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PubMed2002

Bayesian reconstruction of sound localization cues from responses to random spectra.

The directionally sensitive acoustics of the pinnae enable humans to perceive the up-down and front-back direction of sound. This mechanism complements another, independent mechanism that derives sound-source azimuth from interaural difference cues. The pinnae effectively add direction-dependent spectral notches and peaks to the incoming sound, and it has been shown that such features are used to code sound direction in the median plane. However, it is still unclear which of the pinna-induced features play a role in sound localization. The present study presents a method for the reconstruction of the spatially relevant features in the spectral domain. Broadband sounds with random spectral shapes were presented in rapid succession as subjects made saccadic eye movements toward the perceived stimulus locations. The analysis, which is based on Bayesian statistics, indicates that specific spectral features could be associated with perceived spatial locations. Spectral features that were determined by this psychophysical method resemble the main characteristics of the pinna transfer functions obtained from acoustic measurements in the ear canal. Despite current experimental limitations, the approach may prove useful in the study of perceptually relevant spectral cues underlying human sound localization.

Acoustic Stimulation↗

PubMed1999

Effects of altering spectral cues in infancy on horizontal and vertical sound localization by adult ferrets.

We investigated the behavioral consequences of removing the pinna and concha of the external ear bilaterally in infancy on the sound localization ability of adult ferrets. Altering spectral cues in this manner has previously been shown to disrupt the development of the neural representation of auditory space in the superior colliculus. Using broadband noise stimuli, we tested pinnae-removed ferrets and normal ferrets in three sound localization tasks. In each case, we found that both groups of animals performed significantly better when longer duration noise bursts were used. In a relative localization task, we measured the acuity with which the ferrets could discriminate between two speakers in the horizontal plane. The speakers were placed symmetrically either around the anterior midline or around a position 45 degrees lateral to the midline. In this task, the pinnae-removed ferrets achieved very similar scores to the normal ferrets. By contrast, in another relative localization task that measured localization ability in the midsagittal plane, pinnae-removed ferrets performed less well than normals. In an absolute localization task, 12 speakers were spaced at 30 degrees intervals in the horizontal plane at the level of the ferrets' ears. Overall, the pinnae-removed ferrets also performed poorly in this task compared with normal ferrets: they made significantly fewer correct responses, larger localization errors and more front-back errors. Both normal and pinnae-removed animals showed an improvement in performance with practice, although the pattern of improvement differed for each group. The largest improvements in localization accuracy were achieved by the pinnae-removed ferrets, particularly at the frontal positions, and their performance eventually approached that of the normal animals. Nevertheless, some intergroup differences were still present. In particular, the pinnae-removed ferrets continued to make significantly more front-back errors than the normals. These deficits can be attributed to differences in the spectral localization cues available to the animals. Acoustical measurements showed that, compared with normal animals, the head-related transfer functions in the horizontal plane were largely ambiguous around the interaural axis and also contained fewer location-dependent features in the midsagittal plane.

Acoustic Stimulation↗

PubMed1996

Sound orientation behavior in cats. II. Mid-frequency spectral cues for sound localization.

The cat's head-related transfer function creates a directionally dependent mid-frequency notch in the amplitude spectrum of a broadband sound as the stimulus propagates to the tympanic membrane [Rice et al., Hear. Res. 58, 132-152 (1992)]. Our previous behavioral studies [May and Huang, J. Acoust. Soc. Am. 100, 1059-1069 (1996)] have indicated that the cat's sound-evoked orientation responses are well directed to the azimuth and elevation of broadband noise bursts in the frontal sound field, where pinna-based spectral notches are prominent and change systematically with sound direction. In the present study, the importance of mid-frequency directional cues in the cat's sound localization behavior was further evaluated by manipulating the frequency and bandwidth of orientation stimuli. The accurate pattern of orientation behavior seen previously with bursts of broadband noise was relatively unaffected when stimulus bandwidth was decreased to mid-frequency bandpass noise of 5-18 kHz. In contrast, poorly directed head orientation responses were observed in tests with high-pass noise (> 18 kHz) and mid-frequency pure tones. When tested with narrow bands of mid-frequency noise, cats oriented toward the spatial location where HRTF-filtering properties most closely matched the stimulus spectrum. These results suggest that important sound localization cues are derived from mid-frequency spectral features of the cat's HRTF.

Animals↗

PubMed1996

Sound localization in egocentric space following hemispheric lesions.

In this paper the performance of patients with unilateral hemispheric lesions (n = 10 with right brain damage; n = 10 with left brain damage) on a free-field sound localization task was contrasted with that of healthy controls (n = 10). Sound stimuli were presented binaurally in the horizontal plane from eight loudspeakers set at azimuths between -105 degrees and +105 degrees. Whereas performance of both patient groups was generally less accurate than controls, no evidence suggested that this was specific to the contralateral hemisphere. The results indicate that both hemispheres play a role in sound localization, with systematic directional errors made towards the ipsilateral hemifield following unilateral lesions. Furthermore, particular location difficulties at pericentral positions (+15 degrees and -15 degrees) following right hemisphere damage, may indicate a specific function for the right hemisphere in determining personal frames of spatial reference within pericentral space.

Adult↗

PubMed1998

Early-blind human subjects localize sound sources better than sighted subjects.

Do blind persons develop capacities of their remaining senses that exceed those of sighted individuals? Besides anecdotal suggestions, two views based on experimental studies have been advanced. The first proposes that blind individuals should be severely impaired, given that vision is essential to develop spatial concepts. The second suggests that compensation occurs through the remaining senses, allowing them to develop an accurate concept of space. Here we investigate how an ecologically critical function, namely three-dimensional spatial mapping, is carried out by early-blind individuals with or without residual vision. Subjects were tested under monaural and binaural listening conditions. We find that early-blind subjects can map the auditory environment with equal or better accuracy than sighted subjects. Furthermore, unlike sighted subjects, they can correctly localize sounds monaurally. Surprisingly, blind individuals with residual peripheral vision localized sounds less precisely than sighted or totally blind subjects, confirming that compensation varies according to the aetiology and extent of blindness. Our results resolve a long-standing controversy in that they provide behavioural evidence that totally blind individuals have better auditory ability than sighted subjects, enabling them to compensate for their loss of vision.

Adaptation, Physiological↗

PubMed2002

Encoding of sound localization cues by an identified auditory interneuron: effects of stimulus temporal pattern.

An important cue for sound localization is binaural comparison of stimulus intensity. Two features of neuronal responses, response strength, i.e., spike count and/or rate, and response latency, vary with stimulus intensity, and binaural comparison of either or both might underlie localization. Previous studies at the receptor-neuron level showed that these response features are affected by the stimulus temporal pattern. When sounds are repeated rapidly, as occurs in many natural sounds, response strength decreases and latency increases, resulting in altered coding of localization cues. In this study we analyze binaural cues for sound localization at the level of an identified pair of interneurons (the left and right AN2) in the cricket auditory system, with emphasis on the effects of stimulus temporal pattern on binaural response differences. AN2 spike count decreases with rapidly repeated stimulation and latency increases. Both effects depend on stimulus intensity. Because of the difference in intensity at the two ears, binaural differences in spike count and latency change as stimulation continues. The binaural difference in spike count decreases, whereas the difference in latency increases. The proportional changes in response strength and in latency are greater at the interneuron level than at the receptor level, suggesting that factors in addition to decrement of receptor responses are involved. Intracellular recordings reveal that a slowly building, long-lasting hyperpolarization is established in AN2. At the same time, the level of depolarization reached during the excitatory postsynaptic potential (EPSP) resulting from each sound stimulus decreases. Neither these effects on membrane potential nor the changes in spiking response are accounted for by contralateral inhibition. Based on comparison of our results with earlier behavioral experiments, it is unlikely that crickets use the binaural difference in latency of AN2 responses as the main cue for determining sound direction, leaving the difference in response strength, i.e., spike count and/or rate, as the most likely candidate.

Acoustic Stimulation↗

PubMed2005

Acoustical cues for sound localization by the Mongolian gerbil, Meriones unguiculatus.

The present study measured the head-related transfer functions (HRTFs) of the Mongolian gerbil for various sound-source directions, and explored acoustical cues for sound localization that could be available to the animals. The HRTF exhibited spectral notches for frequencies above 25 kHz. The notch frequency varied systematically with source direction, and thereby characterized the source directions well. The frequency dependence of the acoustical axis, the direction for which the HRTF amplitude was maximal, was relatively irregular and inconsistent between ears and animals. The frequency-by-frequency plot of the interaural level difference (ILD) exhibited positive and negative peaks, with maximum values of 30 dB at around 30 kHz. The ILD peak frequency had a relatively irregular spatial distribution, implying a poor sound localization cue. The binaural acoustical axis (the direction with the maximum ILD magnitude) showed relatively orderly clustering around certain frequencies, the pattern being fairly consistent among animals. The interaural time differences (ITDs) were also measured and fell in a +/- 120 micros range. When two different animal postures were compared (i.e., the animal was standing on its hind legs and prone), small but consistent differences were found for the lower rear directions on the HRTF amplitudes, the ILDs, and the ITDs.

Acoustic Stimulation↗

PubMed2005

Development of sound localization mechanisms in the mongolian gerbil is shaped by early acoustic experience.

Sound localization is one of the most important tasks performed by the auditory system. Differences in the arrival time of sound at the two ears are the main cue to localize low-frequency sound in the azimuth. In the mammalian brain, such interaural time differences (ITDs) are encoded in the auditory brain stem; first by the medial superior olive (MSO) and then transferred to higher centers, such as the dorsal nucleus of the lateral lemniscus (DNLL), a brain stem nucleus that gets a direct input from the MSO. Here we demonstrate for the first time that ITD sensitivity in gerbils undergoes a developmental maturation after hearing onset. We further show that this development can be disrupted by altering the animal's acoustic experience during a critical period. In animals that had been exposed to omnidirectional white noise during a restricted time period right after hearing onset, ITD tuning did not develop normally. Instead, it was similar to that of juvenile animals 3 days after hearing onset, with the ITD functions not adjusted to the physiological range. Animals that had been exposed to omnidirectional noise as adults did not show equivalent abnormal ITD tuning. The development presented here is in contrast to that of the development of neuronal representation of ITDs in the midbrain of barn owls and interaural intensity differences in ferrets, where the representations are adjusted by an interaction of auditory and visual inputs. The development of ITD tuning presented here most likely depends on normal acoustic experience and may be related to the maturation of inhibitory inputs to the ITD detector itself.

Acoustic Stimulation↗

PubMed1984

Neural mechanisms for sound localization.

Although the efforts to find a place map of sound direction within the auditory system of mammals has been reinspired by the recent discoveries in owl, progress to date has not been encouraging. Neither the inferior colliculus nor auditory cortex has yielded immediate evidence of such a map, despite ingenious and persistent efforts to find it. Thus, at present, the evidence suggests that a head-referenced map of auditory space is more likely to be found in structures more motor than sensory in function--in the deep layers of the superior colliculus or brainstem tegmentum, for example. Insofar as these structures have been implicated in eye, ear, and head orientation toward a sound source, one might expect that premotor units for orienting would be sensitive to sound direction and thus, collectively, constitute a map of auditory azimuth isomorphic to the map of motor azimuth. However, even for these structures, the possibility for significant variation among mammalian species exists. Because many candidate motor structures (such as the deep superior colliculus) receive input from the cerebral cortex, and because the role of auditory cortex in sound localization seems to vary widely among mammals (38, 51) an equal amount of variation in auditory-motor maps may also exist.

Animals↗

PubMed2004

The bat head-related transfer function reveals binaural cues for sound localization in azimuth and elevation.

Directional properties of the sound transformation at the ear of four intact echolocating bats, Eptesicus fuscus, were investigated via measurements of the head-related transfer function (HRTF). Contributions of external ear structures to directional features of the transfer functions were examined by remeasuring the HRTF in the absence of the pinna and tragus. The investigation mainly focused on the interactions between the spatial and the spectral features in the bat HRTF. The pinna provides gain and shapes these features over a large frequency band (20-90 kHz), and the tragus contributes gain and directionality at the high frequencies (60 to 90 kHz). Analysis of the spatial and spectral characteristics of the bat HRTF reveals that both interaural level differences (ILD) and monaural spectral features are subject to changes in sound source azimuth and elevation. Consequently, localization cues for horizontal and vertical components of the sound source location interact. Availability of multiple cues about sound source azimuth and elevation should enhance information to support reliable sound localization. These findings stress the importance of the acoustic information received at the two ears for sound localization of sonar target position in both azimuth and elevation.

Animals↗

PubMed2004

An investigation of the role of auditory cortex in sound localization using muscimol-releasing Elvax.

Lesion studies suggest that primary auditory cortex (A1) is required for accurate sound localization by carnivores and primates. In order to elucidate further its role in spatial hearing, we examined the behavioural consequences of reversibly inactivating ferret A1 over long periods, using Elvax implants releasing the GABA(A) receptor agonist muscimol. Sub-dural polymer placements were shown to deliver relatively constant levels of muscimol to underlying cortex for >5 months. The measured diffusion of muscimol beneath and around the implant was limited to 1 mm. Cortical silencing was assessed electrophysiologically in both auditory and visual cortices. This exhibited rapid onset and was reversed within a few hours of implant removal. Inactivation of cortical neurons extended to all layers for implants lasting up to 6 weeks and throughout at least layers I-IV for longer placements, whereas thalamic activity in layer IV appeared to be unaffected. Blockade of cortical neurons in the deeper layers was restricted to < or = 500 microm from the edge of the implant, but was usually more widespread in the superficial layers. In contrast, drug-free Elvax implants had little discernible effect on the responses of the underlying cortical neurons. Bilateral implants of muscimol-Elvax over A1 produced significant deficits in the localization of brief sounds in horizontal space and particularly a reduced ability to discriminate between anterior and posterior sound sources. The performance of these ferrets gradually improved over the period in which the Elvax was in place and attained that of control animals following its removal. Although similar in nature, these deficits were less pronounced than those caused by cortical lesions and suggest a specific role for A1 in resolving the spatial ambiguities inherent in auditory localization cues.

Acoustic Stimulation↗

PubMed1993

Sound-localization deficits induced by lesions in the barn owl's auditory space map.

Barn owls possess a two-dimensional map of auditory space. The map appears in its final form in the external nucleus of the inferior colliculus (ICx) and is projected from there to the optic tectum (OT). To determine the role of the map in ICx or its projected version in OT in the localization of acoustic stimuli, head movements of three adult owls were recorded before and after lesioning parts of the map either in ICx or in OT. Small electrolytic lesions caused sound-localization deficits that were characterized by failures to turn, turns away from the sound source, increased response latencies, and reduced turning amplitudes. These deficits occurred at azimuths expected from the physiological data obtained at the lesion site before passing current. They extended over an azimuthal range of some 20 degrees. Thus, this is the first unambiguous demonstration, for sound localization, of a deficit covering a well circumscribed area within an auditory hemifield. Since the major lesion deficits were restricted to one hemifield, a second lesion was made in the space map of the other side of the brain in each animal. The second lesion resulted in behavioral deficits qualitatively similar to those produced by the first lesion. In total, the study is based on nine lesions, three in each owl. Two lesions were sham lesions. These and two further lesions did not produce a measurable deficit. In four of the five remaining cases, in which a behavioral deficit was observed initially, the behavioral performance recovered with time. In two cases, the deficit disappeared completely. Although improvement occurred also in the remaining two cases, prelesion response amplitudes were not reached in one case, and response latency did not reach prelesion values in the other case. Because the behavioral deficits induced by the second lesions disappeared in the same way as did the deficits induced by the first lesions, the possibility is excluded that the animals learned to respond tactically, that is, that they learned to associate "unlocatable sounds" with the lesioned side. The deficits were also not due to a generalized motor impairment, because immediately after the lesion the animals responded as they did before the lesion to stimulation from outside the well circumscribed affected region of space. The possibility is discussed that plasticity after small central (neural) injuries is maintained longer in the lifetime of an animal than plasticity after peripheral (mechanical) manipulations.

Animals↗

PubMed1992

Hearing in large mammals: sound-localization acuity in cattle (Bos taurus) and goats (Capra hircus).

Sound localization acuity of 3 cattle (Bos taurus) and 2 goats (Capra hircus) was determined for brief complex sounds in a two-choice procedure. Thresholds around the median sagittal plane averaged 30 degrees and 18 degrees, respectively. For comparison, thresholds were obtained in the same test apparatus for humans (0.8 degrees) and a dog (8 degrees). Although the relatively poor acuity of cattle and goats compared with most mammals comes as some surprise, given their large interaural distances and the large binaural locus cues available to them, it is not unexpected when other factors are considered. Like other poor localizers (both domesticated and nondomesticated), cattle and goats are prey species with their best vision directed throughout nearly the entire horizon. In contrast to mammals with very narrow foveal fields, they may not need very accurate locus information from their auditory systems to direct their gaze to a sound source.

Adult↗

PubMed1999

A spike-based model of binaural sound localization.

This paper describes a spike-based model of binaural sound localization using interaural time differences (ITDs). To handle the problem of temporal coding and to facilitate a hardware implementation all neurons are simulated by a spike response model, which includes postsynaptic potentials (PSPs) and a refractory period. A winner-take-all (WTA) network selects the dominant source from the representation of the sound's angles of incidences, and can be biased by a multisensory support. We use simulations on real audio data to investigate the function and the practical application of the system.

Acoustic Stimulation↗

PubMed2002

Opposing effects of head position on sound localization in blind and sighted human subjects.

Up to now, there is an unsolved contradiction between the view that the development of an auditory spatial representation needs calibration by vision and the psychophysical demonstration of quite precise sound localization in early blind humans. The present study provides a link between these two competing conceptions. Two experiments were conducted with congenitally or early blind subjects and sighted controls. In the first experiment, subjects pointed with their head to actual sound sources located in the azimuthal plane. In the second experiment, lateralization of dichotic sound stimuli, presented via headphones, was investigated with variation of head-to-trunk position. The results showed opposing systematic errors of sound localization or lateralization, depending on head position, made by blind and sighted subjects. These differences suggest that audiomotor feedback replaces vision so as to calibrate auditory space in blind individuals. That is, in contrast to the widespread opinion of compensation of visual loss by a general sharpening of audition, compensatory plasticity in the blind may specifically be related to enhanced processing of proprioceptive and vestibular information with the auditory spatial input.

Adaptation, Physiological↗

PubMed2006

Perceptual recalibration in human sound localization: learning to remediate front-back reversals.

The efficacy of a sound localization training procedure that provided listeners with auditory, visual, and proprioceptive/vestibular feedback as to the correct sound-source position was evaluated using a virtual auditory display that used nonindividualized head-related transfer functions (HRTFs). Under these degraded stimulus conditions, in which the monaural spectral cues to sound-source direction were inappropriate, localization accuracy was initially poor with frequent front-back reversals (source localized to the incorrect front-back hemifield) for five of six listeners. Short periods of training (two 30-min sessions) were found to significantly reduce the rate of front-back reversal responses for four of five listeners that showed high initial reversal rates. Reversal rates remained unchanged for all listeners in a control group that did not participate in the training procedure. Because analyses of the HRTFs used in the display demonstrated a simple and robust front-back cue related to energy in the 3-7-kHz bandwidth, it is suggested that the reductions observed in reversal rates following the training procedure resulted from improved processing of this front-back cue, which is perhaps a form of rapid perceptual recalibration. Reversal rate reductions were found to generalize to untrained source locations, and persisted at least 4 months following the training procedure.

Acoustic Stimulation↗

PubMed2000

Effect of normobaric hypoxia on sound localization.

BACKGROUND: Three-dimensional (3D) audio displays have considerable potential for conveying spatial information in the aviation environment. How reliably these displays will function in that environment will depend in part on the extent to which sound localization is affected by hypoxia. Many aircraft systems operators are routinely exposed to mild hypoxia, and all are at risk of exposure to more severe hypoxia. METHODS: We have examined hypoxia's effect on localization by measuring the localization performance of four subjects exposed to simulated altitudes of 0, 1200, 2400 and 3700 m above sea level for about 30 min. Sounds to be localized were presented from a free-field source at locations covering 360 degrees of azimuth and extending from -47.6 to + 80 degrees elevation. RESULTS: Localization performance was statistically indistinguishable across the altitudes tested. Average localization errors ranged from 12.6 degrees +/- 0.7 SE at 2400 m to 14 degrees +/- 0.84 SE at 0 m. CONCLUSION: The finding that hypoxia induced by exposure to simulated altitudes as high as 3700 m has no effect of sound localization is encouraging with respect to the continued development of 3D audio displays for use in the aviation environment.

Adult↗

PubMed2001

Sound localization in a new-world frugivorous bat, Artibeus jamaicensis: acuity, use of binaural cues, and relationship to vision.

Passive sound-localization acuity and its relationship to vision were determined for the echolocating Jamaican fruit bat (Artibeus jamaicensis). A conditioned avoidance procedure was used in which the animals drank fruit juice from a spout in the presence of sounds from their right, but suppressed their behavior, breaking contact with the spout, whenever a sound came from their left, thereby avoiding a mild shock. The mean minimum audible angle for three bats for a 100-ms noise burst was 10 degrees-marginally superior to the 11.6 degrees threshold for Egyptian fruit bats and the 14 degrees threshold for big brown bats. Jamaican fruit bats were also able to localize both low- and high-frequency pure tones, indicating that they can use both binaural phase- and intensity-difference cues to locus. Indeed, their ability to use the binaural phase cue extends up to 6.3 kHz, the highest frequency so far for a mammal. The width of their field of best vision, defined anatomically as the width of the retinal area containing ganglion-cell densities at least 75% of maximum, is 34 degrees. This value is consistent with the previously established relationship between vision and hearing indicating that, even in echolocating bats, the primary function of passive sound localization is to direct the eyes to sound sources.

Animals↗