Temporal Modulation Transfer Functions of Amplitude-Modulated Cervical Vestibular-Evoked Myogenic Potentials in Young Adults.

OBJECTIVES: Cervical vestibular-evoked myogenic potentials (cVEMPs) are widely used to evaluate saccular function in clinical and research applications. Typically, transient tonebursts are used to elicit cVEMPs. In this study, we used bone-conducted amplitude-modulated (AM) tones to elicit AMcVEMPs. This new approach allows the examination of phase-locked vestibular responses across a range of modulation frequencies. Currently, cVEMP temporal modulation transfer functions (TMTFs) are not well defined. The purposes of the present study were (1) to characterize the AMcVEMP TMTF in young, healthy individuals, (2) to compare AMcVEMP TMTFs across different analysis approaches, and (3) to determine the upper frequency limit of the AMcVEMP TMTF. DESIGN: Young adults (ages 21 to 25) with no history of vestibular lesions or middle ear pathologies participated in this study. Stimuli were amplitude-modulated tones with a carrier frequency of 500 Hz and modulation frequencies ranging from 7 to 403 Hz. Stimuli were presented at 65 dB HL via a B81 bone-oscillator. RESULTS: AMcVEMP waveforms consisted of transient onset responses, steady-state responses, and transient offset responses; the behavior of these different types of responses varied with modulation frequency. Differences in the TMTF shape were noted across different measures. The amplitude TMTF had a sharp peak, while signal-to-noise ratio and phase coherence TMTFs had broader shapes with plateaus across a range of modulation frequencies. Amplitude was maximal at modulation frequencies of 29 and 37 Hz. Signal-to-noise ratio maintained its peak value at modulation frequencies between 17 Hz and 127 Hz. Phase coherence and modulation gain maintained their peak values at modulation frequencies between 17 Hz and 143 Hz. CONCLUSIONS: AMcVEMPs reflect transient onset and offset responses, as well as a sustained response with the periodicity of an amplitude-modulation frequency. AMcVEMP TMTFs had variable shapes depending on the analysis being applied to the response; amplitude had a narrow shape while others were broader. Average upper frequency limits of the AMcVEMP TMTF were as high as approximately 300 Hz in young, healthy adults.

Nonlinearity in bone-conducted amplitude-modulated cervical vestibular-evoked myogenic potentials: harmonic distortion products.

Otolith organs of the balance system, the saccule and utricle, encode linear acceleration. Integrity of the saccule is commonly assessed using cervical vestibular-evoked myogenic potentials (cVEMPs) arising from an inhibitory reflex along the vestibulospinal pathway. Conventional approaches to eliciting these responses use brief, transient sounds to elicit onset responses. Here we used long-duration amplitude-modulated (AM) tones to elicit cVEMPs (AMcVEMPs) and analyzed their spectral content for evidence of nonlinear processing consistent with known characteristics of vestibular hair cells. Twelve young adults (ages 21-25) with no hearing or vestibular pathologies participated in this study. AMcVEMPs were elicited by bone-conducted AM tones with a 500-Hz carrier frequency. Eighteen modulation frequencies were used between 7 and 403 Hz. All participants had robust distortion products at harmonics of the modulation frequency. Total harmonic distortion ranged from approximately 10 to 80%. AMcVEMPs contain harmonic distortion products consistent with vestibular hair cell nonlinearities, and this new approach to studying the otolith organs may provide a noninvasive, in vivo method to study nonlinearity of vestibular hair cells in humans.NEW & NOTEWORTHY The otolith balance organs of humans are assessed for basic science and clinical applications by using vestibular-evoked myogenic potentials (VEMPs). Traditionally, VEMPs are elicited with brief, transient sounds to study onset responses. We used long-duration sounds to elicit steady-state VEMPs. This allowed us to measure nonlinear distortion products, consistent with nonlinear processing in vestibular hair cells. This new approach may help to better understand links between otolith organs and balance function.

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

BACKGROUND: Traditional approaches to cervical vestibular-evoked myogenic potentials use a transient stimulus to elicit an onset response. However, alternate approaches with long duration stimuli may allow the development of new methodologies to better understand basic function of the vestibular system, as well as potentially developing new clinical applications. PURPOSE: The objective of this study was to examine the effects of stimulus polarity on response properties of amplitude-modulated cervical vestibular-evoked myogenic potentials (AMcVEMPs). RESEARCH DESIGN: Prospective, repeated-measures, within-subjects design. STUDY SAMPLE: Participants were 16 young, healthy adults (ages 21-38 years). DATA COLLECTION AND ANALYSIS: Amplitude-modulated tones, with carrier frequency of 500 Hz and modulation frequency of 37 Hz, were used to elicit AMcVEMPs. Responses were analyzed in three different stimulus polarity conditions: condensation, rarefaction, and alternating. The resulting data were analyzed for differences across polarity conditions. RESULTS: AMcVEMP amplitudes, both raw and corrected for tonic muscle activation, were equivalent across the different stimulus phase conditions. In addition, response signal-to-noise ratio and phase coherence were equivalent across the different phases of the stimulus. CONCLUSION: Analyses of AMcVEMPs are stable when the carrier frequency starting phase is altered and the phase of the temporal envelope is constant.