Applications of neuromodulation to explore vestibular cortical processing; new insights into the effects of direct current cortical modulation upon pursuit, VOR and VOR suppression

Abstract

Functional imaging, lesion studies and behavioural observations suggest that vestibular processing is lateralised to the non-dominant hemisphere. Moreover, disruption of interhemispheric balance via inhibition of left parietal cortex using transcranial direct current stimulation (tDCS) has been associated with an asymmetric suppression of the vestibulo-ocular reflex (VOR). However, the mechanism by which the VOR was modulated remains unknown. In this paper we review the literature on noninvasive brain stimulation techniques which have been used to probe vestibular function over the last decade. In addition, we investigate the mechanisms whereby tDCS may modulate VOR, e.g. by acting upon pursuit, VOR suppression mechanisms or direct VOR modulation. We applied bi-hemispheric parietal tDCS in 11 healthy subjects and only observed significant effects on VOR gain (tdcs * condition p = 0.041) -namely a trend for VOR gain increase with right anodal/left cathodal stimulation, and a decrease with right cathodal/left anodal stimulation. Hence, we suggest that the modulation of the VOR observed both here and in previous reports, is directly caused by top-down cortical control of the VOR as a result of disruption to interhemispheric balance, likely parietal.

Role of Alpha-Band Oscillations in Spatial Updating across Whole Body Motion

“…Because the motion was passively induced in the present study, we consider the alpha power modulations primarily due to vestibular processing, or the subsequent self-motion estimate derived from it ( Seemungal et al, 2008 ). Because subjects had to maintain fixation of a body-fixed fixation point during the motion, it is also possible that the suppression of the vestibular ocular reflex (VOR), which is likely a top-down effect, originated from this activity ( Ahmad et al, 2014 ).…”

Polarity-Dependent Misperception of Subjective Visual Vertical during and after Transcranial Direct Current Stimulation (tDCS)

“…Transcranial direct current stimulation (tDCS), transcranial magnetic stimulation (TMS) and galvanic vestibular stimulation (GVS) are non-invasive methods for modulating cortical excitability that have the potential to influence different brain functions depending on the stimulated region and specific techniques used [ 8 ]. Non-invasive neuromodulation techniques in vestibular research have been recently reviewed [ 9 ]. The vestibular ocular reflex (VOR) and movement perception during rotational stimulation are significantly altered by tDCS [ 10 ].…”

“…The vestibular ocular reflex (VOR) and movement perception during rotational stimulation are significantly altered by tDCS [ 10 ]. Cathodal stimulation over left posterior parietal cortex has been shown to induce modulation of the VOR [ 9 , 11 ]. In addition, GVS can alter haptic and visual vertical perceptions in healthy subjects and stroke patients [ 12 , 13 ].…”