Stapes Vibration in the Chinchilla Middle Ear: Relation to Behavioral and Auditory-Nerve Thresholds.

The vibratory responses to tones of the stapes and incus were measured in the middle ears of deeply anesthetized chinchillas using a wide-band acoustic-stimulus system and a laser velocimeter coupled to a microscope. With the laser beam at an angle of about 40 ° relative to the axis of stapes piston-like motion, the sensitivity-vs.-frequency curves of vibrations at the head of the stapes and the incus lenticular process were very similar to each other but larger, in the range 15-30 kHz, than the vibrations of the incus just peripheral to the pedicle. With the laser beam aligned with the axis of piston-like stapes motion, vibrations of the incus just peripheral to its pedicle were very similar to the vibrations of the lenticular process or the stapes head measured at the 40 ° angle. Thus, the pedicle prevents transmission to the stapes of components of incus vibration not aligned with the axis of stapes piston-like motion. The mean magnitude curve of stapes velocities is fairly flat over a wide frequency range, with a mean value of about 0.19 mm(.)(s Pa(-1)), has a high-frequency cutoff of 25 kHz (measured at -3 dB re the mean value), and decreases with a slope of about -60 dB/octave at higher frequencies. According to our measurements, the chinchilla middle ear transmits acoustic signals into the cochlea at frequencies exceeding both the bandwidth of responses of auditory-nerve fibers and the upper cutoff of hearing. The phase lags of stapes velocity relative to ear-canal pressure increase approximately linearly, with slopes equivalent to pure delays of about 57-76 µs.

Basilar-membrane responses to broadband noise modeled using linear filters with rational transfer functions.

Basilar-membrane responses to white Gaussian noise were recorded using laser velocimetry at basal sites of the chinchilla cochlea with characteristic frequencies near 10 kHz and first-order Wiener kernels were computed by cross correlation of the stimuli and the responses. The presence or absence of minimum-phase behavior was explored by fitting the kernels with discrete linear filters with rational transfer functions. Excellent fits to the kernels were obtained with filters with transfer functions including zeroes located outside the unit circle, implying nonminimum-phase behavior. These filters accurately predicted basilar-membrane responses to other noise stimuli presented at the same level as the stimulus for the kernel computation. Fits with all-pole and other minimum-phase discrete filters were inferior to fits with nonminimum-phase filters. Minimum-phase functions predicted from the amplitude functions of the Wiener kernels by Hilbert transforms were different from the measured phase curves. These results, which suggest that basilar-membrane responses do not have the minimum-phase property, challenge the validity of models of cochlear processing, which incorporate minimum-phase behavior.

Wiener-kernel analysis of responses to noise of chinchilla auditory-nerve fibers.

Responses to broadband Gaussian white noise were recorded in auditory-nerve fibers of deeply anesthetized chinchillas and analyzed by computation of zeroth-, first-, and second-order Wiener kernels. The first-order kernels (similar to reverse correlations or "revcors") of fibers with characteristic frequency (CF) <2 kHz consisted of lightly damped transient oscillations with frequency equal to CF. Because of the decay of phase locking strength as a function of frequency, the signal-to-noise ratio of first-order kernels of fibers with CFs >2 kHz decreased with increasing CF at a rate of about -18 dB per octave. However, residual first-order kernels could be detected in fibers with CF as high as 12 kHz. Second-order kernels, 2-dimensional matrices, reveal prominent periodicity at the CF frequency, regardless of CF. Thus onset delays, frequency glides, and near-CF group delays could be estimated for auditory-nerve fibers innervating the entire length of the chinchilla cochlea.