Effects of Tonic Muscle Activation on Amplitude-Modulated Cervical Vestibular Evoked Myogenic Potentials (AMcVEMPs) in Young Females: Preliminary Findings

Abstract

Cervical vestibular evoked myogenic potentials (cVEMPs) are usually elicited by transient tonebursts, but when elicited by amplitude-modulated (AM) tones, they can provide new information about cVEMPs. Previous reports of cVEMPs elicited by AM tones, or AMcVEMPs, have not systematically examined the effects of tonic EMG activation on their response properties. Fourteen young, healthy female adults (ages 20-24) with clinically normal audiograms participated in this study. AMcVEMPs were elicited with bone-conducted 500 Hz tones amplitude modulated at a rate of 37 Hz and recorded for five different EMG targets ranging from 0 to 90 μV. Amplitude increased linearly as tonic EMG activation increased. Signal-to-noise ratio (SNR) was minimal at 0 μV, but robust and with equivalent values from 30 to 90 μV; phase coherence and EMG-corrected amplitude had findings similar to SNR across EMG target levels. Interaural asymmetry ratios for SNR and phase coherence were substantially lower than those for raw or corrected amplitude. AMcVEMP amplitude scaled with tonic EMG activation similar to transient cVEMPs. Signal-to-noise ratio, phase coherence, and EMG-corrected amplitude plateaued across a range of EMG values, suggesting that these properties of the response reach their maximum values at relatively low levels of EMG activation and that higher levels of EMG activation are not necessary to record robust AMcVEMPs.

Effects of Stimulus Polarity on Amplitude-Modulated Cervical Vestibular-Evoked Myogenic Potentials

“…3 However, a consistent finding in transient cVEMP IAA literature is a relatively high upper limit of normal for a diagnostic criterion (e.g., upward of 40–45% asymmetry). Clinard et al 4 reported AMcVEMP IAAs for amplitude measures that were similar to IAAs of transient cVEMPs, with upper limits of normal at ∼45%; AMcVEMP interaural asymmetries for SNR and phase coherence had upper limits of ∼15%. This smaller variability in AMcVEMP interaural asymmetries could potentially lead to more sensitive and specific diagnostic tests for unilateral vestibular pathologies.…”

“…Borrowing from the auditory-evoked potential literature, we recently elicited cVEMPs using bone-conducted, amplitude-modulated (AM) tones, a response termed an amplitude-modulated cervical vestibular-evoked myogenic potential (AMcVEMP). 4 AMcVEMPs have the potential to provide great insight regarding otolith physiology as phase-based analyses can be examined alongside amplitude-based measures yielding new information about human otolith neural synchrony. Further, AMcVEMPs allow for better ways of examining sustained activity and aspects of otolith hair cell nonlinearity.…”

“…Current clinical applications of VEMPs focus on eliciting onset responses, but new approaches to studying VEMPs show distortion products consistent with saccular hair cell nonlinearity. 4…”

“…Our previous work showed that AMcVEMP amplitude measures increase linearly with increases in EMG activity from the SCM just as conventional cVEMPs. 4 5 However, signal-to-noise ratio (SNR) and phase coherence plateaued across a range of EMG values, suggesting these response metrics do not require high levels of EMG to be recorded and are stable across a range of tonic muscle activations. 4 There is potential that AMcVEMPs could be a clinically viable test of saccular function even in individuals who cannot achieve large EMG activation (e.g., history of cervical spine pathologies).…”

“…10 Effects of stimulus polarity on AMcVEMPs are not reported in the published literature. One study has used alternating-polarity stimuli, 4 while others have not reported what stimulus polarity was used. 6 7 11 Auditory steady-state responses (ASSRs) elicited by tones with 100% amplitude-modulation depth have been shown to have equivalent amplitude and phase delay across single and alternating polarities when stimulation involved air conduction, bone conduction, or electrical stimulation.…”