GROWTH OF THE 2F(1)-F(2) DISTORTION-PRODUCT OTOACOUSTIC EMISSION FOR LOW-LEVEL STIMULI IN HUMAN NEONATES

Distortion product otoacoustic emissions (DPOAEs) for low stimulus lev els (<60 dB SPL) have been reported in adult humans under ideal condit ions. In neonates, DPOAEs have been reported only for high-level stimu li. The purpose of this paper was to determine characteristics of the 2f(1)-f(2) DPOAE for low-level stimuli in neonates and to assess the f easibility of obtaining such measures in a noisy environment. Subjects were 19 premature neonates presumed to have normal hearing based on s ystematic pneumatic otoscopy measures and evoked auditory brainstem re sponses. For stimuli centered at 2000 and 6000 Hz and presented over a range of 30 to 75 dB SPL, DPOAEs were measured employing linear time averaging for up to 128 time frames at each level. In quiescent subjec ts, the level of the noise floor was as low as that reported in cooper ative adults under ideal conditions (similar to-30 dB SPL), and the fu nctions were identical. That is, valid measures were obtained for very low stimulus levels (30 dB SPL), the rate of growth approached 1 dB/d B, and identical nonmonotonicities (saturation, plateaus, and notches) were observed as those reported for adults. When the noise floor was elevated due to subject activity, no valid data could be obtained for low-level stimuli even though the DPOAEs were at expected levels for h igh-level stimuli. These results have important implications for the u se of such measures in this population because the DPOAEs associated w ith the metabolically active nonlinear cochlear processes at low stimu lus levels may be contaminated with DPOAEs associated with other proce sses at high stimulus levels.