MYOGENIC POTENTIALS GENERATED BY A CLICK-EVOKED VESTIBULOCOLLIC REFLEX

Electromyograms (EMGs) were recorded from surface electrodes over the sternomastoid muscles and averaged in response to brief (0.1 ms) click s played through headphones. In normal subjects, clicks 85 to 100 dB a bove our reference (45 dB SPL: close to perceptual threshold for norma l subjects for such clicks) evoked reproducible changes in the average d EMG beginning at a mean latency of 8.2 ms. The earliest potential ch ange, a biphasic positive-negativity (p13-n23), occurred in all subjec ts and the response recorded from over the muscle on each side was pre dominantly generated by afferents originating from the ipsitateral ear . Later potentials (n34, p44), present in most but not all subjects, w ere generated bilaterally after unilateral ear stimulation. The amplit ude of the averaged responses increased in direct proportion to the me an level of tonic muscle activation during the recording period. The p 13-n23 response was abolished in patients who had undergone selective section of the vestibular nerve but was preserved in subjects with sev ere sensorineural hearing loss. It is proposed that the p13-n23 respon se is generated by activation of vestibular afferents, possibly those arising from the saccule, and transmitted via a rapidly conducting oli gosynaptic pathway to anterior neck muscles. Conversely, the n34 and p 44 potentials do not depend on the integrity of the vestibular nerve a nd probably originate from cochlear afferents.