VESTIBULAR SYSTEM: Anatomy and Functional Magnetic Resonance Imaging

SUMMARY Convincing evidence exists that the multi-sensory neurons in the posterior insula represent the PIVC in both human and non-human primates. They receive sensory input from vestibular (otolith and semicircular canals), visual, and somatosensory systems; however, animal and human studies have found functional discrepancies with respect to the semicircular canals and otoliths. In monkeys, Grüsser et al 27 , 28 found PIVC neurons that responded only to semicircular canal stimulation and not to otolith stimulation. In contrast, patients who have acute infarctions of the PIVC typically present with otolith dysfunction (tilts of perceived verticality) without associated rotatory vertigo. 7 , 17 The authors expect the signals from the semicircular canals and the otoliths to project to the same cortical loci, because both types of afferents converge at the level of the vestibular nuclei. 1 Electrical stimulation of the utricular and saccular nerves in the cat has been shown to activate the contralateral vestibular cortex in the same regions as does horizontal ampullar nerve stimulation. 31