Index of content:
Volume 126, Issue 5, November 2009
- PHYSIOLOGICAL ACOUSTICS 
A phenomenological model of the synapse between the inner hair cell and auditory nerve: Long-term adaptation with power-law dynamics126(2009); http://dx.doi.org/10.1121/1.3238250View Description Hide Description
There is growing evidence that the dynamics of biological systems that appear to be exponential over short time courses are in some cases better described over the long-term by power-law dynamics. A model of rate adaptation at the synapse between inner hair cells and auditory-nerve (AN) fibers that includes both exponential and power-law dynamics is presented here. Exponentially adapting components with rapid and short-term time constants, which are mainly responsible for shaping onset responses, are followed by two parallel paths with power-law adaptation that provide slowly and rapidly adapting responses. The slowly adapting power-law component significantly improves predictions of the recovery of the AN response after stimulus offset. The faster power-law adaptation is necessary to account for the “additivity” of rate in response to stimuli with amplitude increments. The proposed model is capable of accurately predicting several sets of AN data, including amplitude-modulation transfer functions, long-term adaptation, forward masking, and adaptation to increments and decrements in the amplitude of an ongoing stimulus.
Contralateral acoustic stimulation alters the magnitude and phase of distortion product otoacoustic emissions126(2009); http://dx.doi.org/10.1121/1.3224716View Description Hide Description
Activation of medial olivocochlear efferents through contralateral acoustic stimulation (CAS) has been shown to modulate distortion product otoacoustic emission (DPOAE) level in various ways (enhancement, reduction, or no change). The goal of this study was to investigate the effect of a range of CAS levels on DPOAE fine structure. The DPOAE was recorded (, , and ) from eight normal-hearing subjects, using both a frequency-sweep paradigm and a fixed frequency paradigm. Contamination due to the middle earmuscle reflex was avoided by monitoring the magnitude and phase of a probe in the test ear and by monitoring DPOAE stimulus levels throughout testing. Results show modulations in both level and frequency of DPOAE fine structure patterns. Frequency shifts observed at DPOAE level minima could explain reports of enhancement in DPOAE level due to efferent activation. CAS affected the magnitude and phase of the DPOAE component from the characteristic frequency region to a greater extent than the component from the overlap region between the stimulus tones. This differential effect explains the occasional enhancement observed in DPOAE level as well as the frequency shift in fine structure patterns.
126(2009); http://dx.doi.org/10.1121/1.3224762View Description Hide Description
A nonlinear and non-local cochlear model has been efficiently solved in the time domain numerically, obtaining the evolution of the transverse displacement of the basilar membrane at each cochlear place. This information allows one to follow the forward and backward propagation of the traveling wave along the basilar membrane, and to evaluate the otoacoustic response from the time evolution of the stapes displacement. The phase/frequency relation of the response can be predicted, as well as the physical delay associated with the response onset time, to evaluate the relation between different cochlear characteristic times as a function of the stimulus level and of the physical parameters of the model. For a nonlinear cochlea, simplistic frequency-domain interpretations of the otoacoustic response phase behavior may give inconsistent results. Time-domain numerical solutions of the underlying nonlinear and non-local full cochlear model using a large number (thousands) of partitions in space and an adaptive mesh in time are rather time and memory consuming. Therefore, in order to be able to use standard personal computers for simulations reliably, the discretized model has been carefully designed to enforce sparsity of the matrices using a multi-iterative approach. Preliminary results concerning the cochlear characteristic delays are also presented.
126(2009); http://dx.doi.org/10.1121/1.3238239View Description Hide Description
The cues used by mammals to localize sound can become corrupted when multiple sound sources are present due to the interference of sound waves. Under such circumstances these localization cues become spurious and often fluctuate rapidly . By contrast, rapid fluctuations in sound pressure level do not indicate a corrupted signal, but rather may convey important information about the sound source. It is proposed that filtering in the auditory brainstem acts to selectively attenuate signals associated with the presence of rapidly fluctuating (spurious) localization cues, but not those associated with slowly varying cues. Further it is proposed that specific inhibitory circuitry in the auditory brainstem, centered on the dorsal nucleus of the lateral lemniscus (DNLL), contributes to this selective filtering. Data from extra-cellular recordings in anesthetized Mongolian gerbils are presented to support these hypotheses for a subpopulation of DNLL neurons. These results provide new insights into how the mammalian auditory system processes information about multiple sound sources.