Olivocochlear reflex effect on human distortion product otoacoustic emissions is largest at frequencies with distinct fine structure dips.

Activity of the medial olivocochlear efferents can be inferred by measuring the change of the level of distortion product otoacoustic emissions (DPOAE) during ipsilateral or contralateral acoustic stimulation, the so-called medial olivocochlear reflex (MOCR). A limitation of this measurement strategy, however, is the distinct variability of MOCR values depending on DPOAE primary tone levels and frequency, which makes selection of the stimulus parameters difficult. The objective of this study was to evaluate the dependence of MOCR values on DPOAE fine structure in humans. MOCR during contralateral acoustic stimulation was measured at frequencies with distinct non-monotonicity ("dip") in the DPOAE fine structure, and in frequencies with flat fine structure. One hundred and twenty one different primary tone level combinations were used (L(1)=50-60dB SPL, L(2)=35-45dB SPL, 1dB steps). The measurement was repeated on another day. The major findings were: (1) Largest MOCR effects can be found in frequencies which exhibit a distinct dip in DPOAE fine structure. (2) Primary tone levels have a critical influence on the magnitude of the MOCR effect. MOCR changes of up to 23dB following a L(1) change of only 1dB were observed. Averages of the maximum MOCR change per 1dB step were in the 3-5dB-range. Both findings can be interpreted in the light of the DPOAE two-generator model [Heitmann, J., Waldmann, B., Schnitzler, H.U., Plinkert, P.K., Zenner, H.P. 1998. Suppression of distortion product otoacoustic emissions (DPOAE) near 2f1-f2 removes DP-gram fine structure - evidence for a secondary generator. Journal of the Acoustical Society of America 103, 1527-1531]. According to the present results we propose, that assessing MOCR specifically at frequencies with a distinct dip in the DPOAE fine structure, in combination with fine variation of the stimulus tone levels, allows for a more targeted search for maximum MOCR effects. Future studies must show if this approach can contribute to the further clarification of the physiological roles of the olivocochlear efferents.

Olivocochlear activity and temporary threshold shift-susceptibility in humans.

STUDY OBJECTIVES: Animal studies (guinea pig, cat, chinchilla) have shown that activity of the medial olivocochlear efferents can exert noise-protective effects on the cochlea. It is not yet known whether such effects are also existent in humans. Olivocochlear activity can be estimated indirectly by contralateral suppression (CS) of otoacoustic emissions (OAE). MATERIAL AND METHODS: We measured Input/Output functions of distortion products of OAE (DPOAE), with and without contralateral acoustic stimulation by white noise, in 94 normal hearing young male subjects. Seven stimuli with L2 between 20 and 60 dB SPL and L1 = 39 dB + 0.4 L2 ("scissor paradigm") were used at f2 = 2, 3, 4, 5, and 6 kHz. The measurement was repeated 2 weeks later. In 83 subjects of the same group, pure tone audiometry was registered before and 6 minutes after shooting exercises to evaluate individual susceptibility to develop a temporary threshold shift (TTS). RESULTS: Test-retest repeatability of CS was generally good. CS averaged 0.98 dB SPL (SD 1.19 dB, median 0.56 dB). As expected, CS was greatest at low stimulus levels (median 1.06 dB at L2 = 20 dB, as compared with 0.33 dB at L2 = 60 dB). The smallest average CS was found at 4 kHz, and the greatest CS appeared at 2 kHz. A TTS occurred in 7 of 83 (8.5%) subjects. Statistical analysis did not reveal any correlation between the amount of CS and individual TTS susceptibility. CONCLUSIONS AND OUTLOOK: 1) Measurement of CS of DPOAE using an extensive measurement paradigm revealed good test-retest repeatability, confirming the reliability of this audiologic tool. 2) CS of DPOAE does not predict individual susceptibility to mild TTS induced by impulse noise in humans. Possible explanations for the missing association are discussed. Future perspectives include longitudinal studies to further elucidate the association between medial olivocochlear bundle-activity and permanent threshold shift in humans. The goal is to develop a diagnostic tool for the prediction of individual noise vulnerability in humans, thereby preventing noise-induced hearing loss.