The long-term goal of this project is to investigate the development of human cochlear function. This understanding will provide a framework for more precise identification of cochlear dysfunction, provide support for development and refinement of auditory prostheses and contribute t general theory on human auditory development. The long-term goal will be accomplished by investigating the timecourse and frequency dependent development of (1) cochlear tuning, (2) active cochlear mechanics, and (3) cochlear efferent function using distortion product otoacoustic emission (DPOAE) methodology. DPOAEs are intermodulation products generated by the normal cochlea when it is presented with two tones (f1, f2) simultaneously. They provide a noninvasive, frequency-specific probe of cochlear physiology in humans. This project includes three experiments: 1) In Experiment 1, DPOAE ipsilateral suppression tuning curves will provide an indirect measure of cochlear tuning in neonates ranging from 29 to 36 weeks conceptional age, term infants and normal-hearing adult subjects at five f2 frequencies: 1100, 2500, 5600, 9000 and 12,500 Hz. Suppression tuning curves will be recorded with low-level primary tones to include influence of active cochlear mechanics, and high-leve primary tones to minimize contribution of active cochlear mechanics. Measures of tuning curve width, slope and tip characteristics will be analyzed for age, frequency, and level differences; 2) In Experiment 2, DPOAE growth functions (DPOAE amplitude as a function of primary tone level) will be generated as a measure of active cochlear mechanics or cochlear amplifier activity in human neonates at the same developmental stages and f2 frequencies applied in Experiment 1. DPOAE growth function shape, slope and notch threshold (level of amplitude plateau or decrease) will be analyzed for age and frequency differences; 3) in Experiment 3, contralateral DPOAE suppression will be measured as an index of medial olivocochlear efferent function in human neonates at the same developmental stages and f2 frequencies applied in Experiments 1 and 2. DPOAE growth functions will be recorded with and without broadband noise presented to the opposite ear. The difference between average noise and no noise conditions will be tested to detect contralateral DPOAE suppression. Contralateral DPOAE suppression will be analyzed for age, frequency and level effects. In addition to investigating the independent development of cochlear tuning, active cochlear mechanics and efferent function, these studies will elucidate the relationship among these three aspects of cochlear physiology during human maturation.
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