Frequency Tuning of the Cervical Vestibular-Evoked Myogenic Potential (cVEMP) Recorded from Multiple Sites along the Sternocleidomastoid Muscle in Normal Human Subjects

Abstract

Frequency tuning of tone burst-evoked myogenic potentials recorded from the sternocleidomastoid muscle (cervical VEMP or cVEMP) is used clinically to assess vestibular function. Understanding the characteristics of cVEMP is important for improving the specificity of cVEMP testing in diagnosing vestibular deficits. In the present study, we analyzed the frequency tuning properties of the cVEMPs by constructing detailed tuning curves and examining their morphology and dependence on SCM tonic level, sound intensity, and recording site along the SCM. Here we report two main findings. First, by employing nine tone frequencies between 125 and 4,000 Hz, some tuning curves exhibited two distinct peaks, which cannot be modeled by a single mass spring system as previously suggested. Instead, the observed tuning is better modeled as linear summation of two mass spring systems, with resonance frequencies at 300 and~1,000 Hz. Peak frequency of cVEMP tuning curves was not affected by SCM tonic level, sound intensity, and location of recording site on the SCM. However, sharpness of cVEMP tuning was increased at lower sound intensities. Second, polarity of cVEMP responses recorded from the lower quarter of the SCM was reversed as compared to that at the two upper sites. While more studies are needed, these results suggest that cVEMP tuning is mediated through multiple generators with different resonance frequencies. Future studies are needed to explore implications of these results on development of selective VEMP tests and determine the nature of polarity inversion at the lower quarter of SCM.

The Cervical Vestibular-Evoked Myogenic Potentials (cVEMPs) Recorded Along the Sternocleidomastoid Muscles During Head Rotation and Flexion in Normal Human Subjects

“…Figure 1 shows that there was a clear polarity reversal in the ipsi-cVEMPs responses between the upper and lower sites in the sterno-mastoid branch of the SCM (CH3 vs. CH5), but not along the cleidomastoid branch of the SCM (CH3 and CH6). This is consistent with results of our previous study (Wei et al 2013). Figure 2 further quantitatively compares the amplitudes of Pk1 and Pk2 from the seven recording sites in the HR and CD conditions (A 350 Hz, B 1000 Hz).…”

“…On the other hand, we have examined whether the SCM is compartmented and has multiple innervation zones (McLoon 1998). We found that the cVEMPs recorded at the sites close to the sterno head exhibit reversed polarity compared to that recorded at the upper sites (Wei et al 2013), which is consistent with the idea of SCM compartmentation. In the current study, we extended our investigation of the SCM-related factors by further testing three specific hypotheses, i.e., the cVEMP response is ipsilateral and dependent on how the SCM tonic level is achieved (e.g., by head rotation or head flexion), and the polarity of the cVEMP response is dependent on the recording site.…”

“…In contrast to the earlier reports that identified the saccule as the sole source of the cVEMPs (Murofushi et al 1995;Todd et al 2000), recent animal studies have demonstrated sound sensitivity in vestibular afferents from the semicircular canals and the utricle (Zhou et al 2004(Zhou et al , 2005(Zhou et al , 2007Xu et al 2009;Zhu et al 2011 and2014). Our recent human study (Wei et al 2013) also showed that cVEMP frequency tuning curves are better modeled as a summation of two mass spring systems with resonance frequencies of 300 and 1000 Hz. Furthermore, it is currently assumed that there is a single innervation zone in the SCM and that the cVEMPs exhibit the same positive-negative waveforms when recorded at different locations along the SCM.…”

“…Furthermore, it is currently assumed that there is a single innervation zone in the SCM and that the cVEMPs exhibit the same positive-negative waveforms when recorded at different locations along the SCM. In a recent study, however, we showed that the cVEMPs recorded at the lower portion of the SCM exhibited reversed polarity compared with that recorded at upper portions of the SCM during head rotation (Wei et al 2013), suggesting compartmentation of SCM neural innervation. These new results suggest that in order to develop standardized cVEMP testing protocols, continued efforts are needed to systematically identify the factors that affect the cVEMP responses in normal human subjects.…”

“…On the one hand, we have conducted neurophysiological studies in monkeys and rats and showed that acoustic stimulation similar to that used in clinics activates multiple vestibular end organs (Zhou et al 2004(Zhou et al , 2005(Zhou et al , 2007Xu et al 2009;Zhu et al 2011Zhu et al , 2014. Consistent with the animal studies, our recent human study showed that frequency tuning of the cVEMPs is better described as a summation of at least two massspring-damper systems, rather than a single mass spring model (Wei et al 2013). These results argue against the prevailing hypothesis that the saccule is the sole generator of the cVEMPs and suggest that sound-activation of other vestibular end organs contributes to the cVEMPs.…”