Binaural Auditory Processing Research Articles

Interdependence of spatial and temporal coding in the auditory midbrain.

To date, most physiological studies that investigated binaural auditory processing have addressed the topic rather exclusively in the context of sound localization. However, there is strong psychophysical evidence that binaural processing serves more than only sound localization. This raises the question of how binaural processing of spatial cues interacts with cues important for feature detection. The temporal structure of a sound is one such feature important for sound recognition. As a first approach, we investigated the influence of binaural cues on temporal processing in the mammalian auditory system. Here, we present evidence that binaural cues, namely interaural intensity differences (IIDs), have profound effects on filter properties for stimulus periodicity of auditory midbrain neurons in the echolocating big brown bat, Eptesicus fuscus. Our data indicate that these effects are partially due to changes in strength and timing of binaural inhibitory inputs. We measured filter characteristics for the periodicity (modulation frequency) of sinusoidally frequency modulated sounds (SFM) under different binaural conditions. As criteria, we used 50% filter cutoff frequencies of modulation transfer functions based on discharge rate as well as synchronicity of discharge to the sound envelope. The binaural conditions were contralateral stimulation only, equal stimulation at both ears (IID = 0 dB), and more intense at the ipsilateral ear (IID = -20, -30 dB). In 32% of neurons, the range of modulation frequencies the neurons responded to changed considerably comparing monaural and binaural (IID =0) stimulation. Moreover, in approximately 50% of neurons the range of modulation frequencies was narrower when the ipsilateral ear was favored (IID = -20) compared with equal stimulation at both ears (IID = 0). In approximately 10% of the neurons synchronization differed when comparing different binaural cues. Blockade of the GABAergic or glycinergic inputs to the cells recorded from revealed that inhibitory inputs were at least partially responsible for the observed changes in SFM filtering. In 25% of the neurons, drug application abolished those changes. Experiments using electronically introduced interaural time differences showed that the strength of ipsilaterally evoked inhibition increased with increasing modulation frequencies in one third of the cells tested. Thus glycinergic and GABAergic inhibition is at least one source responsible for the observed interdependence of temporal structure of a sound and spatial cues.

Chronic conductive hearing loss in adults: effects on the auditory brainstem response and masking-level difference.

To determine whether chronic conductive hearing loss in adults results in changes in the auditory brainstem response (ABR) similar to those observed in children with histories of otitis media with effusion. Test of effect of unilateral conductive hearing loss on adult ABR using age-matched control group and subjects as their own controls. Twelve adults with a history of unilateral conductive ear disease. An age-matched control group of 21 adults was also tested. The ABR, an electrophysiologic test of auditory brainstem functioning, was used to evaluate possible brainstem abnormalities in the impaired listeners. In addition, the masking-level difference, a behavioral test of binaural auditory processing in the brainstem, was used. When comparing the patients' diseased ears with their healthy ears, significant delays were seen for wave V as well as for the I-V and III-V interwave intervals. For comparison with the control population, significant prolongations were again seen for wave V and for the III-V interwave intervals. In addition, reduced masking-level differences and significant correlations between the masking-level differences and the ABRs, independent of hearing threshold, were noted. The results suggest that chronic conductive impairment in adults leads to changes in the ABR similar to those observed in children with histories of otitis media with effusion. As such, these changes do not appear to be related to a critical period of development.

Automatic speech recognition using signal processing based on auditory physiology and perception

Signal processing for automatic speech recognition has traditionally been inspired more by models of speech production than by models of auditory perception. While some aspects of human auditory processing have been implicit in traditional signal analysis for speech recognition, there is growing interest in the development of more computationally demanding signal processing strategies that are directly motivated by knowledge of auditory physiology and perception. The use of physiologically motivated signal processing has been shown to improve the accuracy of some automatic speech recognition systems, particularly in difficult acoustical environments. This talk will review and discuss the role that knowledge of human hearing has played in the design of speech recognition systems. The common features of auditory models that have emerged from various laboratories as well as their differences will be discussed. The recognition accuracy obtained using auditory models will be compared with the accuracy obtained using conventional signal analysis techniques, as well as the accuracy obtained using other approaches to robust recognition that are not physiologically based. Finally, an attempt will be made to identify aspects of monaural and binaural auditory processing that appear to be most helpful for robust recognition. [Work supported by ARPA.]

Binaural auditory processing in multiple sclerosis subjects

In order to relate human auditory processing to physiological and anatomical experimental animal data, we have examined the interrelationships between behavioral, electrophysiological and anatomical data obtained from human subjects with focal brainstem lesions. Thirty-eight subjects with multiple sclerosis were studied with tests of interaural time and level discrimination (just noticeable differences or jnds), brainstem auditory evoked potentials and magnetic resonance (MR) imaging. Interaural testing used two types of stimuli, high-pass (> 4000 Hz) and low-pass (< 1000 Hz) noise bursts. Abnormal time jnds (Tjnd) were far more common than abnormal level jnds (70% vs 11%); especially for the high-pass (Hp) noise (70% abnormal vs 40% abnormal for low-pass (Lp) noise). The HpTjnd could be abnormal with no other abnormalities; however, whenever the BAEPs, LpTjnd and/or level jnds were abnormal HpTjnd was always abnormal. Abnormal wave III amplitude was associated with abnormalities in both time jnds, but abnormal wave III latency with only abnormal HpTjnds. Abnormal wave V amplitude, when unilateral, was associated with a major HpTjnd abnormality, and, when bilateral, with both HpTjnd and LpTjnd major abnormalities. Sixteen of the subjects had their MR scans obtained with a uniform protocol and could be analyzed with objective criteria. In all four subjects with lesions involving the pontine auditory pathway, the BAEPs and both time jnds were abnormal. Of the twelve subjects with no lesions involving the pontine auditory pathway, all had normal BAEPs and level jnds. ten had normal LpTjnds, but only five had normal HpTjnds. We conclude that interaural time discrimination is closely related to the BAEPs and is dependent upon the stimulus spectrum. Redundant encoding of low-frequency sounds in the discharge patterns of auditory neurons, may explain why the HpTjnd is a better indicator of neural desynchrony than the LpTjnd. Encroachment of MS lesions upon the pontine auditory pathway always is associated with abnormal BAEPs and abnormal interaural time discrimination but may have normal interaural level discrimination. Our data provide one of the most direct demonstrations in humans of relationships among auditory performance, evoked potentials and anatomy. We present a model showing that many of these interrelationships can be readily interpreted using ideas developed from work on animals, even though these relationships could not have been predicted with confidence beforehand. This work provides a clear advance in our understanding of human auditory processing and should serve as a basis for future studies.

Connections between the cochlear nuclei in guinea pig

This study provides a detailed analysis of the appearances and distributions of neurons projecting from one cochlear nucleus to the other. Injections of wheatgerm agglutinin conjugated to horseradish peroxidase were made into ventral or dorsal cochlear nucleus of the guinea pig. Retrogradely labeled cells in the opposite cochlear nucleus were examined and quantified. Three major categories of labeled cells were discerned on the basis of their soma shape: elongate, round-to-oval, and polygonal. All injections resulted in widespread labeling of cells in all of these categories, but especially round-to-oval cells, in the opposite ventral cochlear nucleus and sparse labeling in the dorsal cochlear nucleus. The results suggest that there is a significant cochlear nucleus commissural projection involving heterogeneous cell types which could have diverse functions in binaural auditory signal processing.

Effect of conductive hearing loss and middle ear surgery on binaural hearing.

This study investigated whether conductive hearing loss reduces normal binaural hearing advantages and whether binaural hearing advantages are normal in patients who have had hearing thresholds improved by middle ear surgery. Binaural hearing was assessed at a test frequency of 500 Hz using the masking level difference and interaural time discrimination thresholds. Results indicated that binaural hearing is often poor in conductive lesion patients and that the reduction in binaural hearing is not always consistent with a simple attenuation of the acoustic signal. Poor binaural hearing sometimes occurs even when middle ear surgery has resulted in bilaterally normal hearing thresholds. Our preliminary results are consistent with the interpretation that auditory deprivation due to conductive hearing loss may result in poor binaural auditory processing.

Aging and Binaural Auditory Processing

Aging and Binaural Auditory Processing