Although unable to move and respond, some of the patients in a vegetative state following severe brain injury are able to carry out tasks such as imagining playing a game of tennis.
Using a functional magnetic resonance imaging (fMRI) scanner, which measures brain activity, researchers have previously recorded activity in the pre-motor cortex, the part of the brain which deals with movement, in apparently unconscious patients asked to imagine playing tennis.
Now, a team of researchers led by scientists at the University of Cambridge and the Medical Research Council Cognition and Brain Sciences Unit, Cambridge, have used high-density electroencephalographs (EEG) and a branch of mathematics known as 'graph theory' to study networks of activity in the brains of 32 patients diagnosed as vegetative and minimally conscious and compare them to healthy adults.
The researchers showed that the rich and diversely connected networks that support awareness in the healthy brain are typically - but importantly, not always impaired in patients in a vegetative state.
Some vegetative patients had well-preserved brain networks that look similar to those of healthy adults these patients were those who had shown signs of hidden awareness by following commands such as imagining playing tennis.
"Our research could improve clinical assessment and help identify patients who might be covertly aware despite being uncommunicative," said Dr Srivas Chennu from the Department of Clinical Neurosciences at the University of Cambridge.
Researchers could develop a relatively simple way of identifying which patients might be aware whilst in a vegetative state.
Unlike the 'tennis test', which can be a difficult task for patients and requires expensive and often unavailable fMRI scanners, this new technique uses EEG and could therefore be administered at a patient's bedside.
However, the tennis test is stronger evidence that the patient is indeed conscious, to the extent that they can follow commands using their thoughts.     

"Although there are limitations to how predictive our test would be used in isolation, combined with other tests it could help in the clinical assessment of patients," said Dr Tristan Bekinschtein from the MRC Cognition and Brain Sciences Unit and the Department of Psychology, University of Cambridge.
"If a patient's 'awareness' networks are intact, then we know that they are likely to be aware of what is going on around them. But unfortunately, they also suggest that vegetative patients with severely impaired networks at rest are unlikely to show any signs of consciousness," Bekinschtein said.
The findings of the research are published in the journal PLOS Computational Biology.

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