‘Hidden brain signatures’ of consciousness in vegetative state patients discovered

October 21, 2014

Brain networks in two behaviorally similar vegetative patients (left and middle), but one of whom imagined playing tennis (middle panel), alongside a healthy adult (right panel) (credit: Srivas Chennu)

Scientists in Cambridge, England have found hidden signatures in the brains of people in a vegetative state that point to networks that could support consciousness — even when a patient appears to be unconscious and unresponsive. The study could help doctors identify patients who are aware despite being unable to communicate.

Although unable to move and respond, some patients in a vegetative state are able to carry out tasks such as imagining playing a game of tennis, the scientists note. Using a functional magnetic resonance imaging (fMRI) scanner, researchers have previously been able to record activity in the pre-motor cortex, the part of the brain that 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 MRC Cognition and Brain Sciences Unit, Cambridge, have used high-density electroencephalographs (EEG) and 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 connectome — the rich and diversely connected networks that support awareness in the healthy brain — are typically impaired in patients in a vegetative state. But they also found that 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.

Identifying patients who are aware

The findings could help researchers develop a relatively simple way of identifying which patients might be aware while in a vegetative state. The “tennis test” can be a difficult task for patients and requires expensive and often unavailable fMRI scanners. The new technique uses EEG, so it could 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. The researchers believe that a combination of such tests could help improve accuracy in the prognosis for a patient.

The research findings were published in the journal PLOS Computational Biology (open access). The study was funded mainly by the Wellcome Trust, the National Institute of Health Research Cambridge Biomedical Research Centre and the Medical Research Council (MRC).


Abstract of Spectral Signatures of Reorganised Brain Networks in Disorders of Consciousness

Theoretical advances in the science of consciousness have proposed that it is concomitant with balanced cortical integration and differentiation, enabled by efficient networks of information transfer across multiple scales. Here, we apply graph theory to compare key signatures of such networks in high-density electroencephalographic data from 32 patients with chronic disorders of consciousness, against normative data from healthy controls. Based on connectivity within canonical frequency bands, we found that patient networks had reduced local and global efficiency, and fewer hubs in the alpha band. We devised a novel topographical metric, termed modular span, which showed that the alpha network modules in patients were also spatially circumscribed, lacking the structured long-distance interactions commonly observed in the healthy controls. Importantly however, these differences between graph-theoretic metrics were partially reversed in delta and theta band networks, which were also significantly more similar to each other in patients than controls. Going further, we found that metrics of alpha network efficiency also correlated with the degree of behavioural awareness. Intriguingly, some patients in behaviourally unresponsive vegetative states who demonstrated evidence of covert awareness with functional neuroimaging stood out from this trend: they had alpha networks that were remarkably well preserved and similar to those observed in the controls. Taken together, our findings inform current understanding of disorders of consciousness by highlighting the distinctive brain networks that characterise them. In the significant minority of vegetative patients who follow commands in neuroimaging tests, they point to putative network mechanisms that could support cognitive function and consciousness despite profound behavioural impairment.