title: cVEMP morphology changes with recording electrode position, but single motor unit activity remains constant.
abstract: Cervical vestibular evoked myogenic potentials (cVEMPs) recorded over the lower quarter of the sternocleidomastoid (SCM) muscle in normal subjects may have opposite polarity to those recorded over the midpoint. It has thus been suggested that vestibular projections to the lower part of SCM might be excitatory rather than inhibitory. We tested the hypothesis that the SCM muscle receives both inhibitory and excitatory vestibular inputs. We recorded cVEMPs in 10 normal subjects with surface electrodes placed at multiple sites along the anterior (sternal) component of the SCM muscle. We compared several reference sites: sternum, ipsilateral and contralateral earlobes, and contralateral wrist. In five subjects, single motor unit responses were recorded at the upper, middle, and lower parts of the SCM muscle using concentric needle electrodes. The surface cVEMP had the typical positive-negative polarity at the midpoint of the SCM muscle. In all subjects, as the recording electrode was moved toward each insertion point, p13 amplitude became smaller and p13 latency increased, then the polarity inverted to a negative-positive waveform (n1-p1). Changing the reference site did not affect reflex polarity. There was a significant short-latency change in activity in 61/63 single motor units, and in each case this was a decrease or gap in firing, indicating an inhibitory reflex. Single motor unit recordings showed that the reflex was inhibitory along the entire SCM muscle. The cVEMP surface waveform inversion near the mastoid and sternal insertion points likely reflects volume conduction of the potential occurring with increasing distance from the motor point.
authors.affiliation: Neurology Department, Royal Prince Alfred Hospital, Camperdown, New South Wales, Australia; Central Clinical School, University of Sydney, Sydney, New South Wales, Australia; sally@srosengren.org.
authors.forename: Sally M
authors.identifier: http://orcid.org/0000-0002-3830-7214
authors.initials: SM
authors.lastname: Rosengren
authors.affiliation: Prince of Wales Clinical School, University of New South Wales, Sydney, New South Wales, Australia; Neuroscience Research Australia, University of New South Wales, Sydney, New South Wales, Australia;
authors.forename: James G
authors.initials: JG
authors.lastname: Colebatch
authors.affiliation: Prince of Wales Clinical School, University of New South Wales, Sydney, New South Wales, Australia; Neurology Department, Prince of Wales Hospital, Sydney, New South Wales, Australia;
authors.forename: Adeniyi
authors.initials: A
authors.lastname: Borire
authors.affiliation: Department of Neurology, University Hospital Zurich, University of Zurich, Zurich, Switzerland; and.
authors.forename: Dominik
authors.initials: D
authors.lastname: Straumann
authors.affiliation: Department of Neurology, University Hospital Zurich, University of Zurich, Zurich, Switzerland; and Department of Ophthalmology, University Hospital Zurich, University of Zurich, Zurich, Switzerland.
authors.forename: Konrad P
authors.initials: KP
authors.lastname: Weber
mesh_terms: D000328:Adult; D000368:Aged; D004566:Electrodes; D005260:Female; D006801:Humans; D008297:Male; D008416:Mastoid / Q000502:physiology; D008875:Middle Aged; D009132:Muscles / Q000502:physiology*; D012018:Reflex / Q000502:physiology*; D013249:Sternum / Q000502:physiology; D058585:Vestibular Evoked Myogenic Potentials / Q000502:physiology*; D014722:Vestibule, Labyrinth / Q000502:physiology; D055815:Young Adult
keywords: otolith; single motor unit; sternocleidomastoid muscle; ultrasound; volume conduction