Summary: Researchers report that EEG systems may provide a practical and reliable way to assess levels of awareness in patients diagnosed as vegetative following severe brain injury.

Source: University of Birmingham.

EEG as a Practical Indicator of Awareness in Disorders of Consciousness

Recent research indicates that electroencephalography (EEG) can serve as a strong indicator of residual awareness in some patients diagnosed as being in a vegetative state after severe brain injury. While functional magnetic resonance imaging (fMRI) has already demonstrated that a notable minority of these patients retain covert awareness that is not apparent from their behaviour, the new findings suggest a portable, lower-cost EEG test can identify those patients who are able to direct attention to sensory input and, crucially, those who can follow commands either overtly or covertly.

The study, published in Annals of Neurology, examined whether an EEG marker of somatosensory attention corresponds with the ability to follow verbal commands. The EEG task demanded less cognitive effort than the fMRI paradigms used to detect covert command following. Because EEG is inexpensive, widely available, and portable, the researchers propose that it could expand access to more accurate assessments of awareness in clinical settings where fMRI is impractical.

Study design and participants

Fourteen patients with severe brain injuries were recruited for the study to represent a spectrum of behavioural and awareness states: seven diagnosed as vegetative, four in a minimally conscious state (MCS), two emerging from MCS, and one with locked-in syndrome. Informed written consent for participation was provided by each patient’s surrogate decision maker. For comparison, a control group of fifteen healthy volunteers completed the same tasks.

Experimental tasks

Patients completed two complementary sets of neuroimaging assessments:

EEG vibrotactile attention task: Small vibratory stimulators were attached to each wrist and to the upper back. Non-painful pulses were delivered at a steady rate, with 80% of vibrations occurring on the upper back and 20% delivered to the wrists. In neurologically healthy individuals, the infrequent vibrations to the wrists evoke changes in the EEG that reflect shifts of attention toward the new location of stimulation. The EEG protocol was designed to capture a hierarchy of somatosensory and cognitive responses, including steady-state evoked responses and event-related potentials associated with bottom-up and top-down attention.
fMRI command-following tasks: During separate fMRI scans, participants completed three established covert command-following paradigms: imagining playing tennis, imagining walking around their house, and counting target words amid distracting speech. These tasks have previously revealed awareness in patients who cannot reliably demonstrate command following through behaviour alone.

Illustration of a brain and head — A simple bedside EEG assessment may diagnose a patient’s level of awareness without the need for costly and complex fMRI scans, potentially increasing the number of patients who receive an accurate diagnosis. Image for illustrative purposes only.

Key findings

The central result was a clear correspondence between EEG evidence of attentional orienting to infrequent tactile stimuli and evidence of command following on fMRI. Every patient whose EEG showed attentional responses to the infrequent wrist stimuli also demonstrated the ability to follow commands in the fMRI tasks, whether that command following was overt (behavioural) or covert (detected only with neuroimaging).

Conversely, most patients who failed to generate the EEG attentional marker (five of six) also showed no evidence of command following on fMRI. Across the cohort, no patient demonstrated the EEG marker of top-down attention (the P3b event-related potential), but several showed bottom-up attentional orienting (P3a) and sensory responses.

Dr Damian Cruse of the University of Birmingham commented that a bedside EEG could be a cost-efficient, portable method to improve diagnostic accuracy in disorders of consciousness. He noted that although many clinical diagnoses are accurate, previous estimates suggest that up to 15% of patients labelled vegetative may retain awareness that is not visible through behaviour alone. Improved bedside testing could help identify those patients and guide appropriate rehabilitation and care.

About this research

The research was conducted by teams from the University of Birmingham, UK; the Brain and Mind Institute at the University of Western Ontario, Canada; the University of Kent, UK; and the Department of Clinical Neurosciences at the University of Cambridge, UK.

Disorders of consciousness arise when a person emerges from coma after a severe brain injury. Patients in a vegetative state show no purposeful behaviour and are typically considered unaware, whereas patients in a minimally conscious state demonstrate fluctuating but reproducible purposeful behaviours indicative of some awareness. Accurately assigning patients to the correct diagnostic group is clinically challenging but essential for prognosis, ethical decision-making, and targeting rehabilitation.

In the study cohort, six patients had sustained traumatic brain injuries, primarily from motor vehicle accidents, while eight had non-traumatic injuries from causes such as cardiac arrest and near-drowning.

Conclusions and implications

This multimodal investigation supports the use of somatosensory EEG markers—specifically event-related potentials reflecting attentional orienting—to differentiate patients who can follow commands from those who cannot. Importantly, the differentiation holds whether command following is detected overtly or only through covert neuroimaging measures. Because EEG is more accessible than fMRI, bedside EEG assessments could corroborate or extend the reach of more resource-intensive methods, improving diagnostic precision for patients with disorders of consciousness and helping to allocate rehabilitation resources to those most likely to benefit.

Research article information

Original research: “Somatosensory attention identifies both overt and covert awareness in disorders of consciousness” by Raechelle M. Gibson, Srivas Chennu PhD, Davinia Fernández-Espejo PhD, Lorina Naci PhD, Adrian M. Owen PhD, and Damian Cruse PhD. Published in Annals of Neurology, online July 27, 2016.