a box of tricks that's going to monitor its absence is difficult," says Dr Kevin Fong, a consultant in anaesthesia at University College London Hospitals.
"But awareness during anaesthesia is something we've learnt to guard against through layers and layers of monitoring. We check surrogates of awareness such as heart rate, blood pressure and rates of respiration, and also constantly monitor the exact concentration of drugs in the blood."
Indeed having memory of your surgery is one of the most important complications that both anaesthetists and patients wish to avoid.
Although a national survey this year found the risk of waking up under the knife was extremely low, the effects for patients who do experience it can be acutely traumatic.
For Prof Pandit, the ultimate aim of understanding human unconsciousness is to prevent such episodes from occurring.
"The risk of experiencing pain or distress during accidental awareness is very low at around 1 in 45,000," he explains. "But when it does happen it can have a very bad effect on the individual."
"So it's not just the numbers that are driving us to research dysanaesthesia, but also the impact."