Vestibular inputs modulate somatosensory cortical processing

The vestibular system is unique among the senses because of the entirely multisensory nature of its cortical projections. Neuroanatomical and neuroimaging studies show that vestibular stimulation activates somatosensory areas, and particularly the so-called parieto-insular vestibular cortex (PIVC) in the monkey, while deactivating visual areas. Further, recent psychophysical studies showed that vestibular stimulation facilitates detection of electrocutaneous stimuli, suggesting a vestibular-somatosensory perceptual interaction. However, the functional mechanism underlying this perceptual facilitation remains unclear. We therefore recorded somatosensory potentials evoked by left median nerve stimulation, before and immediately after left cold caloric vestibular stimulation (CVS), in a small-scale study of eight healthy volunteers. CVS selectively enhanced the N80 component recorded over both ipsilateral and contralateral somatosensory areas, without significantly affecting earlier or later components. Interestingly, the N80 component has been localised to the parietal operculum, which includes the human homologue of the monkey PIVC, and is thus a prime neuroanatomical candidate for vestibular-somatosensory convergence. As a control, we measured visual evoked potentials to reversing checkerboard patterns and found no effects of vestibular stimulation. This rules out explanations based on indirect effects of vestibular modulations, such as general arousal or supramodal spatial attention. We believe our results provide the first clue linking brain structure to function for the interaction between vestibular and somatosensory systems.