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 health y term-born neonates for three f(2) frequencies. The 2f(1)-f(2) DPOAE was elicited using f(2)/f(1) = 1.2, L1 = 65 and L2 = 50 dB SPL. A supp ressor tone was presented at frequencies ranging from 1 octave below t o 1/4 octave above fz and varied in level until DPOAE amplitude was re duced by 6 dB. The suppressor level required for 6 dB suppression was plotted as function of suppressor frequency to generate a DPOAE STC. F orward-masked psychoacoustic tuning curves (PTC) were obtained for thr ee 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 f(2) region and STCs are simil ar in shape, width (Q(10)) and slope to VIIIth-nerve TCs. PTCs and STC s measured in the same subject showed similar trends, although PTCs ha d narrower width and steeper slope. Neonatal STCs were recorded at 300 0 and 6000 Hz only and were comparable in shape, width and slope to ad ult STCs. Results suggest: (1) suppression of the 2f(1)-f(2) DPOAE may provide an indirect measure of cochlear frequency resolution in human s and (2) cochlear tuning, and associated active processes in the coch lea, 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.