Olivocochlear efferent vs. middle-ear contributions to the alteration of otoacoustic emissions by contralateral noise

The medial olivocochlear efferent bundle is the key element of a bilateral efferent reflex activated by sound in either ear and acting directly on coc hlear outer hair cells (OHC) via numerous cholinegic synapses. It probably contributes to regulating the mechanical activity of the cochlea Otoacousti c emissions, being sounds emitted by the cochlea as a reflection of its act ivity and suppressed by efferent activation, are increasingly considered to be the privileged tool for a noninvasive assessment of the efferent reflex . However, confounding effects on otoacoustic emissions can occur. A primar y influence is middle-ear muscle reflex activation, which shares common fea tures with the effects of cochlear efferent activation. We report a systema tic comparison of the responses of human otoacoustic emissions to efferent activation by low-level noise in the contralateral ear to various middle-ea r manipulations (reflex contractions of the stapedius muscle induced by hig h-level contralateral noise, moderate middle-ear pressure changes). The pro files of level and phase changes of otoacoustic emissions as a function of frequency were highly specific to the origin of the effects. The changes in duced by middle-ear manipulations matched the predictions computed from a s tandard lumped-element middle-ear model, with one or two peaks around the r esonance frequency(ies) of the involved subsystem, stapes or tympanic membr ane. In contrast, the efferent effect was completely different, exhibiting a broadband-level suppression associated with a small phase lead. We propos e that a careful vector analysis of otoacoustic emission modifications enab les the identification of the contribution of the efferent reflex without a mbiguity even when it is mixed with middle-ear effects. Thereby, otoacousti c emissions can be used more reliably as noninvasive probes of efferent oli vocochlear function. (C) 2000 Published by Elsevier Science B.V. All rights reserved.