The long-term goals of this project are: (1) to investigate the final maturational stages of human cochlear function during early postnatal life and (2) to investigate the mechanism or sources underlying immaturity of cochlear function in neonates. This understanding will provide a framework for more precise identification of cochlear dysfunction and support for development and refinement of auditory prostheses. It will also elucidate the importance of specific cochlear elements to hearing and hearing maturation. The first goal will be accomplished by studying suppression and amplitude growth of distortion product otoacoustic emissions (DPOAEs) during early postnatal life. Recent results have clearly shown an immaturity in DPOAE suppression and suppression growth persists beyond term birth, thus compelling investigators to examine the postnatal period to define maturational time course. The second goal will be accomplished by investigating three potential sources of this DPOAE-based immaturity: outer hair cell function, medial efferent system function and middle ear function. Experiment 1 is a longitudinal experiment to record DPOAE ipsilateral suppression and amplitude growth as measures of cochlear function and non-linearity in neonates from birth through 6 months of age at two f2 frequencies (1500 and 6000 Hz) and three primary tone levels. Prematurely born and term-born subjects will be included to examine whether maturation proceeds similarly in these two groups. Measures of suppression tuning curve width, slope, tip and suppression growth slope, as well as characteristics of the DPOAE I/O function, will be analyzed for session effects as neonates increase in age from birth through 6 months. Experiment 2 applies the same DPOAE paradigms (as described above) to subjects with known or induced outer hair cell dysfunction and to neonates. Neonatal DPOAE suppression data will be compared to data from groups with OHC dysfunction to test the hypothesis that OHC immaturity contributes to immature DPOAE suppression in neonates. In Experiment 3 contralateral suppression of DPOAEs, as a test of medial olivocochlear (MOC) system function, will be measured in normal hearing adults and neonates. A continuum of MOC response strength (from """"""""weak"""""""" to """"""""strong"""""""") will be established in these subjects and correlations between DPOAE suppression characteristics and MOC function will be examined. This analysis will test the hypothesis that immature MOC function contributes to immature DPOAE suppression in neonates. Experiment 4 includes the measurement of acoustic energy reflectance as a test of middle ear function in all neonatal subjects. Measures of energy reflectance have been shown to effectively predict variance in OAE level and are sensitive to middle ear immaturity and pathology. Correlations between variance in energy reflectance and variance in DPOAE suppression and amplitude will be examined to elucidate the contribution of an immature middle ear system to immaturities in DPOAE suppression. ? ?
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