FUNCTIONAL MRI OF GALVANIC VESTIBULAR STIMULATION

The cortical processing of vestibular information is not hierarchicall y organized as the processing of signals in the visual and auditory mo dalities. Anatomic and electrophysiological studies in the monkey reve aled the existence of multiple interconnected areas in which vestibula r signals converge with visual and/or somatosensory inputs. Although r ecent functional imaging studies using caloric vestibular stimulation (CVS) suggest that vestibular signals in the human cerebral cortex may be similarly distributed, some areas that apparently form essential c onstituents of the monkey cortical vestibular system have not yet been identified in humans. Galvanic vestibular stimulation (GVS) has been used for almost 200 years for the exploration of the vestibular system By contrast with CVS, which mediates its effects mainly via the semic ircular canals (SCC), GVS has been shown to act equally on SCC and oto lith afferents. Because galvanic stimuli can be controlled precisely, GVS is suited ideally for the investigation of the vestibular cortex b y means of functional imaging techniques. We studied the brain areas a ctivated by sinusoidal GVS using functional magnetic resonance imaging (fMRI). An adapted set-up including LC filters tuned for resonance at the Larmor frequency protected the volunteers against bums through ra dio-frequency pickup by the stimulation electrodes. Control experiment s ensured that potentially harmful effects or degradation of the funct ional images did not occur. Six male, right-handed volunteers particip ated in the study. In all of them, GVS induced clear perceptions of bo dy movement and moderate cutaneous sensations at the electrode sites. Comparison with anatomic data on the primate cortical vestibular syste m and with imaging studies using somatosensory stimulation indicated t hat most activation foci could be related to the vestibular component of the stimulus. Activation appeared in the region of the temporoparie tal junction, the central sulcus, and the intraparietal sulcus. These areas may be analogous to areas PIVC, 3aV, and 2v, respectively, which form in the monkey brain, the ''inner vestibular circle''. Activation also occurred in premotor regions of the frontal lobe. Although undet ected in previous imaging-studies using CVS, involvement of these area s could be predicted from anatomic data showing projections from the a nterior ventral part of area 6 to the inner vestibular circle and the vestibular nuclei. Using a simple paradigm, we showed that GVS can be implemented safely in the fMRI environment. Manipulating stimulus wave forms and thus the GVS-induced subjective vestibular sensations in fut ure imaging studies may yield further insights into the cortical proce ssing of vestibular signals.