DPOAE suppression tuning: cochlear immaturity in premature neonates or auditory aging in normal-hearing adults?

Previous work has shown that distortion product otoacoustic emission (DPOAE) suppression tuning curves (STCs) recorded from premature neonates are narrower than adult STCs at both low and high frequencies. This has been interpreted to indicate an immaturity in cochlear function prior to term birth. However, an alternative explanation for this finding is that adult DPOAE STCs are broadened and reflect cochlear hair cell loss in normal-hearing adults due to aging, and natural exposure to noise and ototoxins. This alternative hypothesis can be tested by studying suppression tuning in normal-hearing school-aged children. If normal-hearing children, who have not aged significantly or been exposed to noise/ototoxins, have DPOAE suppression tuning similar to adults, the auditory aging hypothesis can be ruled out. However, if children have tuning similar to premature neonates and dissimilar from adults, it implicates aging or other factors intrinsic to the adult cochlea. DPOAE STCs were recorded at 1500, 3000, and 6000 Hz using optimal parameters in normal-hearing children and adults. DPOAE STCs collected previously from premature neonates were used for age comparisons. In general, results indicate that tuning curves from children are comparable to adult STCs and significantly different from neonatal STCS at 1500 and 6000 Hz. Only the growth of suppression was not adultlike in children and only at 6000 Hz. These findings do not strongly support the auditory aging hypothesis as a primary explanation for previously observed neonatal-adult differences in DPOAE suppression tuning. It suggests that these age differences are most likely due to immaturities in the neonatal cochlea. However, nonadultlike suppression growth observed in children at 6000 Hz warrants further attention and may be indicative of subtle alternations in the adult cochlea at high frequencies.