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

Myogenic artifacts masquerade as neuroplasticity in the auditory frequency-following response (FFR)

The frequency-following response (FFR) is an evoked potential that provides a "neural fingerprint" of complex sound encoding in the brain. FFRs have been widely used to characterize speech and music processing, experience-dependent neuroplasticity (e.g., learning, musicianship), and biomarkers for hearing and language-based disorders that distort receptive communication abilities. It is widely assumed FFRs stem from a mixture of phase-locked neurogenic activity from brainstem and cortical structures along the hearing neuraxis. Here, we challenge this prevailing view by demonstrating upwards of ~50% of the FFR can originate from a non-neural source: contamination from the postauricular muscle (PAM) vestigial startle reflex. We first establish PAM artifact is present in all ears, varies with electrode proximity to the muscle, and can be experimentally manipulated by directing listeners' eye gaze toward the ear of sound stimulation. We then show this muscular noise easily confounds auditory FFRs, spuriously amplifying responses by 3-4x fold with tandem PAM contraction and even explaining putative FFR enhancements observed in highly skilled musicians. Our findings expose a new and unrecognized myogenic source to the FFR that drives its large inter-subject variability and cast doubt on whether changes in the response typically attributed to neuroplasticity/pathology are solely of brain origin.

Inter-trial coherence as a measure of synchrony in cervical vestibular evoked myogenic potentials

BACKGROUND

Cervical vestibular evoked myogenic potentials (cVEMPs) are surface-recorded responses that reflect saccular function. Analysis of cVEMPs has focused, nearly exclusively, on time-domain waveform measurements such as amplitude and latency of response peaks, but synchrony-based measures have not been previously reported.

NEW METHOD

Time-frequency analyses were used to apply an objective response-detection algorithm and to quantify response synchrony. These methods are new to VEMP literature and have been adapted from previous auditory research. Air-conducted cVEMPs were elicited using a 500Hz tone burst in twenty young, healthy participants.

RESULTS

Time-frequency characteristics of cVEMPs and time-frequency boundaries for response energy were established. An inter-trial coherence analysis approach revealed highly synchronous responses with representative inter-trial coherence values of approximately 0.7.

COMPARISON WITH EXISTING METHODS

Inter-trial coherence measures were highly correlated with conventional amplitude measures in this group of young, healthy adults (R² = 0.91 - 0.94), although the frequencies at which these measures had their largest magnitude were unrelated (R² =.02). Conventional measures of peak-to-peak amplitude and latency were consistent with previous literature. Interaural asymmetry ratios were comparable between amplitude- and synchrony-based measures.

CONCLUSIONS

Synchrony-based time-frequency analyses were successfully applied to cVEMP data and this type of analysis may be helpful to differentiate synchrony from amplitude in populations with disrupted neural synchrony.