Multisensory vestibular cortex activated by otolith stimulation (fMRI)

Vestibular evoked myogenic potentials (VEMPs) triggered by tone bursts represent a vestibulocollic reflex of the sacculus otolith with descending pathways for head and trunk stabilization in space. Hence, no ascending pathways to the vestibular cortex have been investigated in humans. Therefore, aim of this fMRI study was to determine whether VEMPs can also activate the multisensory vestibular cortex in humans, for which a network of temporo-insular-parietal areas in both hemispheres with a dominance of the non-dominant hemisphere was characterized by imaging studies during galvanic stimulation of the vestibular nerve.

We studied 21 right-handed healthy volunteers (8 M, 6 F; mean age 24y) lying supine, eyes closed, in a clinical 1.5 T scanner wearing MRI-suitable piezo-electric headphones (Jaencke, Zuerich) that suppressed scanner noises to below 30 dB during 3 unilateral auditory stimulation conditions: 1) VEMP stimulation (95 dB 500 Hz tone burst for which VEMPs were recorded outside the scanner), 2) a similar sub-threshold 65 dB 500 Hz tone burst not triggering VEMPs, and 3) a 95 dB white noise signal to control for pure loudness of auditory stimulation. The protocol included 172 volumes, each consisting of 40 slices of a T2*-weighted interleaved EPI sequence in alternating blocks. Random effects statistical analysis was done with SPM2 (p < 0.001, uncorrected).

In a paired t-test excluding the auditory effects (condition 1 vs. 2) activations were found in a cortical network within both hemispheres but predominantly in the right hemisphere. This network included the middle frontal (BA 10), inferior frontal (BA 44), superior and medial temporal gyri (BA 22, 29 and 42) bilaterally, the inferior parietal lobule with the supramarginal gyrus (BA 40), as well as the posterior insula and the right cerebellar nodulus.

This is the first demonstration by means of fMRI that monaural otolith stimulation in right-handers causes bilateral multisensory vestibular cortex activation with a dominance in the non-dominant right hemisphere similar to the activation pattern known from the stimulation of the entire vestibular nerve.

Supported by DFG (German Research Foundation) DI 379/4-3.