Effects of Galvanic Vestibular Stimulation on EEG Correlates of Attention

Galvanic Vestibular Stimulation (GVS) is a non-invasive form of neural stimulation that involves applying weak, electric current over the mastoid processes. Brief periods of GVS have been shown to provide transient relief from symptoms of inattention in hemi-spatial neglect and extinction patients. In addition, GVS has been shown to have effects on spatial- and reward-based attention in healthy participants. Despite this growing body of evidence, the underlying mechanisms associated with GVS-induced changes remain largely unknown. Better understanding of the underlying mechanisms of GVS may help to generate more efficient brain stimulation protocols and to direct treatment to those most likely to respond.

Recently, studies have demonstrated that EEG may be a useful tool in investigating underlying changes with GVS. The current thesis therefore measured electrophysiological correlates known to be associated with attention in response to GVS. Chapters 2 and 3 investigated the impact of GVS on two ERP components, the N2pc and the P3, both shown to be associated with mechanisms involved in tasks often impaired in neglect patients. In these two chapters, GVS was shown to impact the N2pc relative to a sham condition. This may indicate that GVS impacted target detection and suppression of irrelevant distractors. In contrast, GVS did not impact the amplitude of the P3 relative to a sham group. Selective modulation of the N2pc, but not the P3 despite the same conditions, suggests a specific mechanism underlying GVS behavioural effects.

Based on literature demonstrating differing efficiency of GVS effects using different waveforms and stimulation intensities, Chapter 4 aimed to investigate maximally effective protocols of GVS on behavioural measures of accuracy and reaction time. This chapter also aimed to extend and resolve discrepancies in the behavioural findings of Chapters 2 and 3. Chapter 4, however, demonstrated no differences between the various conditions. This indicated that GVS does not impact behavioural measures of accuracy and reaction time in the change-detection paradigm used.

It has been demonstrated behaviourally that GVS can induce lateral shifts of spatial attention. Furthermore, these shifts have been shown to be polarity specific. Chapter 5 investigated these effects using an electrophysiological measure of spatial distribution of attention, steady state visual evoked potentials (SSVEPs). In this chapter GVS was not found to have any effect on SSVEP amplitude either for left-anodal/right-cathodal or right-anodal/left-cathodal GVS. This may indicate that the previously demonstrated effects are task dependent, and/or that GVS does not influence covert attention. Overall, this work provides new insight into underlying electrophysiological mechanisms influenced by GVS. These findings are presented and discussed with relevance to the theoretical and practical implications.