LOCUS OF GENERATION FOR THE 2F(1)-F(2) VS 2F(2)-F(1) DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS IN NORMAL-HEARING HUMANS REVEALED BY SUPPRESSION TUNING, ONSET LATENCIES, AND AMPLITUDE CORRELATIONS

The present study used distortion-product otoacoustic emission (DPOAE) suppression tuning curves (STCs), DPOAE onset latencies (OLs), and DP OAE amplitude correlations to investigate the locus of generation of t he 2f(1)-f(2) DPOAE versus the 2f(2)-f(1) DPOAE in humans. The results of the tuning study revealed that, for the 2f(1)-f(2) DPOAE, the tips of the STCs tuned consistently below the geometric-mean (GM) frequenc y of the primary tones. In contrast, for the 2f(2)-f(1) DPOAE, STCs tu ned above the GM of the primaries, with 50% of the tip frequencies at, or above, the 2f(2)-f(1) frequency place. When the average ratio of t he 2f(2)-f(1) to the 2f(1)-f(2) tip frequencies was computed, a factor of 1.44 provided an estimate of the frequency shift needed to align t he two DPOAE generation sites. Other results showed that OLs for the 2 f(2)-f(1) DPOAE were uniformly shorter than those for the 2f(1)-f(2), with differences at the low frequencies amounting to as much as 6-7 ms . Further, for both DPOAEs, curves describing latency decreases as a f unction of increasing GM frequencies were best fit by power functions. Shifting the GM frequency producing the 2f(2)-f(1) DPOAE by a factor of 1.6 caused the latency distributions for both DPOAEs to overlap thu s resulting in a single function that described cochlear delay as a fu nction of GM frequency. Finally, for each GM frequency in the DP-gram, sliding correlations from 108 normal ears were performed on both DPOA Es by holding the primaries producing the 2f(1)-f(2) DPOAE constant, w hile all 2f(2)-f(1) DPOAE amplitudes were successively correlated with the 2f(1)-f(2) amplitudes. This procedure demonstrated that, for a gi ven GM frequency producing the 2f(1)-f(2), the correlations between th e two DPOAEs peaked when the primaries of the 2f(2)-f(1) were at a GM frequency that positioned the 2f(2)-f(1) frequency place near the GM o f the primaries that produced the 2f(1)-f(2) DPOAE. As a whole, the ab ove findings strongly suggest that the 2f(2)-f(1) DPOAE in humans is g enerated basal to the primary-tone place on the basilar membrane. (C) 1998 Acoustical Society of America.