Volume 116, Issue 4, October 2004
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
116(2004); http://dx.doi.org/10.1121/1.1784442View Description Hide Description
A phenomenological model with time-varying excitation and inhibition was developed to study possible neural mechanisms underlying changes in the representation of temporal envelopes along the auditory pathway. A modified version of an existing auditory-nerve model [Zhang et al., J. Acoust. Soc. Am. 109, 648–670 (2001)] was used to provide inputs to higher hypothetical processing centers. Model responses were compared directly to published physiological data at three levels: the auditory nerve, ventral cochlear nucleus, and inferior colliculus. Trends and absolute values of both average firing rate and synchrony to the modulation period were accurately predicted at each level for a wide range of stimulus modulation depths and modulation frequencies. The diversity of central physiological responses was accounted for with realistic variations of model parameters. Specifically, enhanced synchrony in the cochlear nucleus and rate-tuning to modulation frequency in the inferior colliculus were predicted by choosing appropriate relative strengths and time courses of excitatory and inhibitory inputs to postsynaptic modelcells. The proposed model is fundamentally different than others that have been used to explain the representation of envelopes in the mammalian midbrain, and it provides a computational tool for testing hypothesized relationships between physiology and psychophysics.
116(2004); http://dx.doi.org/10.1121/1.1785832View Description Hide Description
Reported here is a technique for measuring forward and reverse middle-ear transmission that exploits distortion-product otoacoustic emissions (DPOAEs) to drive the middle ear “in reverse” without opening the inner ear. The technique allows measurement of DPOAEs, middle-ear input impedance, and forward and reverse middle-ear transfer functions in the same animal. Intermodulation distortion in the cochlea generates a DPOAE at frequency measurable in both ear-canal pressure and the velocity of the stapes. The forward transfer function is computed from stapes velocities and corresponding ear-canal pressures measured at the two primary frequencies; the reverse transfer function is computed from velocity and pressure measurements at the DPOAE frequency. Middle-ear input impedance is computed from ear-canal pressure measurements and the measured Thévenin equivalent of the sound-delivery system. The technique was applied to measure middle-ear characteristics in anesthetized cats with widely opened middle-ear cavities (0.2–10 kHz). Stapes velocity was measured at the incudo-stapedial joint. Results on five animals are reported and compared with a published middle-ear model. The measured forward transfer functions and input impedances generally agree with previous measurements, and all measurements agree qualitatively with model predictions. The reverse transfer function is shown to depend on the acoustic load in the ear canal, and the measurements are used to compute the round-trip middle-ear gain and delay. Finally, the measurements are used to estimate the parameters of a two-port transfer-matrix description of the cat middle ear.
Distortion product otoacoustic emission (DPOAE) input/output functions and the influence of the second DPOAE source116(2004); http://dx.doi.org/10.1121/1.1791719View Description Hide Description
Distortion product otoacoustic emissions (DPOAEs) at have two components from different cochlear sources, i.e., a distortion component generated near and a reflection component from the characteristic site of The interaction of the two sources may negatively affect the DPOAE input/output (I/O) functions that are used to predict either auditory thresholds or the compression characteristics of the basilar membrane. This study investigates the influence of the reflection component on DPOAE I/O functions in a frequency range for from 1500 to 4500 Hz in steps of 18 Hz. A time windowing procedure is used to separate the components from the two DPOAE sources. With decreasing stimulus level, the relative contribution of the reflection component increases. I/O functions from the separated distortion component (DCOAE I/O functions) only show smooth changes in shape and slope with frequency, while “standard” DPOAE I/O functions show rapid changes between adjacent frequencies, indicating a strong influence from the interference with the second DPOAE source. A reduced variability for adjacent frequencies can be seen as well for prediction of hearing thresholds, when using DCOAE instead of DPOAE I/O functions.
116(2004); http://dx.doi.org/10.1121/1.1787523View Description Hide Description
This study examines auditory brainstem responses (ABR) elicited by rising frequency chirps. Two chirp stimuli were developed and designed such as to compensate for cochlear travel-time differences across frequency, in order to maximize neural synchrony. One chirp, referred to as the O-chirp, was based on estimates of human basilar membrane (BM) group delays derived from stimulus-frequency otoacoustic emissions (SFOAE) at a sound pressure level of 40 dB [Shera and Guinan, in Recent Developments in Auditory Mechanics (2000)]. The other chirp, referred to as the A-chirp, was derived from latency functions fitted to tone-burst-evoked ABR wave-V data over a wide range of stimulus levels and frequencies [Neely et al., J. Acoust. Soc. Am. 83(2), 652–656 (1988)]. In this case, a set of level-dependent chirps was generated. The chirp-evoked responses, particularly wave-V amplitude and latency, were compared to click responses and to responses obtained with the original chirp as defined in Dau et al. [J. Acoust. Soc. Am. 107(3), 1530–1540 (2000)], referred to here as the M-chirp since it is based on a (linear) cochlea model. The main hypothesis was that, at low and medium stimulation levels, the O- and A-chirps might produce a larger response than the original M-chirp whose parameters were essentially derived from high-level BM data. The main results of the present study are as follows: (i) All chirps evoked a larger wave-V amplitude than the click stimulus indicating that for the chirps a broader range of spectral components contributes effectively to the ABR. (ii) Only small differences were found between the O-chirp and M-chirp responses at low and medium levels. This indicates that SFOAE may not provide a robust estimate of BM group delay, particularly at low frequencies, or that frequency-dependent neural delays exist which are not reflected in the design of these chirps. (iii) The A-chirp produced the largest responses, particularly at low stimulation levels. This chirp might therefore be valuable for clinical applications, particularly in tests where the click stimulus has been used so far.