Supporting evidence for reverse cochlear traveling waves
Illustration of intracochlear pressure measurements and single tone response characteristics close to the BM. (A) Intracochlear pressure was measured in scala tympani (ST), close to or far from the sensory tissue’s BM in the basal turn of the gerbil cochlea. The pressure sensor is shown to scale. Also indicated are the cochlear inner and outer hair cells, the mechanotransducers of audition. (B) and (C) Single-tone ST pressure responses measured close to the BM in turn one of the gerbil cochlea (wg81, from BM, ). (B) Amplitude normalized by the stimulus level in the EC. (C) Phase relative to the pressure in the EC. Significant aspects of cochlear mechanics are: (1) Cochlear nonlinearity: the normalized response curves fan out in the frequency region of the peak . 30 dB SPL responses showed a sharp peak at the BF of (dotted line), the responses showed broader tuning with the peak shifted toward lower frequency. (2) Traveling wave: the phase of the response accumulated three complete cycles of delay relative to the stimulus in the ear canal. The phase changed most rapidly at frequencies around the BF. (3) Compression wave: at frequencies above , the phase changed little with frequency, which indicates that the fast wave dominated the responses. The amplitude in this frequency region was well above the noise floor and scaled linearly with SPL (gray band). (4) Interference between traveling and compression waves: the deep notch at with stimuli of 70 and SPL corresponds to a region in which the phase jumps with level (arrowheads).
The and components measured from an ear with intact cochlea showing the complex nature of the DPOAE. The bulla was open for measuring CAP thresholds at the round window and the cochlea was intact. (A) and (B) The amplitude with ratio of 1.05 and 1.25. (C) and (D) The amplitude with ratio of 1.05 and 1.25. Dotted–dashed line shows the noise floor of the B & K probe-tube microphone. (E)–(H) DPOAE phase referenced to and phases in the EC. Dotted–dashed line shows the average middle-ear round-trip delay from Dong and Olson (2006) . The two primaries were equal intensity of 70 (thin line), 80 (medium), and SPL (thick). ratio was fixed either at 1.05 or 1.25. swept from 1000 to in steps (animal wg92).
Simultaneous recording of DPOAE in the EC and DP in the ST close to the BM with different cochlear conditions: healthy (H) (blue), locally damaged (LD) (green), and postmortem (PM) (red). The pressure sensor was positioned from the BM at the basal turn of the cochlea with . (A) and (C) DPOAE and . (B) and (D) DP and . ( SPL, , swept from 1000 to in steps.) (E) CAP thresholds under healthy and locally damaged conditions. (F)–(H) Single-tone amplitude responses (normalized to SPL) from the same position/conditions with stimulation levels of SPL in steps. Vertical dotted line indicates the BF position. Single-tone nonlinearity and local DPs were severely reduced with damage and the CAP thresholds were elevated, especially in the frequency region of damage (B), (D), (E), and (G). Postmortem, single-tone responses became linear and the DPs were reduced almost to the noise level (B), (D), and (H). DPOAEs changed over a limited frequency region due to local damage, and were greatly reduced postmortem (A), (C) (animal wg95).
Lack of sensor perturbation to DPOAE and CAP thresholds. (A)–(D) and DPOAE amplitude. Dotted–dashed lines indicate the B & K probe-tube microphone noise floor in the EC. (E)–(H) and DPOAE phase relative to EC and phases. Dotted–dashed lines represent the middle-ear round-trip delay. (I) CAP thresholds. Blue lines show data collected with the cochlea intact. Green lines indicate responses with the ST hole and sensor at from BM and red lines represent data with the sensor from BM after tapping it. The primary stimuli were equal-intensity tones of SPL with ratio fixed at either 1.05 or 1.25. frequency was swept from 1000 to in steps of . The DPOAE and CAP thresholds were nearly unchanged after making the ST hole, introducing the sensor into ST, and after tapping the BM (animal wg96).
Scala tympani DP amplitude and phase show both locally/basally and remotely generated components. Sensor was positioned from the BM, , 80, and SPL. was fixed at either 1.05 or 1.25. was swept from 1000 to in steps. (A) and (B) Amplitude of the DP with ratio of 1.05 and 1.25, respectively. (C) and (D) Corresponding phase, referenced to the EC primaries. In (A)–(D) DPs are plotted vs their own frequency. (E) and (F) DP amplitude is plotted vs the frequency, to better understand the basis for tuning. Solid colored lines show the DP responses; dotted lines show single-tone pressure responses at 50, 60, and SPL, measured at the same position, for comparison. Dotted–dashed lines show the noise floor of the sensor and vertical dotted line indicates the BF position (animal wg93).
DP spatial variation shows that the DP drops off with distance from the BM. (A) and DP amplitude measured at distances far (thin) and close (thick) to the BM. Dotted–dashed lines indicate the sensor noise floor. Vertical dotted lines represent the BF position. (B) and Amplitude of primary measured close to (thick dotted) and far from (thin dotted) the BM. (C) and Phase referenced to EC and . DP and primary responses are plotted in solid and dotted lines, respectively. Arrowheads indicate frequency/position combinations for which destructive interference between compression and traveling modes is apparent. (D) and Concurrently measured DPOAE is stable, indicating a lack of significant sensor perturbation. ( SPL, , swept in (animal wg95) or (animal wg92) steps). The DP decreased as the distance from the BM increased, similar to the primary response in the BF frequency region.
Simultaneous recording of the DP and DPOAE with . (blue), 70 (green), and SPL (red), the sensor was positioned from BM. was swept from 1000 to in steps. (A), (C), (F) and (H) The and DP and DPOAE amplitudes. Dotted–dashed lines indicate the noise floor of the sensor and B and K probe-tube microphone. Vertical dotted lines represent the BF position. (B), (D), (G) and (I) Phases of DP and DPOAE referenced to EC and phases. Dotted–dashed lines show the middle-ear round-trip delay and dotted lines show the single-tone forward-traveling-wave phase, for comparison. (E) and (J) Phase of the DPOAE referenced to the DP phase. When this phase overlaid the forward-traveling wave of the single tone, the reverse wave is considered to be detected. Within the BF frequency region (gray bar), the observation of the reverse wave depends upon the DPOAE phase being rapidly varying, as then the phase contains information about the traveling-wave delay. In the sub-BF region, the reverse wave can be expected as long as the DP amplitude and phase behavior [in panels (C) and (D), (H) and (I)] indicates that the DP is not dominated by local distortion. Except for in these two regions, a reverse wave cannot be expected to be detected (animal wg81).
Simultaneous recording of the DP and DPOAE with with the same format, measurement position, and animal as in Fig. 7 . In the relative phase data of panel (E), in the BF region [gray bar in panel (E)] the reverse wave was apparent in the data ( region) but less so at the higher, SPL level. At the higher level, the DPOAE phase was slightly less steep than at the level in the region. The DP was not recorded in the BF region for this case.
Simultaneous recording of the DP and DPOAE with . (blue), 70 (green), and SPL (red), sensor positioned from the BM, swept from 1000 to in steps. 60 dB SPL data were averaged 50 times rather than the usual 20 times. In the sub-BF region (less than ), the double arrows indicate that the fine structure in DP amplitude was echoed in the DPOAE. The DPOAE–DP phase supported the reverse wave in all cases in which data were available. In the BF region (gray bars) the reverse-traveling wave was only apparent when the DPOAE phase was rapidly varying. When the DPOAE phase was not rapidly varying [panel (G)] relating the DPOAE and DP phases caused a paradoxical result [panel (J)] in which the DPOAE appeared to lead the DP at frequencies close to BF (animal wg96).
Simultaneous recording of the DP and DPOAE with with the same format, measurement position, and animal as in Fig. 9 . In the BF region (gray bars), the detection of the reverse-traveling wave depends upon the DPOAE phase, which is level dependent. In the BF region (gray bars) the reverse-traveling wave was only apparent when the DPOAE phase was rapidly varying [ stimulus level of panel (B)]. When the DPOAE phase was not rapidly varying [panels (G) and (J)] relating the DPOAE and DP phases caused a paradoxical result in which the DPOAE appeared to lead the DP at frequencies close to BF. In the BF region, the stimulus level data in panel (B) are subject to a stair–steplike behavior (coupled to sharp fine structure in the amplitude) that gives rise to a messy DPOAE–DP result.
Simultaneous recordings of DP and DPOAE with fixed at BF. SPL, changed from 1.02 to 1.5 in steps of 0.02. The sensor was positioned from the BM. (A) The DPOAE (thick line) and DP (thin line) amplitude. The DPOAE amplitude was moved up to facilitate the comparison. The vertical line indicates the frequency. (B) The DPOAE (thick) and DP (thin) phase, referenced to EC and . (C) Phase of DPOAE–DP. The dotted lines show single-tone stimulus level response phase relative to the pressure stimulus in the EC. Dotted–dashed lines show average middle-ear reverse delay. The reverse wave was detected in the sub-BF region below (C). Above the DPOAE seemed to lead the DP. Because the stimulus paradigm was not a fixed ratio, the DPOAE phase does not allow for identification of wave-fixed and place-fixed emission type, making interpretation of these data less straightforward than with the fixed-ratio paradigm (animal wg96).
The grouped phases of the and DPOAEs from eight animals illustrate the generality of the responses. These preparations had robust DPOAE responses over a wide frequency range for primaries. (When the primaries are at a level of SPL, the phases tend to flatten, and are less interesting.) All phases were referenced to EC and phases. (A) and (B) The phase with ratios of 1.05 and 1.25, respectively. (C) and (D) The phase with ratios of 1.05 and 1.25, respectively. The dotted line is twice the single-tone pressure response phase measured close to the BM from a representative case (wg81, SPL), shown for comparison. The dotted–dashed line also shows the average middle-ear round-trip delay for comparison. Gray bars identify the region that can be compared to the DP data gathered during this study, for which the BF was (animals wg81, wg89, wg90, wg92, wg93, wg94, wg95 and wg96).
The and DP phases from all 17 animals that contributed to this study. The sensor was positioned within of the BM, (gray) and SPL (black), . (A) The phase; (B) phase. All phases referenced to EC and phases. The dotted line shows the single-tone response phase measured close to the BM from a representative case (wg81, SPL), shown for comparison. The phases overlaid the single-tone (forward-traveling wave) phase at frequencies in a broad BF region, indicating that local distortion dominated the DP there. The DP phase sloped up at low frequencies, and was not similar to the single-tone response there. In the region of the BF, the phase typically sloped downward similarly to the single-tone phase, suggesting a forward-traveling-wave DP in this frequency region. The gray bar identifies the BF region probed in this study.
Illustration of the generation and transmission of DPs. Upon two-tone stimulation, distortion products are generated by cochlear nonlinearity and can be detected within the cochlea as DPs and in the ear canal as DPOAEs. The pressure sensor was positioned at the basal turn of the cochlea where the BF was . In (A) the primary and frequencies were well beneath the local BF. In this case the sub-BF DP is likely generated apical of our sensor location, and when detected, is on its way out of the cochlea. In (B) the primary and frequencies are in the BF region. In this case the DP was generated close to our sensor position, and is relatively large due to cochlear tuning of both the primaries and the DP. It has a forward-going character due to the forward-going character of the primaries.
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