Evidence for basal distortion-product otoacoustic emission components
DPOAE L/P maps for a normal rabbit measured at SPL demonstrating the lack of measurable reflection emissions from . DPOAE L/P maps were collected without [(A) and (B)] and with [(C) and (D)] a 45-dB SPL IT placed near . The vector difference is shown as residual maps [(E) and (F)]. Emission levels as a function of DPOAE frequency are shown in the level plots [(A), (C), and (E)], and their corresponding phases as phase plots [(B), (D), and (F)]. In the control phase condition (B), both horizontal (white arrows at ) and vertical (black arrows at ) phase patterns were apparent. In the IT condition [(C) and (D)], there was no obvious effect of the IT 44 Hz below on the DPOAE L/P maps. White arrow on the residual map in (E) points to a small DPOAE component the IT extracted. However, this emission had horizontal phase banding [white arrow in mid-(F)] consistent with a distortion component as opposed to a reflection-based DPOAE. On these DPOAE L/P maps, the black dashed horizontal lines on the level and phase plots demark the ratio for acquiring commonly measured DP-grams, where horizontal phase banding predominates, and narrow-ratio DP-grams where vertical phase banding is most apparent. On all DPOAE L/P maps, data for the and DPOAEs are displayed above and below, respectively, the white dashed horizontal line at (not labeled). The thin white diagonal dashed lines through the phase plots [(B), (D), and (F)] of this figure represent constant trajectories beginning at 15 kHz in the top right of each plot and ending at 1 kHz in the lower left region that can be used as a reference for this and other DPOAE L/P maps. See text for complete details of this and other figures.
Cartoon predicting the effects of basal DPOAE-generator components on corresponding DP-grams for ears exhibiting either a low- (A) or high-frequency (B) OHC lesion. Frequency is depicted along the abscissa from low (apex) to high (base) as indicated by the frequency arrow at the bottom of the figure. Dashed line in depicts a hypothetical low-frequency OHC lesion that abruptly returns to normal . Dashed line at illustrates the opposite situation where normal function exhibits a precipitous drop at the beginning of the high-frequency OHC lesion . A notched-like loss pattern would be expected to resemble a combination of these two situations. In all cases, the presentation of a 1/3-oct IT is assumed to remove DPOAE components generated basal to (, , , and ). The residual in both scenarios represents the basal components that were removed (, , , and ). For the low-frequency loss cartoon, the DP-gram is filled in by basal DPOAE components in , with the loss being accurately revealed by the ADP-gram in . The RDP-gram of reveals the DPOAEs removed by the IT. For the high-frequency loss counterpart, DP- and ADP-grams are relatively unaffected by the presence of basal DPOAE components. However, for the RDP-gram in , the loss appears to extend to lower frequencies, because DPOAE components generated basal to can be envisioned as encountering the loss region first when the primaries are swept from low (apex) to high frequencies (base).
Comparison of DP- (open squares), ADP- (solid circles), and RDP-grams (solid triangles) as a function of for the normal ear of rabbit RV5 showing that basal DPOAE components (RDP-gram) can be extracted in normal ears. Except for the lowest optimal unequal-level primary-tones (F), ADP-gram levels closely tracked DP-grams demonstrating that the 1/3-oct IT had little effect on near- DPOAE components. In most cases, RDP-gram levels were similar to those of the DP-grams indicating that basal DPOAE components removed by the IT were sizable in this normal ear.
Comparison of DP- (open squares), ADP- (solid circles), and RDP-grams (solid triangles) as a function of in the low-frequency loss ear of rabbit RV5-R demonstrating that basal DPOAE components caused the high-frequency extent of the presumed OHC loss to be underestimated. Thin black lines plot DPOAE levels for the normal ear (see Fig. 3) for comparison. For this ear, ADP-grams even at relatively high primary-tone levels, e.g., (A) or 65 (B) dB SPL, more closely tracked the lesion pattern revealed by lower-level primaries [e.g., (E)].
Plots of ADP-gram (solid circles) and ABR threshold (solid squares) differences computed between the normal and noise-exposed ears of rabbit RV5 substantiate the functional losses estimated by the ADP-gram.
Control experiments to rule out the possibility of residuals based on suppression of components by the IT in a normal ear (A), or residuals due to the increased effects of the IT on components resulting from changes in cochlear tuning in a noise-damaged ear (B). In the normal ear (A), the suppressive effect of the IT on the DP-gram (solid circles) can be seen when is at or above 8 kHz. Little or no suppression occurs for this condition below approximately 5 kHz. The existence of the residual (solid triangles) at frequencies almost 3 oct below the IT and the systematic growth of this component as the tails of primary-tone TW envelopes approached the IT strongly support the notion that the residual represents basal DPOAE components removed by the IT. Unlike in the normal ear, in the damaged ear (B), DPOAEs were decreased in the presence of the IT (solid circles vs open squares) at similar low frequencies. Since near- DPOAE components were presumably absent in this damaged region and the residual magnitude matched the DP-gram, these DPOAEs appeared to be largely composed of basal components removed by the remote 8-kHz IT.
DPOAE L/P maps [(A)–(F)] measured at SPL showing that the IT in (D) eliminated vertical phase banding present in the control phase map [(B) black arrows near ] for this normal ear of rabbit RV5-L. Note how the 75-dB SPL 1/3-oct IT [(C) and (D)] also removed amplitude notches [(A) white arrow] and phase discontinuities obvious in (B) indicative of the mixing of DPOAE components.
DPOAE L/P maps [(A)–(F)] collected at SPL in the low-frequency loss ear of rabbit RV5-R provided evidence consistent with the notion that basal DPOAE components removed by the IT filled in the apical side of the lesion. In (C) (white arrows), the 1/3-oct IT removed essentially all DPOAEs present in (A) (white arrows) below the estimated high-frequency extent of the lesion indicated by the bold dashed line along a 5-kHz constant trajectory through the map. Also, vertical phase banding and abnormally wide bands in (B) (black arrow near ) are absent in (D). Panel (E) shows the levels of the DPOAE components and their phases (F) that the IT removed.
DP-grams for rabbit RV12-L with a notched-loss pattern demonstrating that the ADP-grams (solid circles) again more clearly revealed the notched loss [(e.g., (B) and (D)] tracked by the low-level DP-grams [(E) and (F)]. Note the pronounced effects of the 1/3-oct IT in the unequal-level optimal primary-tone conditions [(D) and (F)]. As predicted in Fig. 2(B), the RDP-grams (solid triangles) dropped in level before the high-frequency extent of the lesion are encountered.
Plots of ADP-gram (solid circles) and ABR threshold (solid squares) differences computed between the normal and noise-exposed ears of rabbit RV12-L corroborating the functional changes estimated by the ADP-gram.
DPOAE L/P maps measured at SPL from the normal ear of rabbit RV12-R showing that the presence of a 1/3-oct IT set to 50 dB SPL (C) decreased the overall level of DPOAEs, and dramatically removed low-frequency DPOAEs (white arrow) as compared to the control condition (A). The substantial residual (E) demonstrates that even at low optimal unequal primary-tone levels large DPOAE components appear to be generated basal to . At these less intense primary-tone levels, no DPOAEs were detectable in the lower halves of the DPOAE L/P maps.
DPOAE L/P maps measured at SPL for rabbit RV12-L with a notched-like loss pattern demonstrating that the IT (C) removed basal DPOAE components apparent in the control (A) DPOAE L/P condition (white arrow). The IT aligned the DPOAEs with the estimated high-frequency edge at (bold dashed diagonal at upper right) of the lesion (C), an effect similar to that observed for the DP-gram in Fig. 9(F). In (E), the loss detected by the residual extended to lower frequencies below the bold dashed diagonal at 5 kHz, as predicted in Fig. 2 (line ).
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