Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 94-100).

The time accuracy of the auditory neuronal pathway in its sound localization branch is high, compared to other sensory systems. The time differences in the sound arrival between the left and right ear are distinguished by the neural circuit in this branch. The accuracy achieved here is in the order of tens of microseconds. This phenomenon has not yet been definitively clarified. In this master thesis, a model of a neuron central to this neural circuit is presented. This neuron is called binaural (neuron of the two ears) and is located in the medial superior olive (MSO) neural nucleus. The properties of the MSO neuron are described. Specifically, the neuron acts as a coincidence detector, and this is necessary for the circuit functioning. Main result of the thesis is the theory explaining how the function of the coincidence detector can be described based on the interaction of the post-synaptic potentials on the spike-response model neuron. Generality and implications for the auditory pathway are then discussed.

Sound source localization depends on the auditory system to identify, recognize, and segregate elements of salient sources over distracting noise. My research investigates sensory mechanisms involved in these auditory processing tasks of an insect hearing specialist, to isolate individual sound sources of interest over noise. I first developed quantitative methods to determine signal features that the acoustic parasitoid fly Ormia ochracea (Diptera: Tachinidae) evaluate for host cricket song recognition. With flies subjected to a no-choice paradigm and forced to track a switch in the broadcast location of test songs, I describe several response features (distance, steering velocity, and angular orientation) that vary with song pulse rate preferences. I incorporate these response measures in a phonotaxis performance index that is sensitive to capturing response variation that may underlie song recognition. I demonstrate that Floridian O. ochracea exhibit phonotaxis to a combination of pulse durations and interpulse intervals that combine to a range of accepted pulse periods. Under complex acoustic conditions of multiple coherent cricket songs that overlap in time and space, O. ochracea may experience a phantom source illusion and localize a direction between actual source locations. By varying the temporal overlap between competing sources, I demonstrate that O. ochracea are able to resolve this illusion via the precedence effect: exploitation of small time differences between competing sources to selectively localize the leading over lagging sources. An increase in spatial separation between cricket song and masking noise does not reduce song detection thresholds nor improve song localization accuracy. Instead, walking responses are diverted away from both song and noise. My findings support the idea that the ears of O. ochracea function as bilateral symmetry detectors to balance sound intensity, sound arrive time differences, and temporal pattern input to both sides of the auditory system. Asymmetric acoustic input result in corrective turning behaviour to re-establish balance for successful source localization.

Thesis (Ph. D.)--West Virginia University, 2003. / Title from document title page. Document formatted into pages; contains x, 179 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 158-179).

The most accepted model of mammalian binaural sound localization postulates two neural/perceptual channels with hemifield tuning and overlapping medial borders; the extent to which the two channels are co-activated by the source is the neural "code" for the source's azimuthal location. This model does not take into account physiological data on the existence of a population of cells with spatial receptive fields centered on the azimuthal midline. The following work sought to test the hypothesis that the mammalian binaural sound localization apparatus includes a third, midline-tuned channel. Ten experiments used a selective adaptation paradigm in human listeners to probe for the existence of a midline channel. Psychometric functions were obtained for lateral position based on ITD or ILD both before and after adaptation with high-frequency (2800 and 4200 Hz) or low-frequency (260 and 570 Hz) tones. Listeners experienced highly lateralized adaptor stimuli with different frequencies at each ear (asymmetrical adaptation), highly lateralized adaptor stimuli of the same frequency at each ear (symmetrical adaptation), and single frequency adaptation at the midline (central adaptation). At both high and low frequencies in the domains of both interaural time difference (ITD) and interaural level difference (ILD), location judgements after asymmetrical adaptation shifted away from the fatigued side. These shifts occurred across each adapted hemifield and extended slightly over the midline, as is consistent with the two-channel model. The two-channel model would predict no effect of symmetrical or central adaptation because fatiguing both lateral channels equally would not change their relative activation by a given source. In practice, the result of symmetrical adaptation was a shift in location judgements towards the midline as would be expected if adaptation of the lateral channels resulted in a greater relative contribution of a third, midline channel. Likewise, central adaptation tended to result in shifts in perceived location towards the sides. The evidence for the midline channel was strong for high and low frequencies localized by ILD, and was present for low frequencies, but not for high frequencies, localized by ITD.

Sound source localization depends on the auditory system to identify, recognize, and segregate elements of salient sources over distracting noise. My research investigates sensory mechanisms involved in these auditory processing tasks of an insect hearing specialist, to isolate individual sound sources of interest over noise. I first developed quantitative methods to determine signal features that the acoustic parasitoid fly Ormia ochracea (Diptera: Tachinidae) evaluate for host cricket song recognition. With flies subjected to a no-choice paradigm and forced to track a switch in the broadcast location of test songs, I describe several response features (distance, steering velocity, and angular orientation) that vary with song pulse rate preferences. I incorporate these response measures in a phonotaxis performance index that is sensitive to capturing response variation that may underlie song recognition. I demonstrate that Floridian O. ochracea exhibit phonotaxis to a combination of pulse durations and interpulse intervals that combine to a range of accepted pulse periods. Under complex acoustic conditions of multiple coherent cricket songs that overlap in time and space, O. ochracea may experience a phantom source illusion and localize a direction between actual source locations. By varying the temporal overlap between competing sources, I demonstrate that O. ochracea are able to resolve this illusion via the precedence effect: exploitation of small time differences between competing sources to selectively localize the leading over lagging sources. An increase in spatial separation between cricket song and masking noise does not reduce song detection thresholds nor improve song localization accuracy. Instead, walking responses are diverted away from both song and noise. My findings support the idea that the ears of O. ochracea function as bilateral symmetry detectors to balance sound intensity, sound arrive time differences, and temporal pattern input to both sides of the auditory system. Asymmetric acoustic input result in corrective turning behaviour to re-establish balance for successful source localization.

The central theme of this thesis is a description of information processing in the sound localization circuit of the auditory pathway. The focus is on principal neurons of the medial superior olive (MSO), the first major convergence point for binaural information. Selected properties and relations of MSO neurons are derived and expressed through models. In the thesis we present three modeling studies. The first one clarifies a relation- ship between biophysical parameters of the MSO neuron and its ability to detect coincidental spikes from the left and the right ear. The second study describes the statistical behavior of spike trains on the input and output of the MSO neuron. In the third work, we studied how interaural coherence could guide localization of sound sources in complex listening situations with multiple sound sources in reverberant environments. The main results are analytical and numerical models describing the aforemen- tioned relations and behaviors. Secondary results include that inhibitory input to the MSO neuron narrows and shifts the time range of coincidence detection, that ergodic assumption from statistical physics and circular statistics are beneficial in the description of spike trains in the auditory pathway, and that interaural level difference of parts of the signal with...

This work introduces new binaural beamforming algorithms for hearing aids, with a robustness to errors in the estimate of the target speaker direction of arrival (DOA) and a good trade-off between noise reduction and preservation of the noise/interferers spatial impression. Three robust designs are proposed, and their robustness is confirmed by simulation results. These robust designs are a combination of binaural and monaural beamformers using two different microphone configurations: one for low frequency components and one for high frequency components. The robust designs are also found to be robust to mismatch between the anechoic propagation models used for the beamformers designs and the reverberant propagation models used to generate the signals at the microphones in the simulations. To preserve the binaural cues of the noise/interferers in the binaural beamformer outputs, a method based on a mixing/selection of different available binaural signals is proposed, using a classification from the phase and magnitude of a complex coherence function. This method is added as a post processor to the beamforming designs robust to target DOA mismatch. Simulation results show that the resulting mixed binaural output signals have a good binaural cues preservation level that outperform the benchmark design, with significant noise reduction and low target distortion. Since knowledge of source DOAs is important for beamforming noise reduction, a beamformer-based broadband multi-source DOA detection system is also developed in the thesis, using information from different frequencies or sub‐bands to obtain global estimates of sources DOAs. Simulation results shows that using one beamformer on each side is capable of detecting the DOAs of active sources under several acoustic scenarios, including scenarios with one, two, or three sources, and with or without the presence of some level of diffuse noise.

No description available.

This study investigated automobile drivers' ability to localize auditory and haptic (touch) alarms in terms of speed and accuracy. Thirty-two subjects, balanced across age (20-30 years old and 60-70 years old) and gender, participated in the study. Subjects were screened for minimum hearing of 40 dB for 500 Hz through 4000 Hz auditory tones, and maximum bilateral hearing differences of 10 dB. The experiment consisted of subjects identifying the target location of an alarm while driving a 2001 Buick LeSabre at 55 mph in light traffic. Four alarm modes were tested: 1) an auditory broadband alarm, 2) a haptic seat, 3) a combination of the haptic and the auditory alarm modes, and 4) a combination of the haptic alarm mode with a non-directional auditory alarm played from the front speakers of the vehicle. The alarms were evoked from eight target locations: the front-left, front, front-right, right, back-right, back, back-left, and left. The target locations of the auditory alarm mode existed around the interior of the car cabin using the vehicle's stock sound system speakers. The haptic alarm target locations existed in the bottom of the driver seat using an eight-by-eight grid of actuators. The experimenter evoked the alarms while subjects drove along a two-lane highway, and the alarms were not associated with any actual collision threat. Subjects were instructed to quickly identify the location of the alarm by calling them out, while being as correct as possible. Their choice response time and target location selection was recorded. The alarms were presented approximately every minute during fifteen-minute intervals over the duration of two and a half hours. Subjects completed questionnaires regarding their preference to the alarm modes. Under the conditions investigated, subjects localized the haptic alarm mode faster and more accurately than the auditory alarm mode. Subjects performed equally well with the haptic alarm mode and the two auditory and haptic combination alarm modes in terms of speed and accuracy in identifying their location. Subjects did express a preference for the addition of the auditory component to the haptic alarm mode, perhaps owing to a heightened sense of urgency. However, subjects preferred the haptic alarm mode on its own in response to hypothetical false alarm questions, perhaps because it was less annoying. Alarm mode discriminability was believed to affect localization accuracy and response time owing to its effect on the likelihood of correctly identifying a target location and the attention resources required to differentiate adjacent target locations. / Master of Science