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Aug 2008

Volume 124, Issue 2, pp. 689-EL61

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Sources of variability in distortion product otoacoustic emissions

Cassie A. Garner, Stephen T. Neely, and Michael P. Gorga

J. Acoust. Soc. Am. Volume 124, Issue 2, pp. 1054-1067 (2008); (14 pages) | Cited 3 times

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The goal of this study was to determine the extent to which the variability seen in distortion product otoacoustic emissions (DPOAEs), among ears with normal hearing, could be accounted for. Several factors were selected for investigation, including behavioral threshold, differences in middle-ear transmission characteristics either in the forward or the reverse direction, and differences in contributions from the distortion and reflection sources. These variables were assessed after optimizing stimulus parameters for individual ears at each frequency. A multiple-linear regression was performed to identify whether the selected variables, either individually or in combination, explained significant portions of variability in DPOAE responses. Behavioral threshold at the f2 frequency and behavioral threshold squared at that same frequency explained the largest amount of variability in DPOAE level, compared to the other variables. The combined model explained a small, but significant, amount of variance in DPOAE level at five frequencies. A large amount of residual variability remained, even at frequencies where the model accounted for significant amounts of variance.
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43.64.Jb Otoacoustic emissions
43.64.Kc Cochlear mechanics

Statistics of instabilities in a state space model of the human cochlea

Emery M. Ku, Stephen J. Elliott, and Ben Lineton

J. Acoust. Soc. Am. Volume 124, Issue 2, pp. 1068-1079 (2008); (12 pages) | Cited 2 times

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A state space model of the human cochlea is used to test Zweig and Shera’s [(1995) “The origin of periodicity in the spectrum of evoked otoacoustic emissions,” J. Acoust. Soc. Am. 98(4), 2018–2047 ] multiple-reflection theory of spontaneous otoacoustic emission (SOAE) generation. The state space formulation is especially well suited to this task as the unstable frequencies of an active model can be rapidly and unambiguously determined. The cochlear model includes a human middle ear boundary and matches human enhancement, tuning, and traveling wave characteristics. Linear instabilities can arise across a wide bandwidth of frequencies in the model when the smooth spatial variation of basilar membrane impedance is perturbed, though it is believed that only unstable frequencies near the middle ear’s range of greatest transmissibility are detected as SOAEs in the ear canal. The salient features of Zweig and Shera’s theory are observed in this active model given several classes of perturbations in the distribution of feedback gain along the cochlea. Spatially random gain variations are used to approximate what may exist in human cochleae. The statistics of the unstable frequencies for random, spatially dense variations in gain are presented; the average spacings of adjacent unstable frequencies agree with the preferred minimum distance observed in human SOAE data.
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43.64.Kc Cochlear mechanics
43.64.Jb Otoacoustic emissions
43.40.Yq Instrumentation and techniques for tests and measurement relating to shock and vibration, including vibration pickups, indicators, and generators, mechanical impedance
43.64.Bt Models and theories of the auditory system

Medial olivocochlear efferent inhibition of basilar-membrane responses to clicks: Evidence for two modes of cochlear mechanical excitation

John J. Guinan, Jr. and Nigel P. Cooper

J. Acoust. Soc. Am. Volume 124, Issue 2, pp. 1080-1092 (2008); (13 pages) | Cited 1 time

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Conceptualizations of mammalian cochlear mechanics are based on basilar-membrane (BM) traveling waves that scale with frequency along the length of the cochlea, are amplified by outer hair cells (OHCs), and excite inner hair cells and auditory-nerve (AN) fibers in a simple way. However, recent experimental work has shown medial-olivocochlear (MOC) inhibition of AN responses to clicks that do not fit with this picture. To test whether this AN-initial-peak (ANIP) inhibition might result from hitherto unrecognized aspects of the traveling-wave or MOC-evoked inhibition, MOC effects on BM responses to clicks in the basal turns of guinea pig and chinchilla cochleae were measured. MOC stimulation inhibited BM click responses in a time and level dependent manner. Inhibition was not seen during the first half-cycle of the responses, but built up gradually, and ultimately increased the responses’ decay rates. MOC stimulation also produced small phase leads in the response wave forms, but had little effect on the instantaneous frequency or the waxing and waning of the responses. These data, plus recent AN data, support the hypothesis that the MOC-evoked inhibitions of the traveling wave and of the ANIP response are separate phenomena, and indicate that the OHCs can affect at least two separate modes of excitation in the mammalian cochlea.
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43.64.Kc Cochlear mechanics

Comparison of behavioral and auditory brainstem response measures of threshold shift in rats exposed to loud sound

Henry E. Heffner, Gimseong Koay, and Rickye S. Heffner

J. Acoust. Soc. Am. Volume 124, Issue 2, pp. 1093-1104 (2008); (12 pages)

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The purpose of this study was to determine how closely the auditory brainstem response (ABR) can estimate sensorineural threshold shifts in rats exposed to loud sound. Behavioral and ABR thresholds were obtained for tones or noise before and after exposure to loud sound. The results showed that the ABR threshold shift obtained with tone pips estimated the initial pure-tone threshold shifts to within ±5 dB 11% of the time and the permanent pure-tone threshold shifts 55% of the time, both with large errors. Determining behavioral thresholds for the same tone pips used for the ABR did not improve the agreement between the measures. In contrast, the ABR obtained with octave noise estimated the initial threshold shifts for that noise to within ±5 dB 25% of the time and the permanent threshold shifts 89% of the time, with much smaller errors. Thus, it appears that the noise-evoked ABR is more accurate in estimating threshold shift than the tone-evoked ABR.
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43.64.Ri Evoked responses to sounds
43.64.Wn Effects of noise and trauma on the auditory system
43.66.Sr Deafness, audiometry, aging effects
43.66.Gf Detection and discrimination of sound by animals
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