Maturation of medial efferent system function in humans

Otoacoustic emissions are typically reduced in amplitude when broadband noi se is presented to the contralateral ear. This contralateral suppression is attributed to activation of the medial olivocochlear system, which has an inhibitory effect on outer hair-cell activity. By studying the effects of c ontralateral noise on cochlear output at different stages of auditory matur ation in human neonates, it is possible to describe the timecourse for deve lopment of medial efferent system function in humans. The present study rec orded 2f1-f2 distortion product otoacoustic emissions (DPOAE) in human adul ts, term and premature neonates at three f2 frequencies: 1500, 3000, and 60 00 Hz, using fixed primary tone frequency ratio (f2/f1 = 1.2) and level sep aration (10 dB, L1 > L2). Average DPOAE growth functions were recorded with and without contralateral broadband noise. Results indicate that contralat eral suppression of DPOAEs is absent at 6000 Hz, but present at 1500 and 30 00 Hz for all ages. However, DPOAE amplitude from premature neonates was no t altered by noise in an adult-like manner; in this age group, DPOAE amplit ude was equally likely to be suppressed or enhanced by noise presented cont ralaterally. Contralateral enhancement may reflect a temporary stage of imm aturity in outer hair cell-medial efferent fiber synapses just prior to ter m birth. (C) 1999 Acoustical Society of America. [S0001-4966(99)04504-X].