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

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 modulation 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 non‐invasive 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 bihemispheric
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 previously and herein observed is
directly caused by top‐down cortical control of the VOR as a result of disruption to
interhemispheric balance, likely parietal.

The Vestibular System: A Clinical and Scientific Update in Siena In honor of Professor Daniele Nuti, Siena, Italy, April 5–6, 2013