Rotation of the Eyes (Not the Head) Potentiates the Postauricular Muscle Response

Abstract

The authors investigated how eye and head rotation modulate the human postauricular muscle response (PAMR), to determine the optimal strategy for potentiating the PAMR, or minimizing it to reduce contamination of small neurogenic responses. The authors recorded the PAMR evoked by binaural bipolar clicks (50 dB SL, 360 µsec per phase with 53-msec interval) from behind the right ear of 12 normally hearing adults, and attempted to enhance it with (a) voluntary ear contraction alone, (b) head rotation alone (with the eyes forward-directed and tracking a target attached to the head), or (c) by lateral eye movement alone (toward the right measurement side, with the head facing forward). When the head alone was rotated with eyes fixed relative to the head, the PAMR increased only slightly in some subjects, as did the ongoing electromyography (EMG) (probably due to EMG activity from nearby neck muscles). On returning the head to the forward control position, the PAMR dropped rapidly to control amplitudes. When the eyes alone were rotated, both the EMG and the PAMR increased markedly in most subjects, and returning the eyes to their control position produced a gradual return to control amplitudes. Voluntary PAM contraction (in those subjects who could accomplish it) increased the EMG tone and the PAMR amplitude concomitantly, with vigorous voluntary PAM contraction saturating the PAMR but not the EMG. Head rotation alone was not effective in potentiating the PAMR when lateral eye movement relative to the head was avoided during the head rotation maneuver. When lateral eye movement was controlled appropriately, the PAMR could be potentiated reliably, or reliably avoided when recording smaller neurogenic responses. If head rotation was used to optimize the PAMR without explicit control of eye movement, a false impression of variability was produced.