Basic properties of 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). Although this electrical response has been known for many years, it has been ignored by most clinicians and frequently misreported in the li terature. This paper presents the fundamental properties of the PAM electri cal response (the PAMR) and examines wars in which its measurement can be i mproved by altering the standard electrode position and filtering. The resp onse consists of a simple bipolar compound action potential with a first pe ak latency of between 12.5 and 15 ms, depending on the stimulus intensity a nd PAM muscle tone. The largest recordings can be made with an active elect rode over the PAM and with the reference electrode on the dorsal surface of the pinna. It can be obtained with click and tone-burst stimuli within 20 dB of the subjective detection threshold, can be evoked with tone-bursts be tween 500 Hz and 16 kHz and grows either linearly with the click level or a pproximately exponentially with the tone-burst level, reaching a maximum of as large as 250 mu V pp in some subjects. It has a frequency spectrum most ly between 25 and 200 Hz. The response is often visible in raw recordings, with as few as 20 averages required for obtaining a stable waveform. There is very little amplitude and latency difference in stimulating the ear on t he same side or opposite side to the recording electrodes and the binaurall y evoked response is similar to the simple arithmetic sum of the waveforms obtained with monaural stimulation. The response latency and duration are l onger in very young infants, but reach adult values by 12 months of age. In a companion paper, we describe a method of enhancing the PAMR using latera l eye movement (Patuzzi and O'Beirne, 1999a). (C) 1999 Elsevier Science B.V . All rights reserved.