Somatosensory attention identifies both overt and covert awareness in disorders of consciousness

Objective: Some patients diagnosed with disorders of consciousness retain sensory and cognitive abilities beyond those apparent from their overt behaviour. Characterising these covert abilities is crucial for diagnosis, prognosis, and medical ethics. This multimodal study investigates the relationship between electroencephalographic evidence for perceptual/cognitive preservation and both overt and covert markers of awareness.

Methods: Fourteen patients with severe brain injuries were evaluated with an electroencephalographic vibrotactile attention task designed to identify a hierarchy of residual somatosensory and cognitive abilities: 1) somatosensory steady-state evoked responses, 2) bottom-up attention orienting (P3a event-related potential), and 3) top-down attention (P3b event-related potential). Each patient was also assessed with a clinical behavioural scale and two functional magnetic resonance imaging assessments of covert command following.

Results: Six patients produced only sensory responses, with no evidence of cognitive event-related potentials. A further eight patients demonstrated reliable bottom-up attention orienting responses (P3a). No patient showed evidence of top-down attention (P3b). Only those patients who followed commands, whether overtly with behaviour or covertly with functional neuroimaging, also demonstrated event-related potential evidence of attentional orienting.

Interpretation: Somatosensory attentional orienting event-related potentials differentiated patients who could follow commands from those who could not. Crucially, this differentiation was irrespective of whether command following was evident through overt external behaviour, or through covert functional neuroimaging methods. Bedside electroencephalographic methods may corroborate more expensive and challenging methods such as functional neuroimaging, and thereby assist in the accurate diagnosis of awareness.