Effects of age and hearing loss on overshoot

Abstract

The detection of a brief, sinusoidal probe in a long broadband, simultaneous masker improves as the probe is delayed from the masker's onset. This improvement ("overshoot") may be mediated by a reduction in cochlear amplifier gain over the timecourse of the masker via the medial olivocochlear (MOC) reflex. Overshoot was measured in younger adults with normal hearing and in older adults with normal and impaired hearing to test the hypothesis that aging and cochlear hearing loss result in abnormal overshoot, consistent with changes in certain structures along the MOC pathway. Overshoot decreased with increasing quiet probe thresholds and was only minimally influenced by increasing age. Marked individual differences in overshoot were observed due to differences in masking thresholds for probes presented near the masker's onset. Model simulations support the interpretation that reduced overshoot in hearing-impaired listeners is due to limited cochlear amplifier gain and therefore less gain to adjust over the timecourse of the masker. Similar overshoot among younger and older adults with normal hearing suggests that age-related changes to mechanisms underlying overshoot do not result in significant differences in overshoot among younger and older adults with normal hearing.

FIG. 1.

(Color online) Detection thresholds for 350-ms (left panel) and 6-ms (right panel) pure tone signals. Mean thresholds are shown for the YNH group (squares), ONH group (circles), and OHI group (triangles). Thin lines represent thresholds for individual subjects in these three groups.

FIG. 2.

(Color online) DPOAE measurements. Top panel: mean DPOAE level as a function of f₂, in the YNH group (squares), OHN group (circles), and OHI group (triangles). Error bars are one standard error around the mean. Bottom panel: overall DPOAE response (see text) for each listener is plotted for YNH, ONH, and OHI groups. Asterisks indicate significant differences between groups (p < 0.05).

FIG. 3.

(Color online) Masking thresholds and overshoot. Top and middle panels are masking thresholds for 2− and 198-ms probe delays, respectively. The bottom panels display overshoot, calculated by subtracting the 2-ms thresholds from the 198-ms thresholds. The left and right columns are data for the 2- and 4-kHz probes, respectively. The interpretation of lines is as in Fig. 1. The filled symbols at 100 dB SPL indicate that these thresholds were extrapolated for some OHI subjects due to thresholds being beyond the limits of the equipment.

FIG. 4.

(Color online) GOM slopes plotted against quiet threshold (6-ms probes) for the 2-kHz (left panels) and 4-kHz (right panels) probes, where the probe was delayed by 2 ms (top panels) and 198 ms (bottom panels) from the masker's onset. Results from regression analyses (Pearson's correlation coefficient) are shown in panels. Non-significant correlations are denoted by “n.s.”

FIG. 5.

(Color online) Predicted masking thresholds (lines) in overshoot conditions for the average behavioral data (symbols) obtained with a 2-kHz probe. Top and middle panels show predictions and measured data in the 2− and 198-ms delay conditions. The bottom panels display overshoot, calculated by subtracting the 2-ms thresholds from the 198-ms thresholds.

FIG. 6.

(Color online) The predicted change in maximum OHC gain as a function of masker level based on the model parameter m_GR. Thick lines show gain versus level functions for the best-fitting simulations. Thin lines show additional simulations with rms errors within 0.5 dB of the best-fitting simulation. Dotted, dashed, and solid lines represent YNH, ONH, and OHI listeners, respectively.