Single trial detection of human vestibular evoked myogenic potentials is determined by signal-to-noise ratio

Abstract

Vestibular reflexes in humans can be assessed by means of acoustically evoked responses of myogenic origin. For the vestibular-collic pathway this is termed the vestibular evoked myogenic potential (or VEMP) and for the vestibular-ocular pathway the ocular VEMP (or OVEMP). Usually VEMPs require an averaging process to obtain a clear response against the background myogenic activity, but depending on the combination of target reflex and stimulus mode, in some cases clear responses can be observed in single trials without averaging. We aimed to test whether this difference in detectability was simply related to signal-to-noise ratio (SNR), or a manifestation of some other difference in the reflex pathways. In four healthy subjects we recorded VEMPs and OVEMPs in response to 2-ms, 500-Hz sound pips and 10-ms, 100-Hz transmastoid vibrations at four intensity levels, and also determined thresholds. A plot of probability of detection P vs. SNR for all subjects and conditions fell onto a single sigmoid curve. When fitted by a logistic function after linearization a regression yielded an R(2) of 0.89 (n = 64, p < 0.001), with parameter estimates of mu = 2.9 and sigma = 2.0. Three patients with superior canal dehiscence, characterized by significantly lowered thresholds for sound-activated responses, exhibited a similar detection curve. We conclude that single trial detection of evoked myogenic potentials is a property mainly determined by SNR. Thus vestibular reflexes, differing in both their response magnitude and in their levels of myogenic activity by more than an order of magnitude, can be described by a single relationship when their magnitude is expressed relative to background activity, demonstrating the fundamental importance of the SNR.