Index of content:
Volume 118, Issue 3, September 2005
- PHYSIOLOGICAL ACOUSTICS 
118(2005); http://dx.doi.org/10.1121/1.1993148View Description Hide Description
A general mathematical approach was proposed to study phenomenological models of the inner-hair-cell and auditory-nerve (AN) synapse complex. Two models (Meddis, J. Acoust. Soc. Am.79(3), 702–711 (1986); Westerman and Smith, J. Acoust. Soc. Am.83(6), 2266–2276 (1988)) were studied using this unified approach. The responses of both models to a constant-intensity stimulus were described mathematically, and the relationship between model parameters and response characteristics was investigated. The mathematical descriptions of the two models were essentially equivalent despite their structural differences. This analytical approach was used to study the effects of adaptation characteristics on model parameters and of model parameters on adaptation characteristics. The results provided insights into these models and the underlying biophysical processing. This analytical method was also used to study offset adaptation, and it was found that the offset adaptation of both models was limited by the models’ structures. A modified version of the synapsemodel, which has the same onset adaptation but improved offset adaptation, is proposed here. This modified synapsemodel produces more physiologically realistic offset adaptation and also enhances the modulation gain of model AN fiber responses, consistent with AN physiology.
Phase of neural excitation relative to basilar membrane motion in the organ of Corti: Theoretical considerations118(2005); http://dx.doi.org/10.1121/1.2000770View Description Hide Description
Although the auditory transduction process is dependent on neural excitation of the auditory nerve in relation to motion of the basilar membrane (BM) in the organ of Corti (OC), specifics of this process are unclear. In this study, therefore, an attempt was made to estimate the phase of the neural excitation relative to the BM motion using a finite-element model of the OC at the basal turn of the gerbil, including the fluid-structure interaction with the lymph fluid. It was found that neural excitation occurs when the BM exhibits a maximum velocity toward the scala vestibuli at 10 Hz and shows a phase delay relative to the BM motion with increasing frequency up to 800 Hz. It then shows a phase advance until the frequency reaches 2 kHz. From 2 kHz, neural excitation again shows a phase delay with increasing frequency. From 800 Hz up to 2 kHz, the phase advances because the dominant force exerted on the hair bundle shifts from a velocity-dependent Couette flow-induced force to a displacement-dependent force induced by the pressure difference. The phase delay that occurs from 2 kHz is caused by the resonance process of the hair bundle of the IHC.
Effects of aspirin on distortion product otoacoustic emission suppression in human adults: A comparison with neonatal data118(2005); http://dx.doi.org/10.1121/1.1985043View Description Hide Description
One of the distortion product otoacoustic emission (DPOAE) paradigms used to study cochlear function is DPOAE ipsilateral suppression. Newborns do not have adultlike DPOAE suppression. At 6000 Hz, infants show excessively narrow DPOAE suppression tuning and shallow growth of suppression for low-frequency suppressor tones. The source of this immaturity is not known but the outer hair cell (OHC) is one possible locus. In the present study, DPOAE suppression was measured at and 6000 Hz from two groups with impaired OHC function in an attempt to model the observed immaturity in neonates: adults with aspirin-induced OHC dysfunction and subjects with sensorineural hearing loss(SNHL). Their DPOAE suppression results were compared to those obtained from a group of term newborns to address whether infant DPOAE suppression resembles suppression from individuals with known OHC dysfunction. Results indicate that aspirin systematically alters DPOAE suppression in adults at , but not 1500 Hz. However, neither aspirin-induced OHC dysfunction nor naturally occurring SNHL produces “neonatal-like” DPOAE suppression at either test frequency. This finding does not support the hypothesis that non-adultlike DPOAE suppression characterizing newborns can be explained by minor impairments or alterations of OHC function.
118(2005); http://dx.doi.org/10.1121/1.2000769View Description Hide Description
Transient evoked otoacoustic emissions of a large population of neonates (466 ears) are analyzed, with the aim of establishing if a significant latency difference can be observed between “pass” (333) and “fail” (133) ears, discriminated with a screening protocol based on band and global reproducibility. The ears that did not pass the test in at least one frequency band are named “fail,” for simplicity, but they should be more appropriately defined as “partial pass.” In a previous study, significantly different average latencies had been observed in adult subjects, comparing normal hearing and hearing impaired ears [J. Acoust. Soc. Am.111, 297–308 (2002)], but no similar study has been conducted on neonates yet. An improved wavelet technique was applied to transient evoked otoacoustic emission data, to get accurate experimental measurements of the otoacoustic emission latency. The results show statistically significant differences between the latency distributions of the “pass” and “fail” populations, with the “fail” latencies longer. However, non-Gaussianity of the distributions and systematic errors associated with low reproducibility levels suggested using a conservative approach in the data analysis and interpretation. New otoacoustic estimates of cochlear tuning in neonates are also provided.
118(2005); http://dx.doi.org/10.1121/1.1993147View Description Hide Description
Measurements of motion of an in vitro preparation of the alligator lizard basilar papilla in response to sound demonstrate elliptical trajectories. These trajectories are consistent with the presence of both a translational and rotational mode of motion. The translational mode is independent of frequency, and the rotational mode has a displacement peak near . These measurements can be explained by a simple mechanical system in which the basilar papilla is supported asymmetrically on the basilar membrane. In a quantitative model, the translational admittance is compliant while the rotational admittance is second order. Best-fit model parameters are consistent with estimates based on anatomy and predict that fluid flow across hair bundles is a primary source of viscous damping. The model predicts that the rotational mode contributes to the high-frequency slopes of auditory nerve fiber tuning curves, providing a physical explanation for a low-pass filter required in models of this cochlea. The combination of modes makes the sensitivity of hair bundles more uniform with radial position than that which would result from pure rotation. A mechanical analogy with the organ of Corti suggests that these two modes of motion may also be present in the mammalian cochlea.