Electromyograms (EMGs) were recorded from surface electrodes over the sternomastoid muscles and averaged in response to brief (0.1 ms) clicks played through headphones. In normal subjects, clicks 85 to 100 dB above our reference (45 dB SPL: close to perceptual threshold for normal subjects for such clicks) evoked reproducible changes in the averaged EMG beginning at a mean latency of 8-2 ms. The earliest potential change, a biphasic positive-negativity (p13-n23), occurred in all subjects and the response recorded from over the muscle on each side was predominantly generated by afferents originating from the ipsilateral ear. Later potentials (n34, p44), present in most but not all subjects, were generated bilaterally after unilateral ear stimulation. The amplitude of the averaged responses increased in direct proportion to the mean level of tonic muscle activation during the recording period. The p13-n23 response was abolished in patients who had undergone selective section of the vestibular nerve but was preserved in subjects with severe sensorineural hearing loss. It is proposed that the p13-n23 response is generated by activation of vestibular afferents, possibly those arising from the saccule, and transmitted via a rapidly conducting oligosynaptic pathway to anterior neck muscles. Conversely, the n34 and p44 potentials do not depend on the integrity of the vestibular nerve and probably originate from cochlear afferents. (7 Neurol Neurosurg Psychiatry 1994;57:190-197) The vestibular nuclei have powerful projections to the ocular motor nuclei, the cerebellum, the reticular formation, and the spinal cord.' In humans, the most accessible and best studied vestibular pathway is that between the semicircular canals and the ocular motor nuclei: the standard test of vestibular function, caloric induced nystagmus, measures the effect of horizontal canal activation on eye movements.2 The reflex effects resulting from activation of the otoliths and the function of the direct vestibular projections to the spinal cord in humans are difficult to study and poorly understood.3 The initial muscle excitation after an unexpected fall depends on otolith activation, possibly via connections to the reticular formation and thereby to the spinal cord." Detailed studies on a single patient who had an otolithic Tullio phenomenon (sound-evoked activation of the vestibular apparatus) showed shortlatency activation of leg muscles, probably via vestibulospinal pathways.7 The limitations of our knowledge of vestibular influences on the muscles of the trunk and limbs led us to reinvestigate earlier reports of activation of the vestibular apparatus in normal subjects by loud clicks.Bickford et al 8 described the characteristics of averaged responses to clicks with recordings with an active electrode just below the inion (the "inion response"). They concluded that the short latency potentials that they recorded were not, as they had first supposed, indicative of an auditory projection to the cerebellar vermis, but rather were gener...

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Taps to the forehead on the midline, at the hairline (Fz), with a reflex hammer or powerful bone conduction vibrator caused short-latency surface potentials from beneath both eyes in all healthy subjects. The earliest negative responses were invariably absent from the eye contralateral to the side of a previous vestibular nerve section but were preserved despite sensorineural hearing loss. These responses probably reflect vestibular function via crossed otolith-ocular pathways.

The normal galvanic vestibulocollic responses indicate that sound sensitivity in patients with the Tullio phenomenon is likely to occur distal to the vestibular nerve, probably at the level of the receptors. Both click hypersensitivity and dehiscence of the anterior (superior) semicircular canal are associated with the Tullio phenomenon but as the CT scan abnormality can occur in clinically unaffected ears, click testing is important for specific diagnosis. Abnormal sound sensitivity, as demonstrated by click responses, confirms that the radiologic abnormality is function significant.

Acute vestibular neuritis most often affects both vestibular nerve divisions. The horizontal vHIT alone identifies superior nerve dysfunction in all patients with vestibular neuritis tested acutely, whereas both cervical/vestibular evoked myogenic potentials and posterior vHIT are necessary for diagnosing inferior vestibular nerve involvement.