Thesis (M.Sc.Eng.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Perception of sound laterality (left-right angle) is mediated by both interaural time differences (ITD) and interaural level differences (ILD). Previous localization studies in anechoic settings consistently show that low-frequency ITDs dominate perception of source laterality. However, reverberant energy differentially degrades ITDs and ILDs; the effects of room reflections on the perceptual weight given to ITDs and ILDs are not well understood. Here, we tested the hypothesis that high-frequency envelope ITD cues are important for spatial judgments in reverberant rooms by measuring the perceived laterality of high-pass, low-pass and broadband sounds. Results show that when ILD cues and ITD envelope cues are both available, reverberant energy has the smallest effect on localization of high-pass stimuli. When ILD cues are set to zero, localization of high-pass stimuli with strong envelopes (i.e. click trains and speech tokens) is also minimally affected by reverberant energy; however, as envelope modulation is reduced, subjects show increasing localization bias, responding towards the center. Moreover, for stimuli with strong envelopes, subjects with better modulation detection sensitivity are affected less by the addition of reverberant energy. These results suggest that, in contrast to in anechoic space, high-frequency envelope ITD cues influence localization in reverberant settings. / 2031-01-01

This thesis provides an overview of work conducted to investigate human spatial hearing in situations involving multiple concurrent sound sources. Much is known about spatial hearing with single sound sources, including the acoustic cues to source location and the accuracy of localisation under different conditions. However, more recently interest has grown in the behaviour of listeners in more complex environments. Concurrent sound sources pose a particularly difficult problem for the auditory system, as their identities and locations must be extracted from a common set of sensory receptors and shared computational machinery. It is clear that humans have a rich perception of their auditory world, but just how concurrent sounds are processed, and how accurately, are issues that are poorly understood. This work attempts to fill a gap in our understanding by systematically examining spatial resolution with multiple sound sources. A series of psychophysical experiments was conducted on listeners with normal hearing to measure performance in spatial localisation and discrimination tasks involving more than one source. The general approach was to present sources that overlapped in both frequency and time in order to observe performance in the most challenging of situations. Furthermore, the role of two primary sets of location cues in concurrent source listening was probed by examining performance in different spatial dimensions. The binaural cues arise due to the separation of the two ears, and provide information about the lateral position of sound sources. The spectral cues result from location-dependent filtering by the head and pinnae, and allow vertical and front-rear auditory discrimination. Two sets of experiments are described that employed relatively simple broadband noise stimuli. In the first of these, two-point discrimination thresholds were measured using simultaneous noise bursts. It was found that the pair could be resolved only if a binaural difference was present; spectral cues did not appear to be sufficient. In the second set of experiments, the two stimuli were made distinguishable on the basis of their temporal envelopes, and the localisation of a designated target source was directly examined. Remarkably robust localisation was observed, despite the simultaneous masker, and both binaural and spectral cues appeared to be of use in this case. Small but persistent errors were observed, which in the lateral dimension represented a systematic shift away from the location of the masker. The errors can be explained by interference in the processing of the different location cues. Overall these experiments demonstrated that the spatial perception of concurrent sound sources is highly dependent on stimulus characteristics and configurations. This suggests that the underlying spatial representations are limited by the accuracy with which acoustic spatial cues can be extracted from a mixed signal. Three sets of experiments are then described that examined spatial performance with speech, a complex natural sound. The first measured how well speech is localised in isolation. This work demonstrated that speech contains high-frequency energy that is essential for accurate three-dimensional localisation. In the second set of experiments, spatial resolution for concurrent monosyllabic words was examined using similar approaches to those used for the concurrent noise experiments. It was found that resolution for concurrent speech stimuli was similar to resolution for concurrent noise stimuli. Importantly, listeners were limited in their ability to concurrently process the location-dependent spectral cues associated with two brief speech sources. In the final set of experiments, the role of spatial hearing was examined in a more relevant setting containing concurrent streams of sentence speech. It has long been known that binaural differences can aid segregation and enhance selective attention in such situations. The results presented here confirmed this finding and extended it to show that the spectral cues associated with different locations can also contribute. As a whole, this work provides an in-depth examination of spatial performance in concurrent source situations and delineates some of the limitations of this process. In general, spatial accuracy with concurrent sources is poorer than with single sound sources, as both binaural and spectral cues are subject to interference. Nonetheless, binaural cues are quite robust for representing concurrent source locations, and spectral cues can enhance spatial listening in many situations. The findings also highlight the intricate relationship that exists between spatial hearing, auditory object processing, and the allocation of attention in complex environments.

This thesis provides an overview of work conducted to investigate human spatial hearing in situations involving multiple concurrent sound sources. Much is known about spatial hearing with single sound sources, including the acoustic cues to source location and the accuracy of localisation under different conditions. However, more recently interest has grown in the behaviour of listeners in more complex environments. Concurrent sound sources pose a particularly difficult problem for the auditory system, as their identities and locations must be extracted from a common set of sensory receptors and shared computational machinery. It is clear that humans have a rich perception of their auditory world, but just how concurrent sounds are processed, and how accurately, are issues that are poorly understood. This work attempts to fill a gap in our understanding by systematically examining spatial resolution with multiple sound sources. A series of psychophysical experiments was conducted on listeners with normal hearing to measure performance in spatial localisation and discrimination tasks involving more than one source. The general approach was to present sources that overlapped in both frequency and time in order to observe performance in the most challenging of situations. Furthermore, the role of two primary sets of location cues in concurrent source listening was probed by examining performance in different spatial dimensions. The binaural cues arise due to the separation of the two ears, and provide information about the lateral position of sound sources. The spectral cues result from location-dependent filtering by the head and pinnae, and allow vertical and front-rear auditory discrimination. Two sets of experiments are described that employed relatively simple broadband noise stimuli. In the first of these, two-point discrimination thresholds were measured using simultaneous noise bursts. It was found that the pair could be resolved only if a binaural difference was present; spectral cues did not appear to be sufficient. In the second set of experiments, the two stimuli were made distinguishable on the basis of their temporal envelopes, and the localisation of a designated target source was directly examined. Remarkably robust localisation was observed, despite the simultaneous masker, and both binaural and spectral cues appeared to be of use in this case. Small but persistent errors were observed, which in the lateral dimension represented a systematic shift away from the location of the masker. The errors can be explained by interference in the processing of the different location cues. Overall these experiments demonstrated that the spatial perception of concurrent sound sources is highly dependent on stimulus characteristics and configurations. This suggests that the underlying spatial representations are limited by the accuracy with which acoustic spatial cues can be extracted from a mixed signal. Three sets of experiments are then described that examined spatial performance with speech, a complex natural sound. The first measured how well speech is localised in isolation. This work demonstrated that speech contains high-frequency energy that is essential for accurate three-dimensional localisation. In the second set of experiments, spatial resolution for concurrent monosyllabic words was examined using similar approaches to those used for the concurrent noise experiments. It was found that resolution for concurrent speech stimuli was similar to resolution for concurrent noise stimuli. Importantly, listeners were limited in their ability to concurrently process the location-dependent spectral cues associated with two brief speech sources. In the final set of experiments, the role of spatial hearing was examined in a more relevant setting containing concurrent streams of sentence speech. It has long been known that binaural differences can aid segregation and enhance selective attention in such situations. The results presented here confirmed this finding and extended it to show that the spectral cues associated with different locations can also contribute. As a whole, this work provides an in-depth examination of spatial performance in concurrent source situations and delineates some of the limitations of this process. In general, spatial accuracy with concurrent sources is poorer than with single sound sources, as both binaural and spectral cues are subject to interference. Nonetheless, binaural cues are quite robust for representing concurrent source locations, and spectral cues can enhance spatial listening in many situations. The findings also highlight the intricate relationship that exists between spatial hearing, auditory object processing, and the allocation of attention in complex environments.

Despite the importance of auditory selective attention in everyday communication, the cortical mechanisms underlying the amazing ability of human brain to focus on a sound stimulus and suppress others are not well understood. Recent studies have led to the hypothesis that alpha band oscillation (8-14Hz) is a neural signature of multimodal spatial attention. Research in multiple sensory domains has shown that alpha synchronizes in the hemisphere contralateral to unattended stimuli and desynchronizes on the hemisphere contralateral to attended stimuli, suggesting it is a marker of an inhibition process for filtering out unattended stimuli. However, further research is needed to understand the possible functional role of these alpha oscillations as well as their correlation with other cortical activity. Moreover, it is not clear whether different forms of auditory attention employ different cortical mechanisms, mediated through different brain networks. This study aims to combine brain stimulation methods (transcranial Direct/Alternative Current Stimulation) with electrophysiological measurements of electroencephalography (EEG) to measure and interpret the underlying cortical activity during different forms of auditory selective attention. More specifically, there are four studies, each of which employs behavioral tasks to test specific hypotheses. First, we studied alpha oscillatory activity during auditory spatial attention. Second, we compared and contrast cortical activity during auditory spatial and non-spatial attention. Third, we used brain stimulation to see if we can show a causal relationship between alpha oscillation and selective auditory attention performance. Lastly, we applied the existing results on alpha power to use it as a quantitative biomarker to indicate the level of spatial attention network engagement. Our results contributed to the growing body of knowledge about how the brain employs auditory selective attention for effective communication. / 2021-06-04T00:00:00Z

Listening to speech with competing speech in the background is challenging and becomes harder with age. Three experiments examined the auditory and cognitive aspects of auditory spatial attention in conditions in which the location of the target was uncertain. In all experiments, word identification was measured for target sentences presented with two competitor sentences. On each trial, the three sentences were presented with one from each of three spatially separated loudspeakers. A priori cues specified the location and identity callsign of the target. In Experiments I and II, sentences were also presented in conditions of simulated spatial separation achieved with the precedence effect. Participants were younger and older adults with normal hearing sensitivity below 4 kHz. For both age groups, the contributions of richer acoustic cues (those present when there was real spatial separation, but absent when there was simulated spatial separation) were most pronounced when the target occurred at “unlikely” spatial listening locations, suggesting that both age groups benefit similarly from richer acoustical cues. In Experiment II, the effect of time between the callsign cue and target word on word identification was investigated. Four timing conditions were tested: the original sentences (which contained about 300 ms of filler speech between the callsign cue and the onset of the target words), or modified sentences with silent pauses of 0, 150, or 300 ms replacing the filler speech. For targets presented from unlikely locations, word identification was better for all listeners when there was more time between the callsign cue and key words, suggesting that time is needed to switch spatial attention. In Experiment III, the effects of single and multiple switches of attention were investigated. The key finding was that, whereas both age groups performed similarly in conditions requiring a single switch of attention, the performance of older, but not younger listeners, was reduced when multiple switches of spatial attention were required. This finding suggests that difficulties disengaging attention may contribute to the listening difficulties of older adults. In conclusion, cognitive and auditory factors contributing to auditory spatial attention appear to operate similarly for all listeners in relatively simple situations, and age-related differences are observed in more complex situations.

Listening to speech with competing speech in the background is challenging and becomes harder with age. Three experiments examined the auditory and cognitive aspects of auditory spatial attention in conditions in which the location of the target was uncertain. In all experiments, word identification was measured for target sentences presented with two competitor sentences. On each trial, the three sentences were presented with one from each of three spatially separated loudspeakers. A priori cues specified the location and identity callsign of the target. In Experiments I and II, sentences were also presented in conditions of simulated spatial separation achieved with the precedence effect. Participants were younger and older adults with normal hearing sensitivity below 4 kHz. For both age groups, the contributions of richer acoustic cues (those present when there was real spatial separation, but absent when there was simulated spatial separation) were most pronounced when the target occurred at “unlikely” spatial listening locations, suggesting that both age groups benefit similarly from richer acoustical cues. In Experiment II, the effect of time between the callsign cue and target word on word identification was investigated. Four timing conditions were tested: the original sentences (which contained about 300 ms of filler speech between the callsign cue and the onset of the target words), or modified sentences with silent pauses of 0, 150, or 300 ms replacing the filler speech. For targets presented from unlikely locations, word identification was better for all listeners when there was more time between the callsign cue and key words, suggesting that time is needed to switch spatial attention. In Experiment III, the effects of single and multiple switches of attention were investigated. The key finding was that, whereas both age groups performed similarly in conditions requiring a single switch of attention, the performance of older, but not younger listeners, was reduced when multiple switches of spatial attention were required. This finding suggests that difficulties disengaging attention may contribute to the listening difficulties of older adults. In conclusion, cognitive and auditory factors contributing to auditory spatial attention appear to operate similarly for all listeners in relatively simple situations, and age-related differences are observed in more complex situations.

Small timing differences occur when sounds reach one ear before the other, creating interaural phase differences (IPDs). The phase-locked activity in the auditory nerve can, at low frequencies, preserve IPDs. IPDs are used for localising and separating sounds from different directions. Chapters 3, 5, and 6 report three studies of the independent effects of age and sensorineural hearing loss on the temporal processing of sound that aids spatial hearing. Chapters 2 and 4 describe two supporting methodological studies. Chapter 2 compared the duration of training required for stable IPD-discrimination thresholds for two stimulus presentation procedures. The procedure requiring the least training was adopted for subsequent studies. Age and hearing loss are related and both may affect sensitivity to IPDs. Chapter 3 demonstrated that hearing loss, regardless of listener age, is related to poorer sensitivity to IPDs in the temporal fine structure (TFS), but not in the temporal envelope. Chapter 3 also showed that age, independent of hearing loss, is related to poorer envelope-IPD sensitivity at low modulation rates, and somewhat poorer TFS-IPD sensitivity. In Chapter 5, listener age and IPD sensitivity were both compared to subcortical neural phase locking measured through the frequency-following response (FFR). Phase coherence in the envelope-FFR at 145 Hz modulation and in the TFS-FFR deteriorated with age, suggesting less precise phase locking in old age. However, age-related changes to IPD sensitivity were not strongly related to age-related changes in FFR phase coherence. IPD sensitivity declines may be predominantly caused by deterioration of binaural processing independent of subcortical phase locking. Chapter 4 showed that electrodes at the mastoids recorded TFS-FFR generated earlier in the auditory pathway than electrodes from the nape of the neck to forehead, which recorded FFR generated later in the brainstem. However, these electrode montages did not reveal different age- or hearing-loss-related FFR deficits in Chapter 5. Chapter 6 determined whether hearing loss affected the ability to use TFS IPDs to achieve better speech perception. On average, old hearing-impaired listeners gained a small, but significant, benefit from a lateral separation of the speech sources. Replacing the TFS with binaurally in-phase sine waves (removing the TFS IPDs) significantly reduced the benefit of lateral separation. How much a listener benefitted from intact TFS IPDs in speech perception was strongly related to the extent of their hearing loss at low frequencies and their monaural processing of TFS, but not to their ability to discriminate IPDs. In general, this thesis shows that low-frequency hearing loss is associated with poor sensitivity to TFS IPDs and the ability to benefit from them when sounds are laterally separated. The thesis also shows that old age can reduce sensitivity to IPDs and weaken subcortical temporal coding. Although only partly related, these effects are likely to cause problems for old individuals in challenging listening environments.

Normal-hearing listeners can locate sound sources, using binaural cues for azimuth angle. These binaural differences in the timing and intensity of sound arriving at the two ears, interaural time differences (ITDs) and interaural intensity differences (IIDs), also support selective listening in multi-talker environments. Auditory-brainstem neurons of the medial superior olive (MSO) and lateral superior olive (LSO) encode ITD in the envelope of sound (ITDENV) and in the temporal fine structure of low-frequency sound (ITDTFS); LSO neurons encode IID. Bilateral-cochlear-implant (bCI) listeners generally receive only IID and ITDENV. Experimental bCI pulse-bursts overcome adaptation, and convey electrical ITDTFS. Improving the understanding of mechanisms for ITD sensitivity can help bCI developers convey acoustic ITDTFS. In this dissertation, models for auditory-brainstem neurons are developed that explain human ability to detect small differences in ITD, as neuronal and MSO population mechanisms. Promoting binaural-coincidence detection and limiting backpropagation, model MSO ion-channels set resting potentials that reproduce dendritic and somatic KLT activation, somatic Na+ inactivation, and a lower amount of axonal Na+ inactivation. Sensitivity to ITDTFS in moderately fast and very fast model MSO neurons collectively match physiological data from 150 to 2000 Hz. The best-ITD (the ITD of highest spike rate) can be made contralateral-leading, by contralateral inhibition of moderate speed, or by asymmetric axon location, leveraging dendritic filtering. Leveraging standard binaural-display models, neuronal populations based on these model MSO neurons match normal-hearing human discrimination thresholds for ITDTFS in sine tones from 39 to 1500 Hz. Adaptation before binaural interaction helps model MSO neurons glimpse the ITDTFS of sound direct from a source, before reflected sound arrives from different directions. With inputs from adapting model spherical bushy cells, a moderately fast model MSO neuron reproduces in vivo responses to amplitude-modulated binaural beats, with a frequency-dependent emphasis of rising vs. peak sound-pressure for ITDTFS encoding, which reflects human ITD detection and reverberation times in outdoor environments. Distinct populations of model LSO neurons, spanning the range of electrical membrane impedance as a function of frequency in LSO neurons, collectively reflect discrimination thresholds for ITDENV in transposed tones across carrier frequency (4-10 kHz) and modulation rate (32-800 Hz). / 2022-09-28T00:00:00Z

In bats, the directivity patterns for reception are shaped by the surface geometry of the pinnae. Since many bat species are capable of large ear deformations, these beampatterns can be time-variant. To investigate this time-variance using numerical methods, a digital model that is capable of representing the pinna geometry during the entire deformation cycle has been developed. Due to large deformations and occlusions, some of the surfaces relevant to sound diffraction may not be visible and the geometry of the entire pinna has to be computed from limited data. This has been achieved by combining a complete digital model of the pinna in one position with time-variant sparse sets of three dimensional landmark data. The landmark positions were estimated using stereo vision methods. A finite element model based on elasticity was constructed from CT scans of the pinna post mortem. This elastic model was deformed to provide a good fit to the positions of the landmarks and retain values of smoothness and surface energy comparable to life. This model was able to handle ratios of data to degrees of freedom around 1:5000 and still effect life-like deformations with an acceptable goodness of fit. / Master of Science

The main focus of the research presented within this thesis is, as the title suggests, binaural spatialization. Binaural technology and, especially, the binaural recording technique are not particu-larly recent. Nevertheless, the interest in this technology has lately become substantial due to the increase in the calculation power of personal computers, which started to allow the complete and accurate real-time simulation of three-dimensional sound-fields over headphones. The goals of this body of research have been determined in order to provide elements of novelty and of contribution to the state of the art in the field of binaural spatialization. A brief summary of these is found in the following list: • The development and implementation of a binaural spatialization technique with Distance Simulation, based on the individual simulation of the distance cues and Binaural Reverb, in turn based on the weighted mix between the signals convolved with the different HRIR and BRIR sets; • The development and implementation of a characterization process for modifying a BRIR set in order to simulate different environments with different characteristics in terms of frequency response and reverb time; • The creation of a real-time and offline binaural spatialization application, imple-menting the techniques cited in the previous points, and including a set of multichannel(and Ambisonics)-to-binaural conversion tools. • The performance of a perceptual evaluation stage to verify the effectiveness, realism, and quality of the techniques developed, and • The application and use of the developed tools within both scientific and artistic “case studies”. In the following chapters, sections, and subsections, the research performed between January 2006 and March 2010 will be described, outlining the different stages before, during, and after the development of the software platform, analysing the results of the perceptual evaluations and drawing conclusions that could, in the future, be considered the starting point for new and innovative research projects.