Adaptation of distortion product otoacoustic emission in humans

Previous studies of animals observed a phenomenon of adaptation of distorti on product otoacoustic emission (DPOAE) and found that the phenomenon was m ediated to a large extent by the medial olivocochlear (MOC) reflex. The pre sent study investigated DPOAE adaptation in humans. The following stimuli w ere used: f(2)/f(1) = 1.2; f(2) = 2, 4, or 5.65 kHz; L-2 = 50-65 dB SPL re 20 mu Pa rms, L-1 - L-2 = 0-15 dB, where L-1 and L-2 represent levels of th e f(1) and f(2) tones, respectively; duration of two-tone burst = 5.5 s; in terburst gap 20 or 30 s; number of repetitions = 40 or 64, We analyzed the (2)f(1) - f(2) DPOAE as a function of time using a method of heterodyne env elope detection. The subjects were 20 humans aged from 15 to 54 years (medi an = 21 years) with normal hearing. We observed that (1) humans exhibited D POAE adaptation phenomenon; (2) the time course of DPOAE level was characte rized by a 2-exponential function; (3) distributions of the fast and slow t ime constants were well, separated with their median values being 69 ms and 1.51 s, respectively; (4) distributions of the magnitudes of the fast and slow adaptation components were largely overlapped with their median values being 0.65 and 0.40 dB, respectively; and (5) the combined magnitude of th e adaptation ranged from 0.4 to 3.0 dB with a median of 1.10 dB. To our kno wledge, the present study is the first published article to describe adapta tion of DPOAE in humans. These results should help advance the basic knowle dge of human cochlear mechanics operating under the control of the MOC feed back system and contribute to the development of practical applications suc h as identifying people at high risk of acoustical injury and a clinical te st of the functional status of the MOC system.