Effects of eye rotation on the sound-evoked post-auricular muscle response (PAMR)

One objective electrophysiological test for deafness involves presenting a brief acoustic stimulus to a subject and measuring the electrical activity evoked in the muscle located just behind the ear (the post-auricular muscle or PAM). We describe a method for enhancing this post-auricular muscle response (PAMR) using lateral eye movement, which increases both the tonic EMG activity in the PAM and the magnitude of the PAMR, and decreases response latency. EMG activity in most subjects tested (more than 30) increased almost instantly on rotation of the eyes, and thereafter grew more slowly with maintained lateral gaze, with the largest increase occurring with eye rotation towards rather than away from the measurement electrodes over the PAM. The EMG activity returned rapidly to near pre-rotation levels when the eyes were returned to the forwards position, with full recovery taking some minutes. While there was a similar increase and return of the PAMR amplitude with eye rotation, the time-course of these changes was somewhat different, largely because the EMG activity and the PAMR amplitude were not proportional. Rather the PAMR amplitude was a saturating function of EMG level, so that the PAMR response did not fall as markedly as the EMG when the eyes were returned to a forwards gaze, and the recovery of the PAMR amplitude to pre-rotation levels appeared to take longer. We discuss the neural mechanisms that may be responsible for this PAMR potentiation with eye movement and discuss its probable role in increasing variability in early studies which did not control for eye movement. We also discuss the utility of eye rotation in potentiating and stabilising the PAMR to allow its use in screening for deafness.