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1.  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.
PMCID: PMC3572753  PMID: 20542757
Autoregressive moving-average (ARMA) modeling; basilar membrane (BM); cochlea; Hilbert transform; minimum phase; Wiener kernels
2.  Traveling waves on the organ of Corti of the chinchilla cochlea: spatial trajectories of inner hair cell depolarization inferred from responses of auditory-nerve fibers 
Spatial magnitude and phase profiles for inner hair cell depolarization throughout the chinchilla cochlea were inferred from responses of auditory-nerve fibers to threshold- and moderate-level tones and tone complexes. Firing-rate profiles for frequencies ≤ 2 kHz are bimodal, with the major peak at the characteristic place and a secondary peak at 3–5 mm from the extreme base. Response-phase trajectories are synchronous with peak outward stapes displacement at the extreme cochlear base and accumulate 1.5-period lags at the characteristic places. High-frequency phase trajectories are very similar to the trajectories of basilar-membrane peak velocity toward scala tympani. Low-frequency phase trajectories undergo a polarity flip in a region, 6.5–9 mm from the cochlear base, where traveling-wave phase velocity attains a local minimum and a local maximum and where the onset latencies of near-threshold impulse responses computed from responses to near-threshold white noise exhibit a local minimum. That region is the same where frequency-threshold tuning curves of auditory-nerve fibers undergo a shape transition. Since depolarization of inner hair cells presumably indicates the mechanical stimulus to their stereocilia, the present results suggest that distinct low-frequency forward waves of organ of Corti vibration are launched simultaneously at the extreme base of the cochlea and at the 6.5–9 mm transition region, from where antiphasic reflections arise.
PMCID: PMC3436599  PMID: 22855802
3.  Enhancement and Distortion in the Temporal Representation of Sounds in the Ventral Cochlear Nucleus of Chinchillas and Cats 
PLoS ONE  2012;7(9):e44286.
A subset of neurons in the cochlear nucleus (CN) of the auditory brainstem has the ability to enhance the auditory nerve's temporal representation of stimulating sounds. These neurons reside in the ventral region of the CN (VCN) and are usually known as highly synchronized, or high-sync, neurons. Most published reports about the existence and properties of high-sync neurons are based on recordings performed on a VCN output tract—not the VCN itself—of cats. In other species, comprehensive studies detailing the properties of high-sync neurons, or even acknowledging their existence, are missing.
Examination of the responses of a population of VCN neurons in chinchillas revealed that a subset of those neurons have temporal properties similar to high-sync neurons in the cat. Phase locking and entrainment—the ability of a neuron to fire action potentials at a certain stimulus phase and at almost every stimulus period, respectively—have similar maximum values in cats and chinchillas. Ranges of characteristic frequencies for high-sync neurons in chinchillas and cats extend up to 600 and 1000 Hz, respectively. Enhancement of temporal processing relative to auditory nerve fibers (ANFs), which has been shown previously in cats using tonal and white-noise stimuli, is also demonstrated here in the responses of VCN neurons to synthetic and spoken vowel sounds.
Along with the large amount of phase locking displayed by some VCN neurons there occurs a deterioration in the spectral representation of the stimuli (tones or vowels). High-sync neurons exhibit a greater distortion in their responses to tones or vowels than do other types of VCN neurons and auditory nerve fibers.
Standard deviations of first-spike latency measured in responses of high-sync neurons are lower than similar values measured in ANFs' responses. This might indicate a role of high-sync neurons in other tasks beyond sound localization.
PMCID: PMC3445608  PMID: 23028514
4.  Timing of cochlear responses inferred from frequency-threshold tuning curves of auditory-nerve fibers 
Hearing research  2010;272(1-2):178-186.
Links between frequency tuning and timing were explored in the responses to sound of auditory-nerve fibers. Synthetic transfer functions were constructed by combining filter functions, derived via minimum-phase computations from average frequency-threshold tuning curves of chinchilla auditory-nerve fibers with high spontaneous activity (A. N. Temchin et al., J. Neurophysiol. 100: 2889–2898, 2008), and signal-front delays specified by the latencies of basilar-membrane and auditory-nerve fiber responses to intense clicks (A. N. Temchin et al., J. Neurophysiol. 93: 3635–3648, 2005). The transfer functions predict several features of the phase-frequency curves of cochlear responses to tones, including their shape transitions in the regions with characteristic frequencies of 1 kHz and 3–4 kHz (A. N. Temchin and M. A. Ruggero, JARO 11: 297–318, 2010). The transfer functions also predict the shapes of cochlear impulse responses, including the polarities of their frequency sweeps and their transition at characteristic frequencies around 1 kHz. Predictions are especially accurate for characteristic frequencies < 1 kHz.
PMCID: PMC3039049  PMID: 20951191
5.  Basilar Membrane Responses to Noise at a Basal Site of the Chinchilla Cochlea: Quasi-Linear Filtering 
Basilar membrane responses to clicks and to white noise were recorded using laser velocimetry at basal sites of the chinchilla cochlea with characteristic frequencies near 10 kHz. Responses to noise grew at compressive rates and their instantaneous frequencies decreased with increasing stimulus level. First-order Wiener kernels were computed by cross-correlation of the noise stimuli and the responses. For linear systems, first-order Wiener kernels are identical to unit impulse responses. In the case of basilar membrane responses, first-order Wiener kernels and responses to clicks measured at the same sites were similar but not identical. Both consisted of transient oscillations with onset frequencies which increased rapidly, over about 0.5 ms, from 4–5 kHz to the characteristic frequency. Both first-order Wiener kernels and responses to clicks were more highly damped, exhibited slower frequency modulation, and grew at compressive rates with increasing stimulus levels. Responses to clicks had longer durations than the Wiener kernels. The statistical distribution of basilar membrane responses to Gaussian white noise is also Gaussian and the envelopes of the responses are Rayleigh distributed, as they should be for Gaussian noise passing through a linear band-pass filter. Accordingly, basilar membrane responses were accurately predicted by linear filters specified by the first-order Wiener kernels of responses to noise presented at the same level. Overall, the results indicate that cochlear nonlinearity is not instantaneous and resembles automatic gain control.
PMCID: PMC2774406  PMID: 19495878
Wiener kernels; clicks; laser velocimetry; frequency glides; impulse responses
6.  Wiener Kernels of Chinchilla Auditory-Nerve Fibers: Verification Using Responses to Tones, Clicks, and Noise and Comparison With Basilar-Membrane Vibrations 
Journal of neurophysiology  2005;93(6):3635-3648.
Responses to tones, clicks, and noise were recorded from chinchilla auditory-nerve fibers (ANFs). The responses to noise were analyzed by computing the zeroth-, first-, and second-order Wiener kernels (h0, h1, and h2). The h1s correctly predicted the frequency tuning and phases of responses to tones of ANFs with low characteristic frequency (CF). The h2s correctly predicted the frequency tuning and phases of responses to tones of all ANFs, regardless of CF. Also regardless of CF, the kernels jointly predicted about 77% of the features of ANF responses to “frozen” samples of noise. Near-CF group delays of kernels and signal-front delays of responses to intense rarefaction clicks exceeded by 1 ms the corresponding basilar-membrane delays at both apical and basal sites of the chinchilla cochlea. This result, confirming that synaptic and neural processes amount to 1 ms regardless of CF, permitted drawing a map of basilar-membrane delay as a function of position for the entire length of the chinchilla cochlea, a first for amniotic species.
PMCID: PMC1876724  PMID: 15659530

Results 1-6 (6)