Low-frequency and high-frequency distortion product otoacoustic emission suppression in humans

Abstract

Distortion product otoacoustic emission suppression (quantified as decrements) was measured for f(2)=500 and 4000 Hz, for a range of primary levels (L(2)), suppressor frequencies (f(3)), and suppressor levels (L(3)) in 19 normal-hearing subjects. Slopes of decrement-versus-L(3) functions were similar at both f(2) frequencies, and decreased as f(3) increased. Suppression tuning curves, constructed from decrement functions, were used to estimate (1) suppression for on- and low-frequency suppressors, (2) tip-to-tail differences, (3) Q(ERB), and (4) best frequency. Compression, estimated from the slope of functions relating suppression "threshold" to L(2) for off-frequency suppressors, was similar for 500 and 4000 Hz. Tip-to-tail differences, Q(ERB), and best frequency decreased as L(2) increased for both frequencies. However, tip-to-tail difference (an estimate of cochlear-amplifier gain) was 20 dB greater at 4000 Hz, compared to 500 Hz. Q(ERB) decreased to a greater extent with L(2) when f(2)=4000 Hz, but, on an octave scale, best frequency shifted more with level when f(2)=500 Hz. These data indicate that, at both frequencies, cochlear processing is nonlinear. Response growth and compression are similar at the two frequencies, but gain is greater at 4000 Hz and spread of excitation is greater at 500 Hz.

Figure 1

Top row: Mean DPOAE (squares) and noise (triangles) levels in dB SPL as a function of suppressor level, L₃, in dB SPL. Bottom row: Mean DPOAE decrements (circles) in dB as a function of L₃. Left column shows data when f₂=500 Hz; right column shows data when f₂=4000 Hz. In both cases, L₂=40 dB SL, and an on-frequency suppressor (f₃) was used (f₃=515 and 4100 Hz for f₂=500 and 4000 Hz, respectively). Closed symbols represent the transformed data points and the line represents a linear regression fit to the closed symbols. In all cases, error bars represent ±1 s.d. The short vertical dashed lines in the bottom row of panels are drawn at a decrement of 3 dB, which was used as suppression threshold for the construction of STCs.

Figure 2

DPOAE and noise levels for the control conditions, in which there was no suppressor. Top: Mean DPOAE (squares) and noise (triangles) levels in dB SPL as a function of L₂ in dB SL when f₂=500 Hz. Bottom: Mean DPOAE (squares) and noise (triangles) levels in dB SPL as a function of L₂ in dB SL when f₂=4000 Hz. In both panels, error bars represent ±1 s.d.

Figure 3

Mean DPOAE decrements in dB as a function of L₃ in dB SPL when f₂=500 Hz and L₂=30 dB SL. Error bars represent ±1 s.d. Suppressor frequency is indicated within each panel. Closed symbols represent the transformed data to which a linear regression, represented by the solid line, was fit. The short vertical dashed lines in each panel are drawn at a decrement of 3 dB, which was used as suppression threshold for the construction of STCs.

Figure 4

Mean DPOAE decrements in dB as a function of L₃ in dB SPL when f₂=4000 Hz and L₂=30 dB SL. Error bars represent ±1 s.d. Suppressor frequency is indicated within each panel. Closed symbols represent the transformed data to which a linear regression, represented by the solid line, was fit. The short vertical dashed lines in each panel are drawn at a decrement of 3 dB, which was used as suppression threshold for the construction of STCs.

Figure 5

Mean slopes of the decrement-vs-L₃ functions as a function of f₃ for f₂=500 Hz (circles) and f₂=4000 Hz (triangles). Data are plotted on a log frequency scale in the left column and a relative (octave) frequency scale in the right column.

Figure 6

Mean suppression tuning curves, in which the L₃ for 3 dB of suppression is plotted as a function of f₃, following the conventions used in Fig. 5.

Figure 7

Slopes of the decrement functions as a function of f₃, based on multiple linear regression in which L₂ and L₃ were included in the regression analysis. Open symbols represent the slopes of the decrement-vs-L₃ functions and closed symbols represent the slope of the decrements vs L₂. Circles represent data when f₂=500 Hz and triangles represent data when f₂=4000 Hz.

Figure 8

Mean STCs for 500 and 4000 Hz. Top: STCs constructed using simple linear regressions that were fit to the decrement-vs-L₃ functions. Bottom: STCs constructed using multiple linear regressions that were fit to the decrement-vs-L₃ functions. Within each panel, L₂ increases from the STC with the lowest suppression threshold to the highest. Superimposed in both panels are the mean behavioral thresholds (closed circles) for all the f₂ and f₃ frequencies used in the present study.

Figure 9

L₃ for 3 dB of suppression as a function of L₂ (dB SL) for on-frequency (closed squares) and low-frequency (open squares) suppressors. The low-frequency suppressor was approximately 1 octave below f₂. Top and bottom rows show results for simple and multiple linear regressions, respectively. Left and right columns show data for f₂=500 Hz and f₂=4000 Hz, respectively. The lines represent linear fits to each set of data. Slopes of these lines are provided as insets adjacent to the line to which they apply.

Figure 10

Tip-to-tail difference (in dB) as a function of L₂ (dB SL) when f₂=500 Hz (open circles) and when f₂=4000 Hz (closed circles). Top and bottom panels show the results for simple and multiple linear regressions, respectively.

Figure 11

Q_ERB as a function of L₂ (dB SL) following the conventions used in Fig. 10.

Figure 12

Best frequency in octaves (re: f₂) as a function of L₂ (dB SL), following the conventions used in Figs. 1011. The dashed line is drawn at 0 octaves relative to f₂ and provides a point of reference.

Figure 13

Tip-to-tail difference as a function of Q_ERB, following the convention used in Figs. 101112. Circles represents data for 500 Hz and triangles represent data for 4000 Hz. For both frequencies, the point with the lowest tip-to-tail difference∕Q_ERB represents results for the highest L₂ level (50 dB SL at both frequencies) and the points with the largest Q_ERB∕tip-to-tail difference represent results for the lowest L₂ levels (20 dB SL at 500 Hz and 10 dB SL at 4000 Hz). Lines in each panel (SLR and MLR) represent linear fits to the data.