Mechanical tuning and amplification within the apex of the guinea pig cochlea

Abstract

Key points: A popular conception of mammalian cochlear physiology is that tuned mechanical vibration of the basilar membrane defines the frequency response of the innervating auditory nerve fibres However, the data supporting these concepts come from vibratory measurements at cochlear locations tuned to high frequencies (>7 kHz). Here, we measured the travelling wave in regions of the guinea pig cochlea that respond to low frequencies (<2 kHz) and found that mechanical tuning was broad and did not match auditory nerve tuning characteristics. Non-linear amplification of the travelling wave functioned over a broad frequency range and did not substantially sharpen frequency tuning. Thus, the neural encoding of low-frequency sounds, which includes most of the information conveyed by human speech, is not principally determined by basilar membrane mechanics.

Abstract: The popular notion of mammalian cochlear function is that auditory nerves are tuned to respond best to different sound frequencies because basilar membrane vibration is mechanically tuned to different frequencies along its length. However, this concept has only been demonstrated in regions of the cochlea tuned to frequencies >7 kHz, not in regions sensitive to lower frequencies where human speech is encoded. Here, we overcame historical technical limitations and non-invasively measured sound-induced vibrations at four locations distributed over the apical two turns of the guinea pig cochlea. In turn 3, the responses demonstrated low-pass filter characteristics. In turn 2, the responses were low-pass-like, in that they occasionally did have a slight peak near the corner frequency. The corner frequencies of the responses were tonotopically tuned and ranged from 384 to 668 Hz. Non-linear gain, or amplification of the vibrations in response to low-intensity stimuli, was found both below and above the corner frequencies. Post mortem, cochlear gain disappeared. The non-linear gain was typically 10-30 dB and was broad-band rather than sharply tuned. However, the gain did reach nearly 50 dB in turn 2 for higher stimulus frequencies, nearly the amount of gain found in basal cochlear regions. Thus, our data prove that mechanical responses do not match neural responses and that cochlear amplification does not appreciably sharpen frequency tuning for cochlear regions that respond to frequencies <2 kHz. These data indicate that the non-linear processing of sound performed by the guinea pig cochlea varies substantially between the cochlear apex and base.

Keywords: auditory system; cochlea; hair cell; hearing.

Figure 1. Compound action potential recordings

A, representative raw data collected at 0.6, 1.1 and 14.7 kHz in one guinea pig. Note that the compound action potential (CAP) waveform was clearly visible at 14.7 kHz, but that a superimposed AC response contaminates the CAP response at lower frequencies. This AC response represents a combination of asymmetrical distortion in the cochlear microphonic and the auditory nerve overlapped response (ANOW). B, CAP thresholds over the frequency spectrum recorded in five guinea pigs. Error bars are the SEM. Below 3 kHz, the error bars for the thresholds are smaller than those above 3 kHz because the cochlear microphonic and ANOW waveforms dominate the response.

Figure 2. In vivo imaging and vibratory responses from one representative guinea pig demonstrate low‐pass‐like filter responses

A, 3D volumetric image of the guinea pig cochlea. Two perpendicular cross‐sections are highlighted. These two cross‐sections were selected in every guinea pig we studied to consistently measure vibrations from the same four locations. B and C, the helicotrema (Hel) was identified to determine the apical end of the guinea pig cochlea, and defined the first cross‐section. The second cross‐section was then obtained by rotating the scan angle 90 deg. The auditory neurons sit within the modiolus (Mod). Recording sites are labelled as the percentage distance from the base (95%, turn 3.5; 92%, turn 3.25; 80%, turn 2.5; 75%, turn 2.25). D, enlarged view of the 95% location. The site where vibratory measurements were made is shown (green circle). BM, basilar membrane; RM, Reissner's membrane; OoC, organ of Corti; ST, scala tympani; SV, scala vestibuli. E, plastic‐embedded cross‐section of the 95% location. Osmium stained the Hensen cells particularly dark. F–I, vibratory magnitudes at the four different measurement locations. Data at all stimulus intensities from the living guinea pig are blue and at 70 dB SPL from the dead guinea pig are red. J–M, the sensitivity was calculated by normalizing the vibratory magnitude to the stimulus intensity. Non‐linear gain was found in living (blue), but not dead (red), guinea pigs. N–Q, vibratory phases for the living guinea pig are shown. Post mortem responses were identical and thus not plotted. The phase responses were normalized by subtracting the phase of the ossicular chain. Negative phase indicates a phase delay.

Figure 3. Non‐linear gain is present both below and above the F _3dB corner frequency

A–D, the average sensitivity is shown for the four different measurement locations (live: blue curves; dead: red curves). The stimulus intensity is shown next to the curves, ranging from 20 to 70 dB SPL in live. We only plotted the response to 70 dB SPL stimuli in the dead guinea pigs because the curves all overlapped. The characteristic frequency of the auditory nerve fibres innervating each location is given (red arrows under the x‐axis). The F _3dB corner frequency is shown by the orange dotted line. The average gain was calculated below and above the corner frequency at the highlighted frequencies (dark green and light green arrows). E and F, the average gain, calculated as the ratio of the sensitivities measured between 30 and 70 dB SPL stimuli in live guinea pigs, below and above the corner frequency. The frequency where the gain was calculated is given. All error bars are the SEM. Statistical analyses compare the average gain between the two measurements in turn 3 and the two measurements in turn 2.

Figure 4. Cochlear gain in live guinea pigs using 20 dB SPL stimuli

The sensitivity in response to 20 dB SPL stimuli in live guinea pigs was divided by the sensitivity to 70 dB SPL stimuli in dead guinea pigs. The characteristic frequency of the auditory nerve fibres innervating each location are given (red arrows under the x‐axis). The F _3dB corner frequency is shown by the orange dotted line.

Figure 5. Vibratory thresholds do not match neural thresholds below the characteristic frequency

A, the individual vibratory thresholds from 11 guinea pigs at the 80% location calculated as the minimum sound intensity required to elicit a 5 nm response. B, average vibratory thresholds from all four cochlear locations. C, the average vibratory thresholds for the 95% and 80% locations are shown; superimposed are a series of neural tuning curves collected in guinea pigs (Cooper & Rhode, 1995). Note that while the high‐frequency side of the neural tuning curve matches the mechanical response, the low‐frequency side does not. All error bars are the SEM.

Figure 6. Opening the cochlea reduces the vibratory response to low‐frequency sound stimuli

Data from two different representative guinea pigs are shown in response to 50, 60 and 70 dB SPL stimuli. The gain and phase are normalized to the middle ear (ME) response at the 95% location (A and C) and the 80% location (B and D). In the dead intact cochlea (red), the gain response is low‐pass‐like. After opening the bone over the helicotrema and repeating the measurements, the opened cochleae demonstrated vibratory responses that were more band‐pass. However, there were no substantive changes to the phase response.