Associations between permanent ABR threshold shifts and DPOAE losses following noise exposure. (A) Mean ABR threshold shifts compared to mean DP-gram alterations for four primary-tone levels ( SPL) that are arranged in approximate ascending order, with the lowest level primary-tone combination being closest to the abscissa (thin solid line). Circled letters point to the average DP-gram associated with the corresponding correlation plots below. ABR threshold shifts were largest in the region of the exposure band and decreased toward the high frequencies. DPOAE changes tended to be bimodal with large losses from about and , with a less affected region between approximately 5 and . (B)–(E) correlations between ABR threshold shifts (abscissa) and DPOAE losses (ordinate) at corresponding frequencies for all noise-exposed ears for the primary-tone levels indicated at top left of each plot. Significant correlations were obtained between ARB threshold shifts and DPOAE changes for all primary-tone levels, with the strongest correlation at the lowest primary-tone level (E). In this and subsequent figures, the stippled bar at the lower left indicates the frequency extent of the OBN exposure. Error sd.
Examples of suppression-contour plots showing details of the amount of suppression in a normal rabbit ear [i.e., the left ear (L) of rabbit RD6] as a function of ST level and frequency for two the ST conditions and four primary-tone levels. (A)–(D) Results for an ST at near the place. (E)–(H) results for an ST 1 octave ( oct) below . It can be seen that STs near (left column) were more effective suppressors with lower suppression thresholds (i.e., contours) than STs near the “tail” of the STC (right column). These contour plots reveal many anomalies in the amount of suppression as a function of frequency and irregularities in the spacing of the contour lines across frequency indicative of great variability in SG slope.
Example from the right (R) ear of rabbit RD1 showing “moderate” noise damage. Upper panels show pre- (A, B) and postexposure (C, D) suppression contours corresponding to the DP-gram conditions described below. It is difficult to observe any systematic effects of noise on these plots as a function of , except from about where the DPOAEs were reduced to NF levels. It can be seen for this rabbit that suppression thresholds represented by the contours remained relatively unchanged across frequency for both conditions. Bottom plots (E, F) show DP-grams (repeated in both right and left columns) obtained before noise exposure (solid circles) and at postexposure (open circles). Preexposure SG slopes (solid squares) compared to their postexposure counterparts (open squares) are shown for (E) and oct (F). In this rabbit SG slopes tended to be slightly shallower over the lower frequencies, where noise-induced DPOAE losses were greatest, than at the higher ones. However, it is clear that SG slope changes did not adequately follow the pattern of DPOAE losses as revealed in the related DP-grams.
Examples from the R ear of rabbit RD2 showing “severe” noise damage. In this rabbit DPOAEs were essentially reduced to the NF over the low- and high-frequency regions. Suppression contours [(A)–(D) show postexposure “gaps” (C, D) where DPOAEs approached the NF. In the region of measurable DPOAEs, there were no clear changes in SG, along with little change in suppression thresholds. Below, DP-grams (E, F) obtained before (solid circles) noise exposure are compared to their postexposure (open circles) counterparts. In this rabbit, although DPOAEs were reduced to the NF at low and high frequencies, there was a region of relatively normal DPOAEs centered at . Corresponding measure of SG slopes before (solid squares) and after (open squares) exposure are plotted for the two ST conditions (i.e., , oct). Overall, there was no apparent tendency for SG slopes to track the DPOAE-loss pattern on either the ascending or descending side of the area of normal DPOAEs centered at .
Mean DP-grams and SG slopes computed before and after noise exposure for all four primary-tone levels. Average postexposure (open squares) DP-grams [(A)–(D)] clearly revealed consistent and significant losses as compared to preexposure levels (solid circles) regardless of primary-tone level. Similar comparisons for average SG slopes computed at and oct demonstrated only two significant and seemingly spurious differences. However, at the two lowest primary-tone levels with STs at oct (K, L), SG slopes appeared to show a consistent trend towards reduced values in the region. The double and triple asterisks along the top of the plots indicate statistically significant differences at the and levels, respectively.
Mean suppression thresholds and tip-to-tail measures computed before (solid circles) and after (open squares) noise exposure for four primary-tone levels. It can be seen that there were no systematic changes in these derived DPOAE-suppression measures across rabbits.
Correlations computed between DPOAE losses and SG-slope changes at four primary-tone levels following noise exposure. SG slopes tended to decrease with increasing modifications in DPOAE levels in response to STs near [(A)–(D)] as well as for STs an octave below [(E)–(H)]. This relationship was significant in the majority of instances, thus establishing a tendency for SG slope to become shallower postexposure as DPOAEs decreased from their preexposure levels. In this and the subsequent figure, negative values on the abscissa represent instances when DPOAEs levels were enhanced following noise exposure.
Relation of suppression-threshold and tip-to-tail changes to reductions in DPOAE levels at four primary-tone levels. Suppression thresholds at showed little evidence of a consistent correlation with changes in DPOAEs. However, both suppression thresholds at oct and tip-to-tail measures showed reliable decreases with noise-induced reductions in DPOAE levels.
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