Investigating short latency subcortical vestibular projections in humans: What have we learned?

Vestibular evoked myogenic potentials (VEMPs) are now widely used for the non-invasive assessment of vestibular function and diagnosis in humans. This review focusses on the origin, properties and mechanisms of cervical VEMPs (cVEMPs) and ocular VEMPs (oVEMPs), how these reflexes relate to reports of vestibular projections to brainstem and cervical targets and the physiological role of (otolithic) cervical and ocular reflexes. The evidence suggests that both VEMPs are likely to represent the effects of excitation of irregularly-firing otolith afferents. While the air-conducted (AC) cervical cVEMP appears to mainly arise from excitation of saccular receptors, the oVEMP evoked by bone-conducted (BC) stimulation, including impulsive BC stimuli, mainly arises from utricular afferents. The surface responses are generated by brief changes in motor unit firing. The effects that have been demonstrated are likely to represent otolith-dependent vestibulocollic VCR) and vestibulo-ocular (VOR) reflexes, both linear and torsional. These observations add to previous reports of short latency otolith projections to the target muscles in the neck (sternocleidomastoid and splenius) and extraocular muscles (the inferior oblique). New insights have been provided by the investigation and application of these techniques.