A greater scientific understanding of consciousness is allowing researchers to quantify exactly how conscious we are at any given moment, and the resulting measurements are providing new insight into the likelihood of coma patients to recover.
While consciousness has long been the subject of metaphysical debates, it is only recently that scientists are beginning to develop a picture of how it manifests in our brains.
One of the pioneers of this new field is Stanislas Dehaene, professor of experimental psychology at the College de France in Paris. He and his colleagues were the first to show that the cortex - the topmost brain region associated with high-level functions and thought - is activated by words that aren't consciously perceived.
Researchers have shown that a surprising amount happens in the brain without conscious awareness. Numerical values, meanings, grammatical information, and more, can all be processed non-consciously.
Over the course of a five-year project called NEUROCONSC, funded by the EU's European Research Council (ERC), Prof. Dehaene and colleagues have been exploring the limits of this non-conscious processing and developing a theory of what consciousness is.
A phenomenon called masking is the most common technique used for studying consciousness. If a word is briefly flashed in front of someone's eyes, they can easily read it, but if it is immediately overlaid by a string of letters, they don't perceive it, and can't report what it was.
Researchers can vary the delay between the word and mask to find the threshold of conscious awareness, where the word is perceived half the time. Other techniques include binocular rivalry, where separate images are presented to each eye but participants are conscious of only one, and various methods of manipulating attention.
Using these techniques, Prof. Dehaene and colleagues found that when a participant becomes able to report, say, seeing a word, they can suddenly do a wide range of new things with the information, including naming and remembering it.
This seems to be because information is suddenly shared throughout the brain. When a participant becomes consciously aware of something, the initial neural response is amplified many-fold, and suddenly spreads, as synchronised activity, through a wide network of regions, including many parts of the cortex. Prof. Dehaene calls this the Global Neuronal Workspace.
Now, Prof. Dehaene and colleagues, including Dr Jacobo Sitt at the French Institute of Health and Medical Research (INSERM), have used this insight to develop ways of measuring consciousness. For instance, one measure involves quantifying the degree to which information is shared across distant brain regions.
They have shown that this measure can distinguish between electroencephalography (EEG) recordings of brain activity from patients who were conscious, in a vegetative state, or minimally conscious (where patients are occasionally able to respond to simple commands).
'We found huge differences between patients that were in vegetative state - so unconscious - and patients that were in a minimally conscious state,' said Dr Sitt. Patients with locked-in syndrome, who are fully aware but unable to communicate, register as conscious.
Now, under another ERC-funded project, the team have developed a system, called CoMonIn (Consciousness Monitoring Index) for automatically monitoring consciousness. This includes an offline mode, where EEG recordings are uploaded to a web server, and a real-time version, using a laptop and EEG head-set, which can be used by the bedside.
This version can deliver appropriate audio, video, or even electrical, stimulation when higher levels of consciousness are detected, to try to increase the chances of patients regaining consciousness.
Most recently, they have used machine-learning techniques and a database of EEG recordings to predict who goes on to recover from coma-like states, with around 77% accuracy. They are now looking for commercial partners to take their system to market.
Meanwhile, other researchers are studying altered states of consciousness through a natural long-lasting window that is available at the end of every day.
'We are ignoring the fact that every night we go to sleep and become unconscious of our external environment,' said Dr Hagar Sagiv from Tel Aviv University, Israel. 'We see sleep as another lens through which to look at consciousness.'
Dr Sagiv is a researcher in the lab of Dr Yuval Nir, whose project has been funded by the EU. She is investigating exactly how the brain disconnects the senses during sleep by comparing the brain's activity in wakefulness and REM sleep.
She is particularly interested in REM sleep because of its similarity to waking in terms of metabolism and activity. REM sleep is when we dream, so there are also experiences of a sort, but of course sleepers remain disconnected from external events.
There are reasons to suspect a neurotransmitter called noradrenaline may be involved, partly because the locus coeruleus (the part of the brainstem that produces noradrenaline), is inactive during sleep.
To test this, Dr Sagiv is conducting experiments where participants are given drugs that alter levels of noradrenaline in the brain during wakefulness, to see whether this is sufficient to mimic sleep and change their sensitivity to sensory input. Parallel studies are using cellular recordings in animals, together with a cutting-edge technique for communicating with neurons, called optogenetics, to reveal the underlying mechanisms.
While the project could have similar practical implications as other studies of consciousness, there is also a profound basic science aspect to studying consciousness at such basic biological levels.
'What's special about this is we're trying to explore almost philosophical phenomena, in terms of molecules in the brain,' said Dr Sagiv. 'This is one of the biggest challenges we have as scientists, to understand the biological basis of consciousness - twenty years ago it was a question for philosophers, not scientists.'