Steep and shallow phase gradient distortion product otoacoustic emissions arising basal to the primary tones

Abstract

Distortion product otoacoustic emission (DPOAE) level/phase maps were collected in humans with and without an interference tone (IT) near the DPOAE frequency place (f(dp)) at primary-tone levels of 75 dB SPL. A DPOAE component with the expected steep phase gradient could be extracted at f(dp), however, considerable vertical-phase banding, presumably indicative of reflection emissions, remained. An IT placed 0.33 oct above f(2) removed most of this banding, revealing DPOAE components originating basal to the IT frequency place. These findings suggest that the commonly accepted two-source model of DPOAE generation may need to be qualified when higher primary-tone levels are utilized.

Figures

Figure 1

DPOAE L∕P maps obtained with and without an IT placed 44 Hz below fdp. The residual map in (e) demonstrates that DPOAE components can be extracted near fdp that have vertical-phase banding properties in (f) (arrows). Red∕gray dashed line in (h) demonstrates that these emissions have the expected steep phase gradients consistent with a DPOAE-reflection component. However, in the presence of the IT, large DPOAE components remain (c) associated with similar vertical banding (d) and phase gradients as indicated by the blue∕gray line in (i). Black dashed lines on the DPOAE L∕P maps indicate f2∕f1 ratios where data were extracted for the plots in (g) and (h) (see text for complete details of this and Fig. 2).

Figure 2

DPOAE maps obtained with and without an IT placed 0.33 oct above f2. The residual map in (e) demonstrates that by placing the IT above f2, large DPOAE components can be extracted that have phase properties of both distortion and reflection emissions, depending upon the f2∕f1 ratio (f). Shallow and steep phase gradients associated with these two components were confirmed by extracting the phase curves at a wide f2∕f1 ratio in (h), and at a narrow ratio in (i) (dashed red∕gray lines). In (d), almost all of the narrow f2∕f1-ratio vertical-phase banding was removed by the IT for the narrow ratio 2f1-f2 DPOAE, and similarly for the 2f2-f1 DPOAE, suggesting that both of these reflectionlike components originate in regions of the cochlea that are basal to the IT place.