Effects of Caloric and Galvanic Vestibular Stimulation on Reduced Awareness

Approximately one fifth of those who emerge from coma following an acquired brain injury will be in a minimally conscious or vegetative state. Both conditions are associated with a range of physical co-morbidities, and currently no effective treatments are available. The main aim of this thesis is to seek preliminary evidence that vestibular stimulation can help remediate the reduced awareness that characterises these conditions. This objective was motivated by the fact that vestibular stimulation is already known to reduce lateralised (as opposed to global) disorders of awareness, and increases metabolic activity in the thalamo-cortical network, crucial to awareness. To seek proof-of-principle, five hospital in-patients (three MCS, two PVS) underwent a maximum of 40 minutes of daily left-ear caloric vestibular stimulation (CVS). Stimulation was delivered in four week blocks for a period of 16 weeks, alternating between CVS and sham. Behavioural changes in awareness and cognitive ability were measured weekly via the Wessex Head Injury Matrix (WHIM) and the JFK Coma Recovery Scale – Revised. Electrophysiological correlates of awareness (i.e., EEG power spectra and auditory P3 elicited by an oddball paradigm) were recorded monthly. The behavioural results showed that CVS triggered the emergence of awareness in both MCS patients, along with smaller signs of improvement in both PVS patients. The third MCS patient died three weeks after enrolment due to an unrelated respiratory infection. All participants’ electrophysiological responses were modulated by CVS but not in a manner that could be easily tied to their behavioural improvement. Taken together, these clinical studies establish proof-of-principle and justify a larger, more definitive study to reproduce the treatment effects and investigate the degree to which dose, disease chronicity and level of consciousness influence response. The results also suggest that EEG power spectra and auditory ERPs may not provide the best means of capturing allied neurophysiological change.

A parallel stream of study was also conducted to seek analogous effects of vestibular stimulation on awareness and attention in healthy individuals. Two experiments tested whether galvanic vestibular stimulation (GVS) influences the behavioural and/or electrophysiological responses of participants while they performed a prolonged Go/NoGo task challenging their sustained attention. Four stimulation conditions were compared: supra-sensory GVS, sub-sensory GVS, Sham, and no stimulation. The results indicate that vestibular stimulation can modulate, partly in a dose-dependent way, higher-level processes linked to stimulus processing, motor inhibition and performance monitoring. In broader terms, the results give impetus to the idea that the vestibular sense is not only, as traditionally thought, central to autonomic motor control but also to the formation and maintenance of higher cognitive states. Previous studies indicate that the linear, angular and gravitational accelerations signaled by the vestibular system contribute to egocentric perception and body ownership. The current findings suggest that these signals also help initiate and influence processes involved in global self-awareness and purposeful behaviour, thereby pointing towards an altogether more profound role.