Sound localization under perturbed binaural hearing

Department of Biophysics, Radboud University Nijmegen, Nymegen, Gelderland, Netherlands
Journal of Neurophysiology (Impact Factor: 3.04). 01/2007; 97(1):715-26. DOI: 10.1152/jn.00260.2006
Source: PubMed

ABSTRACT This paper reports on the acute effects of a monaural plug on directional hearing in the horizontal (azimuth) and vertical (elevation) planes of human listeners. Sound localization behavior was tested with rapid head-orienting responses toward brief high-pass filtered (>3 kHz; HP) and broadband (0.5-20 kHz; BB) noises, with sound levels between 30 and 60 dB, A-weighted (dBA). To deny listeners any consistent azimuth-related head-shadow cues, stimuli were randomly interleaved. A plug immediately degraded azimuth performance, as evidenced by a sound level-dependent shift ("bias") of responses contralateral to the plug, and a level-dependent change in the slope of the stimulus-response relation ("gain"). Although the azimuth bias and gain were highly correlated, they could not be predicted from the plug's acoustic attenuation. Interestingly, listeners performed best for low-intensity stimuli at their normal-hearing side. These data demonstrate that listeners rely on monaural spectral cues for sound-source azimuth localization as soon as the binaural difference cues break down. Also the elevation response components were affected by the plug: elevation gain depended on both stimulus azimuth and on sound level and, as for azimuth, localization was best for low-intensity stimuli at the hearing side. Our results show that the neural computation of elevation incorporates a binaural weighting process that relies on the perceived, rather than the actual, sound-source azimuth. It is our conjecture that sound localization ensues from a weighting of all acoustic cues for both azimuth and elevation, in which the weights may be partially determined, and rapidly updated, by the reliability of the particular cue.

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    ABSTRACT: Hoorapparaatgebruikers ervaren vaak grote moeilijkheden om spraak te verstaan in lawaaierige omstandigheden. Om aan dit probleem tegemoet te komen wordt er gebruik gemaakt van ruisonderdrukkingsalgoritmen. De ontwikkeling hiervan gebeurt vaak monauraal. Het auditief systeem is echter een binauraal systeem, dit wil zeggen dat beide oren samenwerken om een geluidsbron waar te nemen als ´e´en enkele entiteit in de ruimte. Een bilaterale aanpassing, bestaande uit twee onafhankelijke monaurale systemen, neemt de binaurale werking van het menselijk auditief systeem niet noodzakelijk in rekening en verstoort daarom mogelijk de binaurale informatie nodig voor het correct lokaliseren van geluidsbronnen en voor een verbeterd spraakverstaan in lawaaierige omstandigheden. In dit project werd eerst de invloed van hedendaagse, bilaterale, ruisonderdrukkingssystemen op het binauraal horen onderzocht. Theoretische, objectieve en perceptuele evaluaties tonen aan dat de twee meest gebruikte commerci¨ele ruisonderdrukkingsalgoritmen, namelijk een bilaterale directionele en een bilaterale adaptief directionele (ADM) microfoonsconfiguratie, de binaurale informatie significant kunnen verstoren. Deze algoritmen bieden typisch geen mogelijkheid om ruisonderdrukking te combineren met het bewaren van alle binaurale informatie. Nadien werden drie nieuwe, binaurale, algoritmen ontworpen en geëvalueerd. Deze zijn gebaseerd op de werking van een meerkanaals Wiener filter (MWF) en veronderstellen de aanwezigheid van een communicatiekanaal tussen beide hoorapparaten. Er werd aangetoond dat de binaurale link tussen de hoorapparaten een significante winst in ruisonderdrukking oplevert. De binaurale MWF, de binaurale MWF met parti¨ele ruisschatting en de binaurale MWF met interaurale transferfunctie zorgen bovendien voor een betere combinatie van ruisonderdrukking met het bewaren van de binaurale informatie in vergelijking met de bilaterale ADM. Hearing aid users experience great difficulty in understanding speech in noisy environments. This has led to the introduction of noise reduction algorithms in hearing aids. The development of these algorithms is typically done monaurally. However, the human auditory system is a binaural system, which compares and combines the signals received by both ears to perceive a sound source as a single entity in space. Providing two monaural, independently operating, noise reduction systems, i.e. a bilateral configuration, to the hearing aid user may disrupt binaural information, needed to localize sound sources correctly and to improve speech perception in noise. In this research project, we first examined the influence of commercially available, bilateral, noise reduction algorithms on binaural hearing. Extensive objective and perceptual evaluations showed that the bilateral adaptive directional microphone (ADM) and the bilateral fixed directional microphone, two of the most commonly used noise reduction algorithms in hearing aids, can significantly distort the binaural properties of the sound signals. These distortions are well within the range used by the human auditory system. In what follows, three binaural algorithms, based on a multichannel Wiener filter (MWF) approach, were developed and evaluated. These algorithms assume a communication link between both hearing aids. It was observed that a binaural hearing aid design significantly increases noise reduction performance. Moreover, the binaural MWF, the binaural MWF with partial noise estimation (MWF-N) and the binaural MWF with interaural transfer function extension (MWF-ITF) provided a better combination of noise reduction performance and preservation of binaural cues compared to the bilateral ADM algorithm. Doctor in de ingenieurswetenschappen Afdeling ESAT - SCD: SISTA/COSIC/DOCARCH Departement Elektrotechniek (ESAT) Faculteit Ingenieurswetenschappen Doctoral thesis Doctoraatsthesis
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    ABSTRACT: Descending projections from sensory areas of the cerebral cortex are among the largest pathways in the brain, suggesting that they are important for subcortical processing. Although corticofugal inputs have been shown to modulate neuronal responses in the thalamus and midbrain, the behavioral importance of these changes remains unknown. In the auditory system, one of the major descending pathways is from cortical layer V pyramidal cells to the inferior colliculus in the midbrain. We examined the role of these neurons in experience-dependent recalibration of sound localization in adult ferrets by selectively killing the neurons using chromophore-targeted laser photolysis. When provided with appropriate training, animals normally relearn to localize sound accurately after altering the spatial cues available by reversibly occluding one ear. However, this ability was lost after eliminating corticocollicular neurons, whereas normal sound-localization accuracy was unaffected. The integrity of this descending pathway is therefore critical for learning-induced localization plasticity.
    Nature Neuroscience 02/2010; 13(2):253-60. DOI:10.1038/nn.2466 · 14.98 Impact Factor
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    ABSTRACT: Localizing a sound source involves the detection and integration of various spatial cues present in the sound waves at each ear. Previous studies indicate that the brain circuits underlying sound localization are calibrated by experience of the cues available to each individual. Plasticity in spatial hearing is most pronounced during development but can also be demonstrated during adulthood under certain circumstances. Investigations into whether adult humans can adjust to reduced input in one ear and learn a new correspondence between interaural differences cues and directions in space have produced conflicting results. Here we show that humans of both sexes can relearn to localize broadband sounds with a flat spectrum in the horizontal plane after altering the spatial cues available by plugging one ear. In subjects who received daily training, localization accuracy progressively shifted back toward their pre-plug performance after 1 week of earplugging, whereas no improvement was seen if all trials were performed on the same day. However, localization performance did not improve on a task that used stimuli in which the source spectrum was randomized from trial to trial, indicating that monaural spectral cues are needed for plasticity. We also characterized the effects of the earplug on sensitivity to interaural time and level differences and found no clear evidence for adaptation to these cues as the free-field localization performance improved. These findings suggest that the mature auditory system can accommodate abnormal inputs and maintain a stable spatial percept by reweighting different cues according to how informative they are.
    The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 04/2010; 30(14):4883-94. DOI:10.1523/JNEUROSCI.5488-09.2010 · 6.75 Impact Factor
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