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  • Title: The development of cochlear frequency resolution in the human auditory system.
    Author: Abdala C, Sininger YS.
    Journal: Ear Hear; 1996 Oct; 17(5):374-85. PubMed ID: 8909885.
    Abstract:
    OBJECTIVE: The objectives of this study were: 1) to evaluate the maturity of cochlear frequency resolution in human neonates, and 2) to further elucidate the differential time course for development of frequency resolution at the cochlear and auditory-neural levels of the auditory system. DESIGN: This paper describes a relatively new technique using distortion product otoacoustic emision (DPOAE) suppression to study cochlear tuning. DPOAE suppression tuning curves (STCs) were generated in 15 normal-hearing adults and 26 healthy, term-born neonates at 1500, 3000, and 6000 Hz. The 2f1-f2 DPOAE was measured in all subjects with primary tones of 65 and 50 dB SPL (L1 > L2) and a 1.22 f2/f1 frequency ratio. Initially, an unsuppressed DPOAE was recorded. After this, a suppressor tone was introduced, and its level varied until DPOAE amplitude was reduced by 6 dB. By plotting the suppressor level required to achieve criterion amplitude reduction by suppressor frequency (for many tones), a DPOAE STC was generated. DPOAE STC shape, width, slope, and tip characteristics were analyzed for both adults and neonates. RESULTS: General shape and appearance of DPOAE STCs were comparable for adults and neonates, as was STC tip frequency and level. Statistical analyses of tuning-curve width (Q) and slope (dB/octave) failed to show age effects, further confirming the similarity between adults and neonates. DPOAE STCs were stable, show minimal intra- and intersubject variability, and closely resemble and behave like physiologic measures of tuning from the VIIIth nerve. CONCLUSIONS: Results suggest that: 1) cochlear tuning and related active processes are basically mature by term birth in the human auditory system, 2) tuning immaturities reported in infants as old as 6 mo of age probably involve auditory-neural immaturities, and 3) suppression of the 2f1-f2 DPOAE seems, to provide an indirect measure of cochlear frequency resolution in humans.
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