Left Cathodal Trans-Cranial Direct Current Stimulation of the Parietal Cortex Leads to an Asymmetrical Modulation of the Vestibular-Ocular Reflex

Abstract

Multi-sensory visuo-vestibular cortical areas within the parietal lobe are important for spatial orientation and possibly for descending modulation of the vestibular-ocular reflex (VOR). Functional imaging and lesion studies suggest that vestibular cortical processing is localized primarily in the non-dominant parietal lobe. However, the role of inter-hemispheric parietal balance in vestibular processing is poorly understood. Therefore, we tested whether experimentally induced asymmetries in right versus left parietal excitability would modulate vestibular function. VOR function was assessed in right-handed normal subjects during caloric ear irrigation (30 °C), before and after trans-cranial direct current stimulation (tDCS) was applied bilaterally over the parietal cortex. Bilateral tDCS with the anode over the right and the cathode over the left parietal region resulted in significant asymmetrical modulation of the VOR, with highly suppressed responses during the right caloric irrigation (i.e. rightward slow phase nystagmus). In contrast, we observed no VOR modulation during either cathodal stimulation of the right parietal cortex or SHAM tDCS conditions. Application of unilateral tDCS revealed that the left cathodal stimulation was critical in inducing the observed modulation of the VOR. We show that disruption of parietal inter-hemispheric balance can induce asymmetries in vestibular function. This is the first report using neuromodulation to show right hemisphere dominance for vestibular cortical processing.

Vestibular stimulation improves insight into illness in schizophrenia spectrum disorders

“…posterior parietal and medial prefrontal cortex) (Gerretsen et al, 2015; Gerretsen et al, 2014a; Levine et al, 2012) via cortical vestibular projections from the thalamus and brain stem vestibular nuclei re-establishing hemispheric balance (Ramachandran, 1996). According with this mechanism, right cold CVS’s worsening of IMP-INS is possibly due to overactivation of the left hemisphere, contributing to left hemisphere dominance (Arshad et al, 2014). In the present study, the physiological effects of CVS were demonstrated by measuring the VOR (i.e.…”

“…10 consecutive days) to determine if repeated CVS can lead to sustained improvement in IMP-INS in schizophrenia. The addition of noninvasive neurostimulation, such as transcranial direct current stimulation or transcranial magnetic stimulation, should also be considered to determine if it can facilitate the effects of CVS (Arshad et al, 2014; Been, 2007; Daskalakis et al, 2006; George et al, 1999). If proven effective, this easy to administer, safe, non-invasive intervention would have the potential to alter individuals’ attitude towards their illness and medication, leading ultimately to an improvement in individuals’ capacity for illness recognition and engagement in treatment, which would in turn, significantly impact the management of this devastating mental disorder.…”

A brief review of the clinical anatomy of the vestibular-ocular connections—how much do we know?

“…Excitation of the right hemisphere (ie, anodal stimulation) and concurrent inhibition of the left hemisphere (ie, cathodal stimulation) induced an asymmetrical VOR response with suppression of left beating nystagmus in response to right cold (30 1C) irrigations. No effect of tDCS was found for the processing of right-beating nystagmus elicited via left cold irrigations 25 as shown in Figure 7. To elucidate the 'active' electrode we applied unipolar tDCS and found that the modulation was specifically attributable to the suppression of the left hemisphere.…”

“…To elucidate the 'active' electrode we applied unipolar tDCS and found that the modulation was specifically attributable to the suppression of the left hemisphere. 25 Further, to ascertain whether the tDCS was impacting upon pursuit and/or VOR suppression mechanisms, that in turn might mediate VOR modulation, we performed a follow-up study and ruled out such an explanation. 26 Hence, we tentatively conclude that cortical modulations of the VOR that we have reported both in behavioural and electrical stimulation studies are subject to dynamic inter-hemispheric competition.…”

“…27 Taken together, our findings in neurologically intact individuals and reports in the literature from brain damaged patients imply that the vestibular cortex is able to down-regulate the VOR and specifically influence the 'low frequency' component of the horizontal velocity storage mechanism. 25 However, much remains unknown; for example, what are the exact anatomical pathways in man, what functionality does such topdown modulation serve, and why is there such a close relationship with spatial attention mechanisms? To conclude, in contrast to the extensive and well-defined knowledge regarding low-level control of the VOR, much is unknown and remains to be elucidated regarding the Figure 6 Shows a summary for results for the effects of binocular rivalry and the visuospatial working memory tasks upon the VOR.…”