Alberto Recio

Study of mechanical motions in the basal region of the chinchilla cochlea

Measurements from the 1-4-mm basal region of the chinchilla cochlea indicate the basilar membrane in the hook region (12-18 kHz) vibrates essentially as it does more apically, in the 5-9-kHz region. That is, a compressive nonlinearity in the region of the characteristic frequency, amplitude-dependent phase changes, and a gain relative to stapes motion that can attain nearly 10,000 at low levels. The displacement at threshold for auditory-nerve fibers in this region (20 dB SPL) was approximately 2 nm. Measurements were made at several locations in individual animals in the longitudinal and radial directions. The results indicate that there is little variability in the phase of motion radially and no indication of higher-order modes of vibration. The data from the longitudinal studies indicate that there is a shift in the location of the maximum with increasing stimulus levels toward the base. The cochlear amplifier extends over a 2-3-mm region around the location of the characteristic frequency.

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.

Responses to cochlear normalized speech stimuli in the auditory nerve of cat

Previous studies of auditory-nerve fiber (ANF) representation of vowels in cats and rodents (chinchillas and guinea pigs) have shown that, at amplitudes typical for conversational speech (60-70 dB), neuronal firing rate as a function of characteristic frequency alone provides a poor representation of spectral prominences (e.g., formants) of speech sounds. However, ANF rate representations may not be as inadequate as they appear. Here, it is investigated whether some of this apparent inadequacy owes to the mismatch between animal and human cochlear characteristics. For all animal models tested in earlier studies, the basilar membrane is shorter and encompasses a broader range of frequencies than that of humans. In this study, a customized speech synthesizer was used to create a rendition of the vowel [E] with formant spacing and bandwidths that fit the cat cochlea in proportion to the human cochlea. In these vowels, the spectral envelope is matched to cochlear distance rather than to frequency. Recordings of responses to this cochlear normalized [E] in auditory-nerve fibers of cats demonstrate that rate-based encoding of vowel sounds is capable of distinguishing spectral prominences even at 70-80-dB SPL. When cochlear dimensions are taken into account, rate encoding in ANF appears more informative than was previously believed.

Multicomponent stimulus interactions observed in basilar-membrane vibration in the basal region of the chinchilla cochlea

Multicomponent stimuli consisting of two to seven tones were used to study suppression of basilar-membrane vibration at the 3-4-mm region of the chinchilla cochlea with a characteristic frequency between 6.5 and 8.5 kHz. Three-component stimuli were amplitude-modulated sinusoids (AM) with modulation depth varied between 0.25 and 2 and modulation frequency varied between 100 and 2000 Hz. For five-component stimuli of equal amplitude, frequency separation between adjacent components was the same as that used for AM stimuli. An additional manipulation was to position either the first, third, or fifth component at the characteristic frequency (CF). This allowed the study of the basilar-membrane response to off-CF stimuli. CF suppression was as high as 35 dB for two-tone combinations, while for equal-amplitude stimulus components CF suppression never exceeded 20 dB. This latter case occurred for both two-tone stimuli where the suppressor was below CF and for multitone stimuli with the third component=CF. Suppression was least for the AM stimuli, including when the three AM components were equal. Maximum suppression was both level- and frequency dependent, and occurred for component frequency separations of 500 to 600 Hz. Suppression decreased for multicomponent stimuli with component frequency spacing greater than 600 Hz. Mutual suppression occurred whenever stimulus components were within the compressive region of the basilar membrane.

Basilar-membrane response to multicomponent stimuli in chinchilla.

The response of chinchilla basilar membrane in the basal region of the cochlea to multicomponent (1, 3, 5, 6, or 7) stimuli was studied using a laser interferometer. Three-component stimuli were amplitude-modulated signals with modulation depths that varied from 25% to 200% and the modulation frequency varied from 100 to 2000 Hz while the carrier frequency was set to the characteristic frequency of the region under study (approximately 6.3 to 9 kHz). Results indicate that, for certain modulation frequencies and depths, there is enhancement of the response. Responses to five equal-amplitude sine wave stimuli indicated the occurrence of nonlinear phenomena such as spectral edge enhancement, present when the frequency spacing was less than 200 Hz, and mutual suppression. For five-component stimuli, the first, third, or fifth component was placed at the characteristic frequency and the component frequency separation was varied over a 2-kHz range. Responses to seven component stimuli were similar to those of five-component stimuli. Six-component stimuli were generated by leaving out the center component of the seven-component stimuli. In the latter case, the center component was restored in the basilar-membrane response as a result of distortion-product generation in the nonlinear cochlea.

Physiological Correlates of Temporal Integration

The decrease in detection and discrimination thresholds with increases in signal duration is usually attributed to long time-constant integration or summation. An alternative view is that no long-term integration occurs but rather that information is combined from multiple, short-duration samples or “looks”. According to this account the phenomenon of temporal integration simply reflects an increase in the number of looks rather than an increase in effective signal energy. To compare predictions of these approaches under reasonable constraints, spike data recorded from the auditory nerve in response to long-duration CF tones are considered. These data are analyzed according to multiple-look and summation schemes, and temporal integration functions are derived. The derived functions are very similar, indicating that a multiple look scheme can show summation-like behavior, and that such behavior does not imply long time constant integration. The analysis also indicates that the detectability of 5-ms looks near the onset of the tone can be considerably higher than those occurring later in the tone and that the derived psychometric functions generally do not show the ✓T dependence that would be expected from a Poisson process.