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The outer-ear's location-dependent pattern of spectral filtering generates cues used to determine a sound source's elevation as well as front-back location. The authors aim to identify these features using a reverse correlation analysis (RCA), combining free-field localization behaviour with the associated head-related transfer functions' (HRTFs) magnitude spectrum from a sample of 73 participants. Localization responses were collected before and immediately after introducing a pair of outer-ear inserts which modified the listener's HRTFs to varying extent. The RCA identified several different features responsible for eliciting localization responses. The efficacy of these was examined using two models of monaural localization. In general, the predicted performance was closely aligned with the free-field localization error for the bare-ear condition; however, both models tended to grossly over-estimate the localization error based on HRTFs modified by the outer-ear inserts. The RCA's feature selection notably had the effect of better aligning the predicted performance of both models to the actual localization performance. This suggests that the RCA revealed sufficient detail for both models to correctly predict localization performance and also limited the influence of filtered-out elements in the distorted HRTFs that contributed to the degraded accuracy of both models.

This study examined the effect of a change in target voice on word recall during a multi-talker conversation. Two experiments were conducted using matrix sentences to assess the cost of a single endogenous switch in non-spatial attention. Performance in a yes-no recognition task was significantly worse when a target voice changed compared to when it remained the same after a turn-taking gap. We observed a decrease in target hit rate and sensitivity, and an increase in masker confusion errors following a change in voice. These results highlight the cognitive demands of not only engaging attention on a new talker, but also of disengaging attention from a previous target voice. This shows that exposure to a voice can have a biasing effect on attention that persists well after a turn-taking gap. A second experiment showed that there was no change in switching performance using different talker combinations. This demonstrates that switching costs were consistent and did not depend on the degree of acoustic differences in target voice characteristics.

The article A Review of the Potential Impacts of Wind Turbine Noise in the Australian Context, written by John Laurence Davy, Kym Burgemeister, David Hillman & Simon Carlile, was originally published electronically on the publisher’s internet portal on 27 June 2020 without open access.

In music, the perception of pitch is governed largely by its tonal function given the preceding harmonic structure of the music. While behavioral research has advanced our understanding of the perceptual representation of musical pitch, relatively little is known about its representational structure in the brain. Using Magnetoencephalography (MEG), we recorded evoked neural responses to different tones presented within a tonal context. Multivariate Pattern Analysis (MVPA) was applied to “decode” the stimulus that listeners heard based on the underlying neural activity. We then characterized the structure of the brain’s representation using decoding accuracy as a proxy for representational distance, and compared this structure to several well established perceptual and acoustic models. The observed neural representation was best accounted for by a model based on the Standard Tonal Hierarchy, whereby differences in the neural encoding of musical pitches correspond to their differences in perceived stability. By confirming that perceptual differences honor those in the underlying neuronal population coding, our results provide a crucial link in understanding the cognitive foundations of musical pitch across psychological and neural domains.

The capacity of healthy adult listeners to accommodate to altered spectral cues to the source locations of broadband sounds has now been well documented. In recent years we have demonstrated that the degree and speed of accommodation are improved by using an integrated sensory-motor training protocol under anechoic conditions. Here we demonstrate that the learning which underpins the localization performance gains during the accommodation process using anechoic broadband training stimuli generalize to environmentally relevant scenarios. As previously, alterations to monaural spectral cues were produced by fitting participants with custom-made outer ear molds, worn during waking hours. Following acute degradations in localization performance, participants then underwent daily sensory-motor training to improve localization accuracy using broadband noise stimuli over ten days. Participants not only demonstrated post-training improvements in localization accuracy for broadband noises presented in the same set of positions used during training, but also for stimuli presented in untrained locations, for monosyllabic speech sounds, and for stimuli presented in reverberant conditions. These findings shed further light on the neuroplastic capacity of healthy listeners, and represent the next step in the development of training programs for users of assistive listening devices which degrade localization acuity by distorting or bypassing monaural cues.

The growing availability of efficient and relatively inexpensive virtual auditory display technology has provided new research platforms to explore the perception of auditory motion. At the same time, deployment of these technologies in command and control as well as in entertainment roles is generating an increasing need to better understand the complex processes underlying auditory motion perception. This is a particularly challenging processing feat because it involves the rapid deconvolution of the relative change in the locations of sound sources produced by rotational and translations of the head in space (self-motion) to enable the perception of actual source motion. The fact that we perceive our auditory world to be stable despite almost continual movement of the head demonstrates the efficiency and effectiveness of this process. This review examines the acoustical basis of auditory motion perception and a wide range of psychophysical, electrophysiological, and cortical imaging studies that have probed the limits and possible mechanisms underlying this perception.

The ability to actively follow a moving auditory target with our heads remains unexplored even though it is a common behavioral response. Previous studies of auditory motion perception have focused on the condition where the subjects are passive. The current study examined head tracking behavior to a moving auditory target along a horizontal 100° arc in the frontal hemisphere, with velocities ranging from 20°/s to 110°/s. By integrating high fidelity virtual auditory space with a high-speed visual presentation we compared tracking responses of auditory targets against visual-only and audio-visual “bisensory” stimuli. Three metrics were measured – onset, RMS and gain error. The results showed that tracking accuracy (RMS error) varied linearly with target velocity, with a significantly higher rate in audition. Also, when the target moved faster than 80°/s, onset and RMS error were significantly worst in audition the other modalities while responses in the visual and bisensory conditions were statistically identical for all metrics measured. Lastly, audio-visual facilitation was not observed when tracking bisensory targets.

This review considers the nature of the sound generated by wind turbines focusing on the low-frequency sound (LF) and infrasound (IS) to understand the usefulness of the sound measures where people work and sleep. A second focus concerns the evidence for mechanisms of physiological transduction of LF/IS or the evidence for somatic effects of LF/IS. While the current evidence does not conclusively demonstrate transduction, it does present a strong prima facia case. There are substantial outstanding questions relating to the measurement and propagation of LF and IS and its encoding by the central nervous system relevant to possible perceptual and physiological effects. A range of possible research areas are identified.

To examine whether auditory streaming contributes to unmasking, intelligibility of target sentences against two competing talkers was measured using the coordinate response measure (CRM) [Bolia et al., J. Acoust. Soc. Am. 107, 1065-1066 (2007)] corpus. In the control condition, the speech reception threshold (50% correct) was measured when the target and two maskers were collocated straight ahead. Separating maskers from the target by +/-30 degrees resulted in spatial release from masking of 12 dB. CRM sentences involve an identifier in the first part and two target words in the second part. In experimental conditions, masking talkers started spatially separated at +/-30 degrees but became collocated with the target before the scoring words. In one experiment, one target and two different maskers were randomly selected from a mixed-sex corpus. Significant unmasking of 4 dB remained despite the absence of persistent location cues. When same-sex talkers were used as maskers and target, unmasking was reduced. These data suggest that initial separation may permit confident identification and streaming of the target and masker speech where significant differences between target and masker voice characteristics exist, but where target and masker characteristics are similar, listeners must rely more heavily on continuing spatial cues.

To hear out a conversation against other talkers listeners overcome energetic and informational masking. Largely attributed to top-down processes, information masking has also been demonstrated using unintelligible speech and amplitude-modulated maskers suggesting bottom-up processes. We examined the role of speech-like amplitude modulations in information masking using a spatial masking release paradigm. Separating a target talker from two masker talkers produced a 20 dB improvement in speech reception threshold; 40% of which was attributed to a release from informational masking. When across frequency temporal modulations in the masker talkers are decorrelated the speech is unintelligible, although the within frequency modulation characteristics remains identical. Used as a masker as above, the information masking accounted for 37% of the spatial unmasking seen with this masker. This unintelligible and highly differentiable masker is unlikely to involve top-down processes. These data provides strong evidence of bottom-up masking involving speech-like, within-frequency modulations and that this, presumably low level process, can be modulated by selective spatial attention.

Because of the slow speed of sound relative to light, acoustic and visual signals from a distant event often will be received asynchronously. Here, using acoustic signals with a robust cue to sound source distance, we show that judgments of perceived temporal alignment with a visual marker depend on the depth simulated in the acoustic signal. For distant sounds, a large delay of sound relative to vision is required for the signals to be perceived as temporally aligned. For nearer sources, the time lag corresponding to audiovisual alignment is smaller and scales at rate approximating the speed of sound. Thus, when robust cues to auditory distance are present, the brain can synchronize disparate audiovisual signals to external events despite considerable differences in time of arrival at the perceiver. This ability is functionally important as it allows auditory and visual signals to be synchronized to the external event that caused them.

The effect of spatial separation on the ability of human listeners to resolve a pair of concurrent broadband sounds was examined. Stimuli were presented in a virtual auditory environment using individualized outer ear filter functions. Subjects were presented with two simultaneous noise bursts that were either spatially coincident or separated (horizontally or vertically), and responded as to whether they perceived one or two source locations. Testing was carried out at five reference locations on the audiovisual horizon (0 degrees, 22.5 degrees, 45 degrees, 67.5 degrees, and 90 degrees azimuth). Results from experiment 1 showed that at more lateral locations, a larger horizontal separation was required for the perception of two sounds. The reverse was true for vertical separation. Furthermore, it was observed that subjects were unable to separate stimulus pairs if they delivered the same interaural differences in time (ITD) and level (ILD). These findings suggested that the auditory system exploited differences in one or both of the binaural cues to resolve the sources, and could not use monaural spectral cues effectively for the task. In experiments 2 and 3, separation of concurrent noise sources was examined upon removal of low-frequency content (and ITDs), onset/offset ITDs, both of these in conjunction, and all ITD information. While onset and offset ITDs did not appear to play a major role, differences in ongoing ITDs were robust cues for separation under these conditions, including those in the envelopes of high-frequency channels.

Changing the shape of the outer ear using small in-ear molds degrades sound localization performance consistent with the distortion of monaural spectral cues to location. It has been shown recently that adult listeners re-calibrate to these new spectral cues for locations both inside and outside the visual field. This raises the question as to the teacher signal for this remarkable functional plasticity. Furthermore, large individual differences in the extent and rate of accommodation suggests a number of factors may be driving this process. A training paradigm exploiting multi-modal and sensory-motor feedback during accommodation was examined to determine whether it might accelerate this process. So as to standardize the modification of the spectral cues, molds filling 40% of the volume of each outer ear were custom made for each subject. Daily training sessions for about an hour, involving repetitive auditory stimuli and exploratory behavior by the subject, significantly improved the extent of accommodation measured by both front-back confusions and polar angle localization errors, with some improvement in the rate of accommodation demonstrated by front-back confusion errors. This work has implications for both the process by which a coherent representation of auditory space is maintained and for accommodative training for hearing aid wearers.

A computational model of auditory localization resulting in performance similar to humans is reported. The model incorporates both the monaural and binaural cues available to a human for sound localization. Essential elements used in the simulation of the processes of auditory cue generation and encoding by the nervous system include measured head-related transfer functions (HRTFs), minimum audible field (MAF), and the Patterson-Holdsworth cochlear model. A two-layer feed-forward back-propagation artificial neural network (ANN) was trained to transform the localization cues to a two-dimensional map that gives the direction of the sound source. The model results were compared with (i) the localization performance of the human listener who provided the HRTFs for the model and (ii) the localization performance of a group of 19 other human listeners. The localization accuracy and front-back confusion error rates exhibited by the model were similar to both the single listener and the group results. This suggests that the simulation of the cue generation and extraction processes as well as the model parameters were reasonable approximations to the overall biological processes. The amplitude resolution of the monaural spectral cues was varied and the influence on the model's performance was determined. The model with 128 cochlear channels required an amplitude resolution of approximately 20 discrete levels for encoding the spectral cue to deliver similar localization performance to the group of human listeners.

In response to the explosion in medical information, there have been considerable recent changes in medical curriculum development. The move to problem based learning (PBL) is, in part, a result of these changes. The Faculty of Medicine at the University of Sydney has exploited a WWW based intranet for the development, delivery, management and evaluation of it's problem based, graduate medical program (GMP). This system has been employed to develop the 72 medical problems that contribute to the first two years of the GMP. The activities of more than 400 members of the faculty have been coordinated using the intranet to develop the wide range of resources to support learning in the program. Daily management of the curriculum is also enabled using Web site posting of bulletins, e-mail and ongoing development of technology training. Coupled with the PBL problems is a formative assessment system that provides questions and feedback that cover the whole range of learning topics. Part of the student and staff evaluation is supported both informally and formally through the use of a 'Feedback' button on each web page and web delivered structured formal evaluations, respectively.

A systematic analysis of the localization of objects in extra-personal space requires a three-dimensional method of documenting location. In auditory localization studies the location of a sound source is often reduced to a directional vector with constant magnitude with respect to the observer, data being plotted on a unit sphere with the observer at the origin. This is an attractive form of data representation as the relevant spherical statistical and graphical methods are well described. In this paper we collect together a set of spherical plotting and statistical procedures to visualize and summarize these data. We describe methods for visualizing auditory localization data without assuming that the principal components of the data are aligned with the coordinate system. As a means of comparing experimental techniques and having a common set of data for the verification of spherical statistics, the software (implemented in MATLAB) and database described in this paper have been placed in the public domain. Although originally intended for the visualization and summarization of auditory psychophysical data, these routines are sufficiently general to be applied in other situations involving spherical data.

Measurement of localization performance will reflect errors that relate to the sensory processing of the cues to sound location and the errors associated with the method by which the subject indicates the perceived location. This study has measured the ability of human subjects to localize a short noise burst presented in the free field with the subject indicating the perceived location by pointing their nose towards the source. Subjects were first trained using a closed loop training paradigm which involved instantaneous feedback as to the accuracy of head pointing which resulted in the reduction of residual localization errors and a rapid acquisition of the task by the subjects. Once trained, 19 subjects localized between 4 and 6 blocks of 76 target locations. The data were pooled and the distribution of errors associated with each target location was examined using spherical methods. Errors in the localization estimates for about one third of the locations were rotationally symmetrical about their mean but the remaining locations were best described by an elliptical distribution (Kent distributed). For about one half of the latter locations the orientations of the directions of the greatest variance of the distributions were not aligned with the azimuth and elevation coordinates used for describing the spatial location of the targets. The accuracy (systematic errors) and the distribution of the errors (variance) in localization for our population of subjects were also examined for each test location. The size of the data set and the methods of analysis provide very reliable measures of important baseline parameters of human auditory localization.

Spectro-temporal variations are a consequence of dynamic structural variations of a sounding body. The human auditory system is highly optimized for the detection, segregation and analysis of one class of such variations—speech. Here, we will examine some consequences of this optimisation in the context of complex listening involving multiple concurrent talkers. In such conversational settings, listeners rapidly shift their attention from one to another talker so foreground and background are defined dynamically by listener intent. Up-regulation of the attended-to talker is generally thought to result from endogenous, top-down attention. Here, we review a recent report indicating that substantial informational masking between concurrent talkers may result from bottom-up interactions between sources within frequency modulation channels. This indicates that temporally dynamic aspects of within-frequency channel processing plays a very important role in speech masking by the sort of “noise” most commonly enc...

These experiments examined the ability to track a moving target with our heads under various stimulus conditions and modalities. While previous studies [1,2] have concentrated on eye tracking within the frontal region; we extended the modes of tracking to include auditory and auditory-visual stimuli and in a significantly wider locus of space. Using a newly developed system that combines high fidelity virtual auditory space with a high speed LED strip we were able to examine head tracking behaviour in a 100° arc around a subject with velocities from ±20°/s to ±110 °/s. This allows us to derive behavioural norms for head tracking, compare differences in tracking ability across the modalities, and determine if cross modal facilitation occurs. Preliminary results show that subjects were better able to track a visual and bimodal target than an auditory one, as evident in the smaller average RMS error (visual = 4°, bimodal = 4.6°, auditory = 7°) and shorter lag (visual = 5.5°, bimodal = 5.9° and, auditory = 8.9°). Furthermore, tracking ability was influenced by stimulus speed, especially in the unimodal auditory situation where a significant increase in both RMS error and lag for speeds >80°/s was observed.

These days it is recognised that for bilateral hearing loss there is generally benefit in fitting two hearing aids, one for each ear (see Byrne, 1980 and Feuerstein, 1992 for clinical studies, see Byrne et al., 1992, Durlach et al., 1981, and Zurek, 1981 for laboratory studies). Bilateral fitting is now standard practice for children with bilateral loss and as of 2005 bilateral fittings account for approximately 75% of all fittings (Libby, 2007). Nonetheless, it is only within the last half-decade that it has become possible to transfer audio signals between bilaterally-fitted hearing aids (Moore, 2007). This is primarily attributed to the technological advances in integrated circuit design, longer lasting batteries and also wireless intercommunication between the two hearing aids, e.g., using near-field magnetic induction (NFMI) communication. The possibility to exchange audio signals between bilaterally-fitted aids opens the door to new types of binaural signal processing algorithms to assist hearingimpaired listeners separate sounds of interest from background noise. In this chapter, we consider whether or not the manipulation of near-field distance cues may provide a viable binaural signal processing algorithm for hearing aids. More specifically, this chapter describes three experiments that explore the spatial unmasking of speech based on nearfield distance cues. In a typical cocktail party setting, listeners are faced with the challenging task of extracting information by sifting through a mixture of multiple talkers overlapping in frequency and time. This challenge arises as a result of interference in the form of energetic masking, where sounds are rendered inaudible due to frequency overlap, and informational masking, where sounds from different sources are confused with one another (Bronkhorst, 2000; Brungart et al., 2001; Kidd et al., 2008). Despite this, listeners are reasonably adept at parsing complex mixtures and attending to separate auditory events. One factor that influences speech intelligibility in mixtures is perceived spatial location. Many studies have established that sounds originating from separate locations are easier to distinguish than sounds which are co-located (Hirsh, 1950; Bronkhorst and Plomp, 1988; Ebata, 2003). Separating sounds in space can result in an increase in the signal-tonoise ratio at one ear (the ‘better ear’). Moreover, sounds that are spatially separated give rise to differences in binaural cues (interaural time and level differences, ITDs/ILDs) that can improve audibility by reducing energetic masking (Durlach and Colburn, 1978

The ability of listeners with bilateral sensorineural hearing loss to localize a speech source in a multitalker mixture was measured. Five simultaneous words spoken by different talkers were presented over loudspeakers in a small room, and listeners localized one target word. Errors were significantly larger in this group compared to a control group with normal hearing. Localization of the target presented alone was not different between groups. The results suggest that hearing loss does not impair spatial hearing per se, but degrades the spatial representation of multiple simultaneous sounds.

This chapter briefly summarizes the results of a meta-analysis that examined auditory localization accuracy for more than 80,000 trials where brief broadband stimuli were presented anechoically in one of four different laboratories. The analyses were aimed at creating a comprehensive map of localization accuracy as a function of sound source location, and characterizing the distribution of responses along the “cone of confusion”. The results reveal trends in auditory localization whilst minimizing the influence of different experimental methodologies and response methods.

Studies of spatial perception during visual saccades have demonstrated compressions of visual space around the saccade target. Here we psychophysically investigated perception of auditory space during rapid head turns, focusing on the “perisaccadic” interval. Using separate perceptual and behavioral response measures we show that spatial compression also occurs for rapid head movements, with the auditory spatial representation compressing by up to 50%. Similar to observations in the visual system, this occurred only when spatial locations were measured by using a perceptual response; it was absent for the behavioral measure involving a nose-pointing task. These findings parallel those observed in vision during saccades and suggest that a common neural mechanism may subserve these distortions of space in each modality.

The phenomenon of binaural interference, where binaural judgments of a high-frequency target stimulus are disrupted by the presence of a simultaneous low-frequency interferer, can largely be explained using principles of auditory grouping and segregation. Evidence for this relationship comes from a number of previous studies showing that the manipulation of simultaneous grouping cues such as harmonicity and onset synchrony can influence the strength of the phenomenon. In this study, it is shown that sequential grouping cues can also influence whether binaural interference occurs. Subjects indicated the lateral position of a high-frequency sinusoidally amplitude-modulated (SAM) tone containing an interaural time difference. Perceived lateral positions were reduced by the presence of a simultaneous diotic low-frequency SAM tone, but were largely restored when the interferer was "captured" in a stream of identical tones. A control condition confirmed that the effect was not due to peripheral adaptation. The data lend further support to the idea that binaural interference is affected by processes related to the perceptual organization of auditory information. Modifications to existing grouping-based models are proposed that may help account for binaural interference effects more successfully.

The acoustic cues to auditory space are referenced to the head which moves through the world, itself composed of moving sources so that our sensation convolves source with self- motion. With the head still, velocity discrimination, a perceptual process, is related to static acuity via the minimum audible movement angle (MAMA). Yet, while we can accurately localize static sounds, we are much less sensitive to velocity, resorting to distance and time cues where available. Interestingly, when velocity changes as a step function, discrimination thresholds and the amount of post-transition stimulus required for detection is greater than the corresponding MAMA. This suggests that when the head is stationary, the window of temporal integration may vary according to the sound's velocity characteristics. We also have evidence that auditory representational momentum scales with velocity, not duration or distance. Facing and following a moving auditory source is an ecologically important behavior. Tracking exhibits ...

This study measured the accuracy with which human listeners can localize spoken words. A broadband (300 Hz-16 kHz) corpus of monosyllabic words was created and presented tolisteners using a virtual auditory environment. Localization was examined for 76 locations ona sphere surrounding the listener. Experiment 1 showed that low-pass filtering the speech sounds at 8 kHz degraded performance, causing an increase in polar angle errors associated with the cone of confusion. In experiment 2 it was found that performance in fact varied systematically with the level of the signal above 8 kHz. Although the lower frequencies (below 8 kHz) are known to be sufficient for accurate speech recognition in most situations, these results demonstrate that natural speech contains information between 8 and 16 kHz that is essential for accurate localization.

The auditory system localizes a sound source on the basis of binaural cues as well as monaural cues provided by the outer ear. These cues are described by the head‐related transfer functions (HRTFs). The individual variation in the frequency and amplitude of perceptually significant features in the human HRTFs raises the issue of whether a given listener can obtain adequate localization cues from stimuli based on another listener’s HRTFs. This question can be addressed in ‘‘virtual auditory space,’’ using headphone stimuli which are convolved with HRTFs and the inverse of the headphone transfer function (HpTF). Using an in‐ear recording system we have measured HpTFs (Sennheiser 250 Linear) for both ears of ten subjects and found that they also display significant individual differences in the 4–10 kHz region. Therefore, the proper reconstruction of both individualized and nonindividualized HRTFs requires that the listener’s HpTF be deconvolved from the stimulus. We have verified this acoustically using the in‐ear recording system for both individualized and nonindividualized HRTFs. Finally, we show that the use of nonindividualized HpTFs creates considerable distortions in the 4–10 kHz range, which could partly account for the increased incidence of mislocalizations reported in previous virtual auditory space localization experiments.

The aim in generating virtual auditory space (VAS) is to create the illusion of a natural free-field sound using a closed-field sound system (see Fig. 1.1, chapter 1) or in another free-field environment, using a loudspeaker system. This technique relies on the general assumption that identical stimuli at a listener’s eardrum will be perceived identically independent of their physical mode of delivery. However, it is important to note that the context of an auditory stimulus also plays a role in the percept generated in the listeners (chapter 1, section 2.1.2) and that auditory and nonauditory factors contribute to this.

This chapter is an introduction to digital signal processing (DSP) concepts used in the generation and analysis of auditory signals. It is not a mathematical exposition on DSP, but rather a qualitative description of the DSP techniques commonly used in the processing of audio signals. Readers should refer to the annotated bibliography for a more detailed coverage of specific DSP techniques.

1. The role of the structures of the outer ear in producing monaural and binaural spectral cues to sound location was examined acoustically in the ferret. A probe microphone was introduced across the wall of the external auditory canal and its responses to digitally constructed wideband signals were recorded for a large number of free field locations. 2. In the intact animal the patterns of both monaural and binaural cues were asymmetrical for horizontal locations about the interaural axis. For anterior sound locations the monaural transformations demonstrated relative gains at middle and high frequencies and a location-dependent frequency notch. Changing elevation resulted in variations in the corner frequencies of these spectral features. Additionally, there was greater front-back asymmetry in the binaural spectral cues for locations in lateral space when compared with locations near the midline. 3. Surgical removal of the pinna and concha (pinnectomy) eliminated all the major front-back asymmetrical features in the horizon monaural and binaural spectral transformations as well as the elevation-dependent variations in the monaural spectra. Thus the residual transformations were ambiguous for sound locations in lateral space, resulting in "cones of confusion" centered on the interaural axis. 4. These cues were reflected in the topographic representation of auditory space in the deeper layers of the superior colliculus (SC). Previous studies have shown that spatial tuning at near-threshold sound levels is based on monaural pinna cues, whereas binaural inputs are utilized at higher levels that stimulate both ears. In the intact ferret we examined statistically the topography of the representation of sound azimuth for near-threshold and suprathreshold stimuli and the alignment of the auditory and visual representations in the SC. The distributions of auditory best positions within the SC for near- and suprathreshold stimulus levels were statistically indistinguishable, suggesting that both monaural and binaural cues are integrated in this neural representation of space. 5. Pinnectomy resulted in a large increase in the number of auditory units that responded best to two distinct locations in space. One lobe of the response was tuned appropriately in terms of the position of the unit within the SC, demonstrating that the residual acoustical cues are sufficient for the construction of a topographic representation of auditory space. However, the second region of space, thereby producing an ambiguous representation.(ABSTRACT TRUNCATED AT 400 WORDS)

Despite the importance of spatial hearing in everyday listening, little is known about the accuracy of sound localization in a complex mixture of sounds. Here we measured, for the frontal audio‐visual horizon, how accurately listeners could localize a female‐voice target amidst four spatially distributed male‐voice interferers in a moderately reverberant room. To examine whether listeners can make use of a priori knowledge about the configuration of the scene, we compared performance when the interferer locations were fixed (in one of five known patterns) to when the locations varied from trial to trial. The presence of interferers disrupted target localization, even after accounting for reduced target detectability. Randomizing the interferer locations had a moderate influence, degrading performance in some configurations but improving it in others. All effects were magnified when the target‐to‐interferer intensity ratio was reduced. The results confirm that spatial perception is disrupted by interfering...

This paper examines the role of biological constraints in the human auditory localization process. A psychophysical and neural system modeling approach was undertaken in which performance comparisons between competing models and a human subject explore the relevant biologically plausible "realism constraints." The directional acoustical cues, upon which sound localization is based, were derived from the human subject's head-related transfer functions (HRTFs). Sound stimuli were generated by convolving bandpass noise with the HRTFs and were presented to both the subject and the model. The input stimuli to the model were processed using the Auditory Image Model of cochlear processing. The cochlear data were then analyzed by a time-delay neural network which integrated temporal and spectral information to determine the spatial location of the sound source. The combined cochlear model and neural network provided a system model of the sound localization process. Aspects of humanlike localization performance were qualitatively achieved for broadband and bandpass stimuli when the model architecture incorporated frequency division (i.e., the progressive integration of information across the different frequency channels) and was trained using variable bandwidth and center-frequency sounds. Results indicate that both issues are relevant to human sound localization performance.

An inherent problem in accurately measuring HRTFs is the use of recording systems placed in the ear canal that may interfere with the sound field. A simple earprint and electroplating method is described for making customized, ultra‐thin (0.2‐mm) holders, which can be used to position a small (1.25‐mm‐diam) polyethylene probe coupled to a miniature microphase deep within the auditory canal. Acoustic recordings using the system on a model ear equipped with a microphone in place of the eardrum show that (1) spectral notches created by standing waves within the canal can be used to precisely place the probe relative to the eardrum; (2) the system does not significantly affect the sound spectrum as it is transformed from the free‐field to the eardrum (±1 dB from 0.2 to 12 kHz, ±3 dB from 12 to 14 kHz); (3) it does not introduce direction‐dependent spectral components; and (4) successive insertions of the set produce highly repeatable recordings. Finally, the system was used to test the reproducibility of headphones placement, a crucial element in the high‐fidelity simulation of auditory space. [Work supported by the Swiss National Science Foundation, the Foundation for the 450th Birthday of the University of Lausanne, the McDonnell–Pew Centre for Cognitive Neurosciences, the Beit Foundation, and the Wellcome Trust.]