(Color online) DPOAE time waveforms observed for a typical f 2/f 1-ratio series for the R ear of rabbit RV36. In particular, at the narrow ratios (f 2/f 1 = 1.13 to 1.01) shown in panels (E)–(H), short-latency onset complexities (black arrows) were observed around 0.5 to 1.5 ms. At about 1.5 ms, the overall amplitude of the waveform decreased, which is consistent with the notion that longer-latency f 2 components were now canceling the early onset of basal-DPOAE components with shorter latencies. At the offset of f 2 [open arrowheads in panels (F)–(H)], as the canceling short-latency basal components decayed, a large component presumably generated nearer to f 2 became apparent. These more apical components required approximately 3.5 ms to decay [dashed arrows in panels (F) and (G)] representing a latency more consistent with the f 2 = 4 kHz cochlear location than the initial 0.5-ms complexity observed as the primary tones were turned on. The bottom trace in panel (I) demonstrates that there was little ringing of the f 2 stimulus in the ear canal, but control experiments demonstrated that it did not affect the results (see Sec. II C above). Vertical lines at 5 and 11 ms in this and subsequent figures indicate the offset and onset of f 2, respectively. Note that for clarity the DPOAE time waveforms are plotted on arbitrary magnitude scales so that the largest component is full scale. Consequently, DPOAE levels cannot be directly compared across experimental conditions. However, values in parentheses at the top right of each plot indicate the level of the corresponding DPOAE at steady state in dB SPL for all time waveforms in each figure.
(Color online) Results for the R ear of rabbit RV35 demonstrating that complexities observed in the DPOAE time waveforms are not due to reflection components from f dp. In panel (A), an onset complexity was present that disappeared as the DPOAE time waveform reached steady state after about 2.5 ms. In panel (B), when f 2 was turned off at 5 ms, a complexity was observed as the canceling of distributed components diminished allowing one group of emission components to dominate. In panel (C), the complexities were unaffected when a 50-dB SPL IT was placed 44 Hz below f dp. In a post-condition shown in panel (D), in the absence of the IT, the waveform complexities (black arrow) were essentially identical to the initial [panel (B)] and IT conditions [panel (C)] displayed above indicating that reflection components from f dp were not responsible for the observed waveform complexities, and that the basis of their origin must lie elsewhere.
(Color online) Results from the R ear of rabbit RV36 demonstrating that DPOAE time waveform complexities originate from a restricted BM region above f 2. In panel (A), DPOAE complexities at the onset of the primaries (black arrow at around 1 ms) and offset of f 2 (vertical line at 5 ms) are easily appreciated. In panel (B), an 8-kHz IT presented an octave above the f 2 = 4-kHz frequency removed these complexities. However, in panel (C), when the IT was increased another octave in frequency to 16 kHz, little effect of the IT was observed. After these manipulations, it was still possible, as seen in panel (D), to reasonably replicate the initial DPOAE time waveform, thus demonstrating the stability of the results. These findings are consistent with the suggestion that at an octave above f 2, the IT removed DPOAE components originating basal to f 2 that were responsible for the short-latency complexity. As these basal components diminished when f 2 was turned off at 5 ms [as seen in panels (A), (C), and (D)], DPOAE components generated nearer f 2 dominated to produce the large offset component.
(Color online) Examples of notches in the DP-grams for two different rabbits that can also be attributed to the cancellation of two groups of distributed DPOAE components. The DP-gram shown in panel (A) for the R ear of rabbit number RV34 was elicited by 75-dB SPL equal-level primary tones, and illustrates a deep notch at 4.287 kHz. DPOAE time waveforms shown in panels (B)–(E) were collected with the f 2-primary tone set to the frequency of the notch. In panel (B), a DPOAE-onset component (black arrow) disappeared after about 2 ms, leaving a low-level DPOAE of about 6.1 dB SPL. When f 2 was turned off in panel (C) at 5 ms, a DPOAE complexity was observed that was mostly in-phase with the synthesized steady-state DPOAE at 15 ms (fine background trace), which was in turn of a different phase to the onset response. In panel (D), an IT at 6 kHz eliminated the onset component and resulted in a much higher-level DPOAE of 12.3 dB SPL. In panel (F) the DP-gram for the R ear of rabbit RV32 showed a similar notch at the lower frequency of f 2 = 2.828 kHz. DPOAE time waveforms shown in panels (G)–(I) were again collected with the f 2-primary tone set to this lower notch frequency. In panel (G), the onset complexity emerged around 1 ms and then descended into the NF by around 3.5 ms. In panel (H), when the f 2 was turned off at 5 ms, a complexity arose that was close to 180° out-of-phase with the initial onset complexity. In panel (I), an IT presented above f 2 at 3.56 kHz eliminated the complexities and brought the DPOAE up out of the NF to approximately 25 dB SPL. These findings provide evidence for instances when sharp notches in DP-grams can be attributed to the cancellation of two groups of DPOAE components that are approximately 180° out-of-phase.
(Color online) Cartoons and representative time waveforms for wide [(A),(B)], narrow [(C),(D)], and narrow [(E),(F)] f 2/f 1 ratios in the presence of an IT illustrating how the interaction of distributed DPOAE components can give rise to observed complexities in DPOAE time waveforms. For the schematic in panel (A), at the optimal f 2/f 1 ratio, apical (#2, white arrows) and basal (#1, black arrows) DPOAE components are largely in-phase. In the time-domain data of panel (B), shorter latency basal components arose at #1 and added to longer latency more apical components (#1&2), but because they were largely in-phase, no complexities were apparent. Likewise, when f 2 was turned off, shorter latency basal components decayed first leaving the long-latency apical components at #2 to decay gradually. The onset process repeated when f 2 was turned back on at 11 ms. As shown in the narrow f 2/f 1-ratio diagram of panel (C), phase varies rapidly and apical components at #2 are mostly out-of-phase with those generated more basally at #1. In panel (D), onset complexities occurred in the time-domain waveforms when basal components at #1 appeared first. At narrow ratios, as f 2 components added out-of-phase in #1&2, the waveform decreased in amplitude. When f 2 was turned off, a large offset complexity was observed composed largely of longer latency apical components as indicated at #2, since canceling short latency more basal components had died out. In the bottom panel, also at narrow f 2/f 1 ratios, the diagram in panel (E) illustrates that the introduction of an IT sufficiently basal to the primary tones leaves mostly apically generated DPOAE components at #2. In this instance, the DPOAE time waveform in panel (F) was similar to the condition when all components were in-phase at standard f 2/f 1 ratios as shown above in panel (B). #1: Region of time waveform largely composed of DPOAE components generated considerably basal to f 2; #2: Region of time waveform dominated by DPOAE components generated more apically closer to f 2; #1&2: Region of time waveform where distributed components are added together with the results dependent upon their phase (see text for more details).
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