Maturation of the human cochlear amplifier: distortion product otoacoustic emission suppression tuning curves recorded at low and high primary tone levels.

The cochlear amplifier shows level-dependent function and works optimally at low levels. For this reason, manipulation of stimulus level is a route through which the human cochlear amplifier can be investigated in a noninvasive manner. Distortion product otoacoustic emissions (DPOAEs) evoked as a function of stimulus level provide a tool for exploration of human cochlear amplifier function and, when applied to neonates, for investigation of cochlear maturation. The current experiment generated 2f1-f2 DPOAE ipsilateral suppression tuning curves (STCs) at three primary tone levels and five f2 frequencies in a large group of premature and term neonates and adults. The differences between tuning generated with low- and high-level primary tones was measured to provide a gross estimate of the "tuning enhancement effect" attributed to the cochlear amplifier. Other features of the DPOAE suppression tuning curves were measured as well. Consistent with previous reports, at 1500 and 6000 Hz, STCs were narrower, with a steeper slope on the low-frequency flank of the tuning curve in premature neonates versus adults. Additionally, only DPOAE STCs from adults and term neonates became markedly broader and more shallow when recorded with high-level primary tones. It has been hypothesized that the excessive narrowness of suppression tuning and the absence of a level effect on DPOAE STCs recorded in premature neonates reflects a subtle immaturity in cochlear amplifier function just prior to term birth.