Figure 1.
Diagram for measuring RW CM, the stapes and BM vibrations.
(A) The CM was measured from the RW niche (red dot). Sound-induced vibrations were measured from the stapes and at a BM location ∼2.5 mm from the base (green dots). The temporal relationships between signals are described by the following delays: the delay from speakers to the stapes (), the forward and backward delays in the cochlea (
and
). (B) The spatial relationship between travelling waves and the electrode locations for recording the RW CM. BM: the basilar membrane; RW: the round window; BF: best frequency.
Figure 2.
Frequency responses of the RW CM at different sound pressure levels.
(A, B) Magnitude and phase of the RW CM as a function of frequency at 10 to 90 dB SPL. (C) The phase of the stapes vibration recorded at 70 and 80 dB SPL. (D) The phase of the RW CM with respect to stapes vibrations as a function of frequency. At sound levels at and above 60 dB SPL or at frequencies above 2 kHz when the sound level is ≤50 dB SPL, the phase is approximately flat, indicating a negligible group delay.
Figure 3.
The BM vibration measured from a longitudinal location ∼2.5 mm from the cochlear base.
The magnitude (A), phase (B), the magnitude ratio of the BM to the stapes (C), and phase difference between the stapes and BM (D) are presented as a function of frequency. Panels A and C show high sensitivity, sharp tuning, and nonlinear compression while panels B and D show that the phase decreases with frequency at an accelerated rate, indicating cochlear dispersion.
Figure 4.
Comparison of RW CM magnitude and phase before (black solid lines) and after (red dotted lines) TTX application.
Compared to solid black lines, TTX-induced CM magnitude decrease at 10 and 30 dB SPL is as much as 20 dB at frequencies below 2 kHz. Corresponding phase curves at this frequency range become less steep in panel B or flat in panel C (red dotted lines). The phase values in panel C were obtained by subtracting stapes phase from panel B.
Figure 5.
Comparison of RW CM magnitude and phase before (black solid lines) and after (red dotted lines) TTX application.
Data were collected from a different sensitive cochlea. After TTX application, CMs induced by 10- and 30-dB SPL tones decreased as much as 20 dB at frequencies below 2 kHz. Panels B and C show that phase slope at low frequencies became less steep or flat (red dotted lines) after TTX application.
Figure 6.
Magnitude of the RW CM as a function of suppressor frequency before (panels A, B, and C) and after (panels D, E, and F) TTX application.
The RW CM was evoked by 500-Hz probe tones at 40 (panels A and D), 60 (panels B and E), and 80 (panels C and F) dB SPL, and suppressing tone levels were 35, 45, 55, 65, 75, and 85 dB SPL. For 40-dB probe tones and intermediate suppressor levels, the RW CM was suppressed mainly at ∼1.8 kHz (panels A and D) and ∼4 kHz (panel A) or ∼10 kHz (panel D). At 60-dB SPL probe level, suppression near ∼10 kHz is more significant than that near ∼1.8 kHz despite suppressor-induced CM increase at other frequencies (panels B and E). At 80-dB SPL probe-tone level, CM suppression occurred dominantly at high frequencies ≥10 kHz. After the ANN is eliminated by TTX, suppression curves in panel D become more regular than those in panel A.
Figure 7.
Magnitude of the RW CM as a function of suppressor frequency in a different sensitive cochlea.
The RW CM was evoked by 500-Hz probe tones at 30 to 80 dB SPL, and suppressed by the second tone at different frequencies and levels. For 30-, 40-, and 50-dB probe tones and at intermediate suppressor levels, the RW CM was suppressed dominantly at frequencies below 5 kHz (panels A–C). At 70- and 80-dB SPL probe-tone levels, suppression occurred mainly at high frequencies near 10 kHz. Suppressor-induced CM increase occurred at frequencies near 9 kHz (panels B–E) and below 1 kHz (panel D).