Amplitude modulated vestibular evoked myogenic responses: a study of carrier and modulating frequencies

Effects of Age on the Bone-Conduction Amplitude-Modulated cVEMP Temporal Modulation Transfer Function

OBJECTIVES

Cervical vestibular evoked myogenic potentials (cVEMPs) reflect saccular stimulation that results in an inhibitory muscle reflex recorded over the sternocleidomastoid muscle. These responses are utilized to study basic vestibular functions and are also applied clinically. Traditionally, cVEMPs have utilized transient stimuli such as clicks and tonebursts to evoke onset responses. Recently, amplitude-modulated tones have been used to elicit cVEMPs (AMcVEMPs). These AMcVEMP responses can provide information about the magnitude, phase synchrony, and nonlinearities from the vestibulo-collic reflexes that cannot be captured using other existing testing techniques. Although temporal modulation transfer functions (TMTFs) of AMcVEMPs for young, healthy adults have been established using different analysis techniques, there is currently no information regarding the effects of age on these responses. Thus, the current study aimed to examine the effects of age on AMcVEMPs across a broad range of modulation frequencies (MFs) using various AMcVEMP metrics including amplitude, signal to noise ratio (SNR), and phase coherence (PC).

DESIGN

The study included 16 (aged 20 to 39 years) young, 17 (aged 40 to 59 years) mid-age, and 16 (60 to 75 years) older adults with no history of neurological, vestibular, or middle-ear complaints. The stimuli consisted of amplitude-modulated tones with a carrier frequency of 500 Hz and 10 MFs ranging from 11 to 397 Hz. These stimuli were presented using a B81 transducer at 123 dB FL. AMcVEMPs were recorded from the sternocleidomastoid muscle (ipsilateral to the stimulating mastoid) using surface electrodes. Response analysis was performed using an FFT-based approach with analyses including amplitude, SNR, and PC.

RESULTS

AMcVEMP waveforms exhibited periodicity corresponding to the stimulus MF, consistent with previous observations. Furthermore, significant age-related degradation in AMcVEMP amplitude, SNR, and PC measures were observed across a broad range of MFs. While AMcVEMPs were elicited across a wide range of MFs (11 to 263 Hz) for young adults, in mid-age and older adults, these metrics were robust only across a narrower range of MFs, resulting in a reduced TMTF. In addition, the MF eliciting the most robust AMcVEMP varied across different AMcVEMP analysis metrics and age groups.

CONCLUSIONS

A significant decline in the AMcVEMP response is seen as an effect of aging; however, the effect of aging is not uniform across measures or across MFs. The TMTF of AMcVEMP gets flatter, and the overall range is reduced as an effect of aging. Results from this study enhance our understanding of age-related changes in the vestibular system. Expansion of AMcVEMP to clinical populations may lead to a deeper understanding of the pathophysiology of vestibular disorders.

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.

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.

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 non-invasive, in vivo method to study nonlinearity of vestibular hair cells in humans.

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

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.