Basilar-membrane responses to clicks at the base of the chinchilla cochlea

Abstract

Basilar-membrane responses to clicks were measured, using laser velocimetry, at a site of the chinchilla cochlea located about 3.5 mm from the oval window (characteristic frequency or CF: typically 8-10 kHz). They consisted of relatively undamped oscillations with instantaneous frequency that increased rapidly (time constant: 200 µs) from a few kHz to CF. Such frequency modulation was evident regardless of stimulus level and was also present post-mortem. Responses grew linearly at low stimulus levels, but exhibited a compressive nonlinearity at higher levels. Velocity-intensity functions were almost linear near response onset but became nonlinear within 100 µs. Slopes could be as low as 0.1-0.2 dB/dB at later times. Hence, the response envelopes became increasingly skewed at higher stimulus levels, with their center of gravity shifting to earlier times. The phases of near-CF response components changed by nearly 180 degrees as a function of time. At high stimulus levels, this generated cancellation notches and phase jumps in the frequency spectra. With increases in click level, sharpness of tuning deteriorated and the spectral maximum shifted to lower frequencies. Response phases also changed as a function of increasing stimulus intensity, exhibiting relative lags and leads at frequencies somewhat lower and higher than CF, respectively. In most respects, the magnitude and phase frequency spectra of responses to clicks closely resembled those of responses to tones. Post-mortem responses were similar to in vivo responses to very intense clicks.

Coding of Sounds in the Auditory System and Its Relevance to Signal Processing and Coding in Cochlear Implants

“…In theory, fast or instantaneous compression would mimic the normal action of the cochlea, because the compression observed on the BM appears to act almost instantaneously (38). However, in practice, problems arise when fast or instantaneous compression is used as the sole means of compression.…”

Reverse Correlation Analysis of Auditory-Nerve Fiber Responses to Broadband Noise in a Bird, the Barn Owl

“…Clicks trigger more non-linear processes, as shown for the chinchilla basilar membrane, where a subpopulation showed double-mode responses (Recio et al 1998), and cat auditory-nerve fibers (Lin and Guinan 2004). In those cases, however, there was a noticeable difference compared with the barn owl: The smaller component consisted of a later ringing, unlike the small early and time-restricted components in our dataset.…”

“…To compare IF slopes independently of frequency, we expressed them in unitless values (Recio et al 1998;Shera 2001;Recio-Spinoso et al 2005). To do so, the slope was divided by BF 2 .…”

“…Direct mechanical measurements at basal sites of the mammalian cochlea are available (de Boer and Nuttall 1997;Recio et al 1998;Recio-Spinoso et al 2009). These invasive techniques are potentially very harmful for the vulnerable cochlea.…”

The Role of Conduction Delay in Creating Sensitivity to Interaural Time Differences

“…Due to the periodicity of the sine wave, both scenarios would produce the same output. This coding strategy remains viable for non-periodic sounds because the tonotopic organization of the auditory system generates ringing properties of the inputs characteristic of the particular frequency (Carney et al 1999;Recio et al 1998;Wagner et al 2005). Previously we had shown that the inputs from the cochlear nuclei act as delay line inputs to form maps of ITD in the barn owl (Carr and Konishi 1990), but could not determine whether the delay lines had identical conduction latencies or were matched for phase only.…”