A small but significant number of those who survive severe brain injury will transition into a state of unconscious wakefulness known as the vegetative state (VS). Patients in a VS remain behaviourally non-responsive during clinical examination and, despite exhibiting clear evidence of wakefulness, do not appear to possess conscious awareness. However, recent neuroimaging research has identified a subpopulation of these patients who reliably produce neural markers indicative of covert awareness. Indeed, brain activity (e.g., EEG and fMRI) in response to motor-imagery paradigms have identified neural signatures of covert command-following ability in nearly 20% of patients tested. More recently, naturalistic movie-watching tasks in fMRI have been used successfully to index higher-order “executive” processing in patients who appear to be entirely unconscious. In this study, we demonstrate the utility of high-density EEG recorded at the bedside during two response-free movie paradigms to detect higher-order stimulus processing in a cohort of patients diagnosed with disorders of consciousness (DOC). We used a correlated components analysis (rCA) to calculate inter-subject neural synchronization—an established measure of stimulus engagement—in 28 healthy volunteers and 13 patients with DOC during a suspenseful audio-visual and an audio-only movie-watching task. Across healthy participants, we observed a remarkably consistent pattern of EEG activity during both movie types, as well as significant and reliable group-wide inter-subject neural synchronization. We then used the common pattern of neural activity from the control group and their overall inter-subject synchrony as a benchmark with which to compare to individual patients with DoC while they watched or listened to the same movie clips. In the audio-visual movie condition, we found that 25% of the patients tested were significantly synchronized to healthy participants. Likewise, we observed significant patient-to-control group synchronization in 30% of patients during the audio-only movie. Consistent with previous findings, our results suggest that at least one-third of patients with DoC retain the capacity to process both the sensory properties of naturalistic stimuli as well as higher-order features like their plot similarly to healthy participants. Our results demonstrate the feasibility of using bedside measures of the brain’s electrophysiological activity to identify patients with intact sensory and cognitive functions who can engage with complex naturalistic stimuli in a way that closely resembles healthy controls.

Using EEG to Identify Markers of Naturalistic Stimulus Processing in Patients with Disorders of Consciousness