In a new paper, we describe evidence that links preserved bottom-up somatosensory attention in prolonged disorders of consciousness with a preserved ability to follow verbal commands. Interestingly, somatosensory attention, indexed by the tactile P3a event-related potential, was evident whether the patient could follow commands to physically move (i.e. diagnosed as at least minimally conscious plus) or could only covertly follow commands by engaging in a mental action (i.e. imagining playing tennis).
The cognitive demands of tasks to elicit a P3a are low, as this component does not require top-down control of attention. Within the clinic, therefore, it may be possible to identify covert awareness - i.e. misdiagnoses of vegetative state - in patients who lack the cognitive capacity to successfully complete more demanding active tasks commonly used in the field, such as mental imagery.
From a basic science perspective, this result now begs the question of what is so special about somatosensory attention that it might be linked to command following abilities? Does it recruit neural circuits that overlap those that support the ability to follow verbal commands? Are oddball tactile events more inherently engaging than oddball visual or auditory events, and therefore more likely to achieve ignition and be associated with conscious processing?
Reference: Gibson, R. M., Chennu, S., Fernández-Espejo, D., Naci, L., Owen, A. M., & Cruse, D. (2016). Somatosensory attention identifies both overt and covert awareness in disorders of consciousness. Annals of Neurology. dx.doi.org/10.1002/ana.24726
Figure: Grand-average P3a potential to rare tactile events in a healthy control group.