Distortion product otoacoustic emission suppression tuning curves in human adults and neonates.

Distortion product otoacoustic emission (DPOAE) iso-suppression tuning curves (STC) were generated in 15 normal-hearing adults and 16 healthy term-born neonates for three f2 frequencies. The 2f1-f2 DPOAE was elicited using f2/f1 = 1.2, LI = 1.2, LI = 65 and L2 = 50 dB SPL. A suppressor tone was presented at frequencies ranging from 1 octave below to 1/4 octave above f2 and varied in level until DPOAE amplitude was reduced by 6 dB. The suppressor level required for 6 dB suppression was plotted as function of suppressor frequency to generate a DPOAE STC. Forward-masked psychoacoustic tuning curves (PTC) were obtained for three of the adult subjects. Results indicate that DPOAE STCs are stable and show minimal inter- and intra-subject variability. The tip of the STC is consistently centered around the f2 region and STCs are similar in shape, width (Q10) and slope to VIIIth-nerve TCs. PTCs and STCs measured in the same subject showed similar trends, although PTCs had narrower width and steeper slope. Neonatal STCs were recorded at 3000 and 6000 Hz only and were comparable in shape, width and slope to adult STCs. Results suggest: (1) suppression of the 2f1-f2 DPOAE may provide an indirect measure of cochlear frequency resolution in humans and (2) cochlear tuning, and associated active processes in the cochlea, are mature by term birth for at least mid- and high-frequencies. These results provide significant impetus for continued study of DPOAE suppression as a means of evaluating cochlear frequency resolution in humans.