Cerebral activation was investigated with functional magnetic resonance imaging (fMRI) during galvanic stimulation of the mastoid in 6 normal volunteers. Cutaneous stimulation at the neck C4-5 level served as a control. During mastoid stimulation, bilateral vestibular activation occurred in the posterior insula (parietoinsular vestibular cortex, PIVC), the transverse temporal (Heschl's) gyrus, and thalamic pulvinar. The cutaneous pain elicited by galvanic stimulation caused bilateral activity of the medial part of the insula and the anterior median thalamus. Thus, galvanic stimulation at the mastoid level activates cortical areas of three different sensory systems in the insulathalamic region, the vestibular, the auditory, and the nociceptive systems.

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Bilateral galvanic vestibular stimulation (GVS) with current intensity of 3 mA was applied at mastoid level in 11 patients with chronic bilateral vestibular failure, in order to determine ocular motor responses by 3-D video-oculography. The following abnormal features were found: (1) a predominantly torsional or mixed torsional-horizontal nystagmus at the onset of stimulation with lower current intensities (1.0-3.0 mA) in nine patients; (2) a reduced amplitude of tonic ocular torsion by about 50% in nine patients (1.3 +/- 0.6 degrees at 3 mA); (3) a nystagmus in the opposite direction at stimulation offset in five patients (rebound); (4) no eye movements at all in a patient with bilateral nerve failure. GVS stimulates the vestibular nerve, thus allowing differentiation of nerve failure from labyrinthine failure. The low thresholds for initiating nystagmus and the rebound, which appear to be the most typical features of bilateral labyrinthine failure, can be explained by central compensation mechanisms.

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