A correlation method for detecting the sound-evoked post-auricular muscle response (PAMR)

We have made detailed measurements of the sound-evoked post-auricular muscl e response (PAMR) in four adults and two infants, in an attempt to understa nd the inter-relationships between sound level, potentiation of the PAMR wi th voluntary PAM contraction or eye rotation, electromyographic (EMG) noise , amplitude of the PAMR, and a correlation measure of the presence of the P AMR. We have found that the amplitude of the PAMR is a simple linear functi on of the decibel level of a monophasic click (0.1 ms duration), and that t he PAMR amplitude is also a saturating power function of the level of tonic EMG. As a result, PAMR = PAM(o).SL.(EMG-EMG(noise))(2)/[(EMG-EMG(noise))(2 )+beta(2) where SL is the decibel level of a click above subjective thresho ld, PAM, is a parameter accounting for the differing PAMR amplitude across individuals or with altered electrode placement, EMG(noise) is the componen t of EMG not associated with PAMR potentiation, and beta determines the ini tial rate of growth of PAMR at low levels of PAM activation. We have also f ound that the correlation measure (C) of the PAMR follows a saturating powe r function of the signal-to-noise ratio (SNR = PAMR/EMG), with C = SNR2/(SN R2+delta(2)), where delta determines the onset of saturation in the correla tion as a function of SNR. The combination of these two relationships means that correlation is a non-monotonic function of the EMG (PAM activation): it can be large for moderate levels of EMG, but small for high levels of EM G, because the PAMR amplitude saturates but the EMG does not. The correlati on is a fast, convenient means of detecting the PAMR, whether using clicks or tone-bursts, and can be used effectively in adults or infants, as long a s the reflex is moderately activated. This moderate activation is most effe ctively produced by eye rotation towards the recording electrodes. (C) 1999 Elsevier Science B.V. All rights reserved.