The response of guinea pig primary utricular and saccular irregular neurons to bone-conducted vibration (BCV) and air-conducted sound (ACS).
Level 5 - mechanism / opinion, no new human data
Animal electrophysiology study (guinea pig primary vestibular afferents)
PubMed 26626360 · doi:10.1016/j.heares.2015.10.019
What was done
Single-unit electrophysiological recordings were performed in guinea pigs to measure resting discharge, rate thresholds, and supra-threshold sensitivity functions of irregular primary utricular and saccular afferents. Responses were tested across frequencies from 100 Hz to 3000 Hz using bone-conducted vibration (BCV) and air-conducted sound (ACS). Neuronal locations were confirmed using neurobiotin labeling.
What was found
Many irregular afferents had low or zero resting discharge, with saccular afferents having higher average resting rates than utricular afferents. Most irregular afferents responded to both BCV and ACS. For BCV, utricular and saccular neurons showed similar low activation thresholds averaging ~0.02 g between 100 Hz and 750 Hz, with steep threshold increases above 750 Hz. For ACS, afferents activated across 250 Hz to 3000 Hz with lowest thresholds around 1000–2000 Hz, and saccular thresholds were 15–20 dB lower on average than utricular thresholds. Phase-locking occurred up to ~1500 Hz for BCV and ~3000 Hz for ACS.
Why it matters
This study demonstrates that mammalian irregular otolith afferents from both the utricle and saccule respond in a cycle-by-cycle phase-locked manner to acoustic and vibratory stimuli. This clarifies the physiological mechanisms underlying clinical vestibular evoked myogenic potential (VEMP) testing.
Limits
The study was conducted entirely in an animal model (guinea pigs) under invasive conditions. The abstract does not report the sample size (number of animals or individual neurons recorded) or statistical variance measures.
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