Nola C. Rich

Basilar membrane mechanics at the base of the chinchilla cochlea. I. Input-output functions, tuning curves, and response phases.

Basilar membrane (BM) velocity was measured at a site 3.5 mm from the basal end of the chinchilla cochlea using the Mössbauer technique. The threshold of the compound action potential recorded at the round window in response to tone bursts was used as an indicator of the physiological state of the cochlea. The BM input-output functions display a compressive nonlinearity for frequencies around the characteristic frequency (CF, 8 to 8.75 kHz), but are linear for frequencies below 7 and above 10.5 kHz. In preparations with little surgical damage, isovelocity tuning curves at 0.1 mm/s are sharply tuned, have Q10s of about 6, minima as low as 13 dB SPL, tip-to-tail ratios (at 1 kHz) of 56 to 76 dB, and high-frequency slopes of about 300 dB/oct. These mechanical responses are as sharply tuned as frequency-threshold curves of chinchilla auditory nerve fibers with corresponding CF. There is a progressive loss of sensitivity of the mechanical response with time for the frequencies around CF, but not for frequencies on the tail of the tuning curve. In some experiments the nonlinearity was maintained for several hours, in spite of a considerable loss of sensitivity of the BM response. High-frequency plateaus were observed in both isovelocity tuning curves and phase-frequency curves.

Application of a commercially-manufactured Doppler-shift laser velocimeter to the measurement of basilar-membrane vibration

A commercially-available laser Doppler-shift velocimeter has been coupled to a compound microscope equipped with ultra-long-working-distance objectives for the purpose of measuring basilar membrane vibrations in the chinchilla. The animal preparation is nearly identical to that used in our laboratory for similar measurements using the Mössbauer technique. The vibrometer head is mounted on the third tube of the microscopes trinocular head and its laser beam is focused on high-refractive-index glass microbeads (10-30 microns) previously dropped, through the perilymph of scala tympani, on the basilar membrane. For equal sampling times, overall sensitivity of the laser velocimetry system is at least one order of magnitude greater than usually attained using the Mössbauer technique. However, the most important advantage of laser-velocimetry vis-à-vis the Mössbauer technique is its linearity, which permits undistorted recording of signals over a wide velocity range. Thus, for example, we have measured basilar-membrane responses to clicks whose waveforms have dynamic ranges exceeding 60 dB.

Chinchilla auditory‐nerve responses to low‐frequency tones

Single unit activity was recorded in the auditory nerves of chinchillas. Period histograms were constructed for responses to tones with frequencies 30-1000 Hz. For low-frequency tones at near-threshold levels, peak period histogram phases for low- and medium-best-frequency (BF) neurons (less than or equal to kHz) ranged from synchronous with condensation at the eardrum to 90 degrees leading it. At near-threshold (but high absolute) levels, high-BF (greater than or equal to 8 kHz) neurons responded in phase with rarefaction. At even higher levels, period histograms for responses of high-BF neurons tended to become bimodal, with one of the modes lagging rarefaction by 90 degrees. Using cochlear microphonics as an indicator of basilar membrane (BM) displacement, at threshold levels, response phase of low- and medium-BF neurons fall within a range between displacement and velocity of the BM toward scala vestibuli. High-BF neurons respond, at threshold (but high) intensities, in phase with BM displacement toward scala tympani. The rates of growth of frequency sensitivity in responses of low-BF (+ 18 dB/oct) and high-BF (+ 12 dB/oct) neurons are consistent with preferred response phases corresponding to BM SV velocity and ST displacement, respectively. At supra-threshold levels high-BF neurons may fire preferentially to both scala tympani displacement and scala vestibuli velocity. These results support the notion that, for high-intensity, low-frequency stimuli, OHC hyperpolarization can induce excitation of the dendrites innervating IHCs.

Spontaneous and impulsively evoked otoacoustic emission: indicators of cochlear pathology? ☆

The first authors right ear produces a spontaneous otoacoustic emission (SOAE) at 7529 Hz and 16 dB SPL. An external continuous tone is able to suppress the SOAE. The 3 dB iso-suppression curve is broadly tuned and displaced, relative to the SOAE, toward higher frequencies. An audiogram notch exists at frequencies just below that of the SOAE. We explain the occurrence of both spontaneous and impulsively evoked OAEs in terms of disruption of active feedback mechanisms of the OHCs upon basilar membrane vibration. According to this hypothesis, each segment of the organ of Corti feeds back positively upon its segment of basilar membrane and negatively upon adjacent segments. If a patch of OHC loss exists, adjacent segments of the basilar membrane are released from the negative feedback and respond to an impulsive stimulus with exaggerated oscillations at their resonance frequencies, thus producing OAEs. At particularly sharp transitions between normal and abnormal regions of the organ of Corti SOAEs may be generated.

Effects of excitatory and non-excitatory suppressor tones on two-tone rate suppression in auditory nerve fibers

Recordings were obtained from individual auditory nerve fibers in anesthetized chinchillas. Rate versus level functions were obtained for best frequency (BF) tones alone and for simultaneously-gated tone pairs comprising a BF tone and a second tone at a fixed intensity that produced evidence of two-tone rate suppression. Care was taken in selecting a range of suppressor tone levels that included excitatory (i.e., the suppressor tone evoked a rate change by itself) and non-excitatory (i.e., no suppressor tone-evoked rate increase) suppressor tone levels. Addition of a suppressor tone produced a shift of the dynamic range portion of the BF rate versus level function to higher test intensities. A parallel shift of the dynamic range portion of the rate versus level function was associated with a non-excitatory suppressor tone. The shift produced by an excitatory suppressor tone was characterized by a decrease in slope. Results indicated that the magnitude of shift increased monotonically as suppressor tone intensity was raised and that there was a gradual transition from a non-excitatory response to an excitatory response. The rate of shift (i.e., dB of shift per dB change in suppressor tone intensity) did not differ for non-excitatory versus excitatory responses, but was substantially greater for below-BF suppressor tones (1.38 dB/dB) than for above-BF suppressor tones (0.54 dB/dB). The rate of shift did not, however, appear to be related systematically to suppressor tone frequency separation from BF. Above- and below-BF suppression was noted for fibers over the range of best frequencies tested (110 Hz to 16.4 kHz).

Basilar membrane mechanics at the base of the chinchilla cochlea. II. Responses to low‐frequency tones and relationship to microphonics and spike initiation in the VIII Nerve

Low-frequency stimuli (40- to 1000-Hz tones) have been used to correlate the motion of the 8-to 9-kHz place of the chinchilla basilar membrane with the cochlear microphonics recorded at the round window and with the responses of auditory nerve fibers with appropriate characteristic frequency. At the lowest stimulus frequencies, maximum displacement of the basilar membrane toward scala tympani occurs in near synchrony with maximum rarefaction at the eardrum and maximum negativity at the round window; at higher frequencies, the mechanical and microphonic response phases progressively lag rarefaction, reaching - 240 deg at 1000 Hz. At most frequencies (40-1000 Hz) near-threshold neural responses, once corrected for neural travel-time and synaptic delays, somewhat lead (by some 40 deg) maximal scala tympani displacement and maximal negativity of the round window microphonics. The variation of sensitivity with frequency is similar for basilar membrane displacement and microphonic responses: Under open-bulla conditions, sensitivity is constant for frequencies between 100 and 1000 Hz; below 100 Hz, sensitivity decreases at rates close to 12 dB/oct toward lower frequencies. Neural response sensitivity matches BM displacement more closely than BM velocity.

Spontaneous otoacoustic emissions in a dog.

Intense (up to 59 dB SPL) spontaneous otoacoustic emissions are produced by both ears of a young dog. The right ear produces a single, very narrow-band (less than 4 Hz) emission at about 9100 Hz. Brainstem evoked-response audiometry suggests that this emission is generated near the transition between normal and abnormal regions of the cochlea.

Mossbauer Measurements of the Mechanical Response to Single-Tone and Two-Tone Stimuli at the Base of the Chinchilla Cochlea

Basilar membrane (BM) motion was measured at a site 3.5 mm from the basal end of the chinchilla cochlea using the Mossbauer technique. In preparations with little surgical damage, mechanical responses were as sharply tuned as auditory nerve fibers with the same characteristic frequency (CF, about 8.4 kHz). High-frequency plateaus were observed in both isovelocity tuning curves and phase-frequency curves. Input-output functions at frequencies around CF were strongly nonlinear. Another type of nonlinearlty, two-tone suppression, was also demonstrated in several cochleas, with suppression effects as large as 28 dB.

Basilar Membrane Responses to Clicks

Basilar membrane responses to clicks were studied in chinchilla cochleae using laser velocimetry. Responses of a basilar membrane site located 3.5 mm from the oval window [characteristic frequency (CF): 8–10 kHz] consisted of relatively undamped transient oscillations with periodicity close to 1/CF. Intense rarefaction clicks caused initial basilar membrane motion toward scala vestibuli after a latency of about 90 microseconds (measured from the onset of inward stapes displacement). The initial response oscillations grew linearly with stimulus intensity but later response cycles grew nonlinearly, at rates less than 1 dB/dB. Thus, with increases in stimulus level, the response envelopes became progressively more asymmetrical, with maxima shifting to earlier times. The magnitude and phase frequency spectra of click responses resembled the corresponding spectra for responses to tones. As click level increased, sharpness of tuning deteriorated and the maximal spectral response component shifted to lower frequencies. The phases of spectral components below characteristic frequency lagged responses evoked by less-intense clicks, while those immediately above characteristic frequency led lower-level responses.

Nonlinear Interactions in the Mechanical Response of the Cochlea to Two-Tone Stimuli

The reduction of the auditory response to one tone due to the simultaneous presence of a second tone is known as two-tone suppression. This nonlinear cochlear response was first described by Galambos and Davis (1944) in electrophysiological recordings from the cat auditory nerve. Since then two-tone suppression has been extensively studied in cochlear afferents of mammals (Nomoto et al., 1964; Kiang et al., 1965; Hind et al., 1967; Sachs and Kiang, 1968) and also of amphibia (Frischkopf and Goldstein, 1963; Feng et al., 1975), birds (Sachs et al., 1974) and reptiles (Holton and Weiss, 1978). Two-tone suppression seems to be demonstrable in all well-studied cochlear fibers, for suppressors at frequencies on both sides of the tuning curve (Sachs and Kiang, 1968; Sachs, 1969; Abbas and Sachs, 1976), and has also been observed in guinea pig inner hair cells (Sellick and Russell, 1979).