Figure 1.
Diagrams for measuring basilar membrane vibrations.
(A) Two measured locations on the BM and one on the stapes (red dots). As the wave travels from the base to its BF location (B), the cochlear amplifier increases the BM vibration at a location basal to the BF site (blue bar in panel C). The local transfer function can specifically quantify the functioning of the amplification region between positions A and B. (D) shows a sharp peak at ∼15.3 kHz at low sound levels, which was >1,000 at 20 dB SPL. As the sound level increased, the peak magnitude decreased, and the peak broadened and shifted toward ∼12.0 kHz. (E and H) Growth rates in dB/dB at the more basal (E) and apical (H) locations. (F) The phase lag progressively increased with frequency. The data in panels G–I, measured at the more apical location, are similar to those in panels D–F (allowing for a lower BF). BMB and BMA are BM vibration magnitudes at the measured basal and apical locations.
Figure 2.
Local transfer functions, delay, velocity, and wavelength of basilar membrane vibration.
(A) The response peak at ∼12.0 kHz decreased, broadened, and shifted toward low frequencies with increasing sound level. The magnitude was smallest at ∼17.0 kHz. (B) Phase response was similar to that in Figures 1F and I but with a smaller phase lag. The delay from the basal to more apical location increased with frequency (C), while the propagation velocity (D) and wavelength (E) decreased over the same frequency range. Red lines show post-mortem data measured at 40 dB SPL.
Figure 3.
Local transfer functions, delay, velocity, and wavelength in a different sensitive cochlea.
Data were collected at longitudinal locations ∼2,650 and ∼2,317 µm with ∼333 µm separation. Allowing a higher peak frequency of 15.0 kHz in panel A, the data in Figure 3 are similar to those in Figure 2, which confirm the existence of magnitude amplification and reduction over the BM region between the two measured locations.
Figure 4.
The relationship between transfer functions and the longitudinal pattern of basilar membrane vibration.
In contrast to the >1,000 gain of the conventional transfer function (thin lines) at the peak frequency, the local transfer function (thick lines) shows a gain of only ∼10 at ∼12.0 kHz and ∼40 dB of reduction at ∼17 kHz in panels A. Response peaks became smaller at 90 dB SPL in panel B. (C) At 20 dB SPL, the highest transmission efficiency was >50 dB/mm at ∼12.0 kHz and the lowest efficiency was <−100 dB/mm at ∼17.0 kHz (thick line). (D) At 90 dB SPL, the response peak at ∼12.0 kHz disappeared and the minimum remained unchanged (thick line). (E) BM response to a 50 dB SPL 11.0-kHz tone increased at the rate of ∼26 dB/mm in the region between 2,450 to 2,750 µm (green arrow), while the 19.0-kHz response decreased at the rate of ∼131 dB/mm over the same distance (red arrow). (F) The increase in low-level response on the basal side of the BF location (solid green arrows) became the decrease at the high sound level (red arrow near 2,300 µm).
Figure 5.
Frequency-dependent amplification and reduction and BM sharp tuning.
(A) The BM response at the basal location is presented by vibration amplitude as a function of frequency (dotted curve), and that at the more apical location is shown by the solid curve. The peak frequency of the basal location was higher than that of the more apical site. As the vibration propagated from base to apex, the BM between the two measured locations increased low-frequency responses (upward arrow) and reduced high-frequency responses (downward arrow), resulting in a sharply tuned response at the apical location (solid curve). (B) Phase at the basal location (dotted curve) leaded that at the apical location (solid curve), indicating that waves propagated from base to apex at frequency-dependent speeds. Data were collected at 30 dB SPL from the same sensitive cochlea as for Figures 1 and 2.