THE DEVELOPMENT OF COCHLEAR FREQUENCY RESOLUTION IN THE HUMAN AUDITORY-SYSTEM

Objective: The objectives of this study were: 1) to evaluate the matur ity of cochlear frequency resolution in human neonates, and 2) to furt her elucidate the differential time course for development of frequenc y resolution at the cochlear and auditory-neural levels of the auditor y system. Design: This paper describes a relatively new technique usin g distortion product otoacoustic emission (DPOAE) suppression to study cochlear tuning. DPOAE suppression tuning curves (STCs) were generate d in 15 normal-hearing adults and 26 healthy, term-born neonates at 15 00, 3000, and 6000 Hz. The 2f(1)-f(2) DPOAE was measured in all subjec ts with primary tones of 65 and 50 dB SPL (L1 > L2) and a 1.22 f(2)/f( 1) frequency ratio. Initially, an unsuppressed DPOAE was recorded. Aft er this, a suppressor tone was introduced and its level varied until D POAE amplitude was reduced by 6 dB. By plotting the suppressor level r equired to achieve criterion amplitude reduction by suppressor frequen cy (for many tones), a DPOAE STC was generated DPOAE STC shape, width, slope, and tip characteristics were analyzed for both adults and neon ates. Results: General shape and appearance of DPOAE STCs were compara ble for adults; and neonates, as was STC tip frequency and level. Stat istical analyses of tuning-curve width (Q) and slope (dB/octave) faile d to show age effects, further confirming the similarity between adult s and neonates, DPOAE STCs were stable, show minimal intra- and inter- subject 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 m ature by term birth in the human auditory system, 2) tuning immaturiti es reported in infants as old as 6 mo of age probably involve auditory -neural immaturities, and 3) suppression of the 2f(1)-f(2) DPOAE seems to provide an indirect measure of cochlear frequency resolution in hu mans.