Azimuthal sound source localization of various sound stimuli under different conditions

AimTo evaluate azimuthal sound-source localization performance under different conditions, with a view to optimizing a routine sound localization protocol.Material and methodTwo groups of healthy, normal-hearing subjects were tested identically, except that one had to keep their head still while the other was allowed to turn it. Sound localization was tested without and then with a right ear plug (acute auditory asymmetry) for each of the following sound stimuli: pulsed narrow-band centered on 250 Hz, continuous narrowband centered on 2000 Hz, 4000 Hz and 8000 Hz, continuous 4000 Hz warble, pulsed white noise, and word (“lac” (lake)). Root mean square error was used to calculate sound-source localization accuracy.ResultsWith fixed head, localization was significantly disturbed by the earplug for all stimuli (P < 0.05). The most discriminating stimulus was continuous 4000 Hz narrow-band: area under the ROC curve (AUC), 0.99 [95% CI, 0.95–1.01] for screening and 0.85 [0.82–0.89] for diagnosis. With mobile head, localization was significantly better than with fixed head for 4000 and 8000 Hz stimuli (P < 0.05). The most discriminating stimulus was continuous 2000 Hz narrow-band: AUC, 0.90 [0.83–0.97] for screening and 0.75 [0.71–0.79] for diagnosis. In both conditions, pulsed noise (250 Hz narrow-band, white noise or word) was less difficult to localize than continuous noise.ConclusionThe test was more sensitive with the head immobile. Continuous narrow-band stimulation centered on 4000 Hz most effectively explored interaural level difference. Pulsed narrow-band stimulation centered on 250 Hz most effectively explored interaural time difference. Testing with mobile head, closer to real-life conditions, was most effective with continuous narrow-band stimulation centered on 2000 Hz.     

声源定位实验研究进展

空间听觉是中枢听觉系统对双耳信息整合处理能力的直观评价指标,声源定位是空间听觉能力的简单有效评估手段,然而由于实验空间及设备要求较高,目前针对声源定位能力的研究依然较少,且实验方案设计及数据分析方法也多种多样,进一步限制了研究结果之间的可比性。本文将对声源定位原理,研究常用的测试条件和数据分析方法,以及目前为止该领域听力正常、听力损失、听力损失干预后和耳鸣人群的研究情况加以综述,以期为后期声源定位实验开展提供借鉴。     

Reweighting of Binaural Localization Cues in Bilateral Cochlear-Implant Listeners

Normal-hearing (NH) listeners rely on two binaural cues, the interaural time (ITD) and level difference (ILD), for azimuthal sound localization. Cochlear-implant (CI) listeners, however, rely almost entirely on ILDs. One reason is that present-day clinical CI stimulation strategies do not convey salient ITD cues. But even when presenting ITDs under optimal conditions using a research interface, ITD sensitivity is lower in CI compared to NH listeners. Since it has recently been shown that NH listeners change their ITD/ILD weighting when only one of the cues is consistent with visual information, such reweighting might add to CI listeners’ low perceptual contribution of ITDs, given their daily exposure to reliable ILDs but unreliable ITDs. Six bilateral CI listeners completed a multi-day lateralization training visually reinforcing ITDs, flanked by a pre- and post-measurement of ITD/ILD weights without visual reinforcement. Using direct electric stimulation, we presented 100- and 300-pps pulse trains at a single interaurally place-matched electrode pair, conveying ITDs and ILDs in various spatially consistent and inconsistent combinations. The listeners’ task was to lateralize the stimuli in a virtual environment. Additionally, ITD and ILD thresholds were measured before and after training. For 100-pps stimuli, the lateralization training increased the contribution of ITDs slightly, but significantly. Thresholds were neither affected by the training nor correlated with weights. For 300-pps stimuli, ITD weights were lower and ITD thresholds larger, but there was no effect of training. On average across test sessions, adding azimuth-dependent ITDs to stimuli containing ILDs increased the extent of lateralization for both 100- and 300-pps stimuli. The results suggest that low-rate ITD cues, robustly encoded with future CI systems, may be better exploitable for sound localization after increasing their perceptual weight via training.     

Selectivity for temporal characteristics of sound and interaural time difference of auditory midbrain neurons in the grassfrog: A system theoretical approach

The selectivity for temporal characteristics of sound and interaural time difference (ITD) was investigated in the torus semicircularis (TS) of the grassfrog. Stimuli were delivered by means of a closed sound system and consisted of binaurally presented Poisson distributed condensation clicks, and pseudo-random (RAN) or equidistant (EQU) click trains of which ITD was varied. With RAN and EQU trains, 86% of the TS units demonstrated a clear selectivity for ITD. Most commonly, these units had monotonically increasing ITD-rate functions. In general, units responding to Poisson clicks, responded also to RAN and EQU trains. One category of units which showed strong time-locking had comparable selectivities for ITD with both stimulus ensembles. A second category of units showed a combined selectivity for temporal structure and ITD. These units responded exclusively to EQU trains in a nonsynchronized way. From the responses obtained with the Poisson click ensemble so-called Poisson system kernels were determined, in analogy to the Wiener-Volterra functional expansion for nonlinear systems. The kernel analysis was performed up to second order. Contralateral (CL) first order kernels usually had positive or combinations of positive and negative regions, indicating that the contralateral ear exerted an excitatory or combined excitatory-inhibitory influence upon the neural response. Ipsilateral (IL), units were characterized by first order kernels which were not significantly different from zero, or kernels in which a single negative region was present. A large variety of CL second order kernels has been observed whereas rarely IL second order kernels were encountered. About 35% of the units possessed nonzero second order cross kernels, which indicates that CL and IL neural processes are interacting in a nonlinear way. Units demonstrating a pronounced selectivity for ITD, were generally characterized by positive CL combined with negative IL first order kernels. Findings suggested that, in the grassfrog, neural selectivity for ITD mainly is established by linear interaction of excitatory and inhibitory processes originating from the CL and IL ear, respectively. Units exhibiting strong time-locking to Poisson clicks and RAN and EQU trains had significantly shorter response latencies than moderately time-locking units. In the first category of units, a substantial higher number of nonzero first and second order kernels was observed. It was concluded that nonlinearr response properties, as observed in TS units, most likely have to be ascribed to nonlinear characteristics of neural components located in the auditory nervous system peripheral to the torus semicircularis.     

Acoustic Cues for Sound Source Distance and Azimuth in Rabbits, a Racquetball and a Rigid Spherical Model

There are numerous studies measuring the transfer functions representing signal transformation between a source and each ear canal, i.e., the head-related transfer functions (HRTFs), for various species. However, only a handful of these address the effects of sound source distance on HRTFs. This is the first study of HRTFs in the rabbit where the emphasis is on the effects of sound source distance and azimuth on HRTFs. With the rabbit placed in an anechoic chamber, we made acoustic measurements with miniature microphones placed deep in each ear canal to a sound source at different positions (10–160 cm distance, ±150° azimuth). The sound was a logarithmically swept broadband chirp. For comparisons, we also obtained the HRTFs from a racquetball and a computational model for a rigid sphere. We found that (1) the spectral shape of the HRTF in each ear changed with sound source location; (2) interaural level difference (ILD) increased with decreasing distance and with increasing frequency. Furthermore, ILDs can be substantial even at low frequencies when distance is close; and (3) interaural time difference (ITD) decreased with decreasing distance and generally increased with decreasing frequency. The observations in the rabbit were reproduced, in general, by those in the racquetball, albeit greater in magnitude in the rabbit. In the sphere model, the results were partly similar and partly different than those in the racquetball and the rabbit. These findings refute the common notions that ILD is negligible at low frequencies and that ITD is constant across frequency. These misconceptions became evident when distance-dependent changes were examined.     

Concurrent Development of the Head and Pinnae and the Acoustical Cues to Sound Location in a Precocious Species,the Chinchilla (<Emphasis Type="Italic">Chinchilla lanigera)</Emphasis>

Sounds are filtered in a spatial- and frequency-dependent manner by the head and pinna giving rise to the acoustical cues to sound source location. These spectral and temporal transformations are dependent on the physical dimensions of the head and pinna. Therefore, the magnitudes of binaural sound location cues—the interaural time (ITD) and level (ILD) differences—are hypothesized to systematically increase while the lower frequency limit of substantial ILD production is expected to decrease due to the increase in head and pinna size during development. The frequency ranges of the monaural spectral notch cues to source elevation are also expected to decrease. This hypothesis was tested here by measuring directional transfer functions (DTFs), the directional components of head-related transfer functions, and the linear dimensions of the head and pinnae for chinchillas from birth through adulthood. Dimensions of the head and pinna increased by factors of 1.8 and 2.42, respectively, reaching adult values by ~6 weeks. From the DTFs, the ITDs, ILDs, and spectral shape cues were computed. Maximum ITDs increased by a factor of 1.75, from ~160 μs at birth (P0-1, first postnatal day) to 280 μs in adults. ILDs depended on source location and frequency exhibiting a shift in the frequency range of substantial ILD (>10 dB) from higher to lower frequencies with increasing head and pinnae size. Similar trends were observed for the spectral notch frequencies which ranged from 14.7–33.4 kHz at P0-1 to 5.3–19.1 kHz in adults. The development of the spectral notch cues, the spatial- and frequency-dependent distributions of DTF amplitude gain, acoustic directionality, maximum gain, and the acoustic axis were systematically related to the dimensions of the head and pinnae. The dimension of the head and pinnae in the chinchilla as well as the acoustical properties associated with them are mature by ~6 weeks.     

Human Long-Latency Potentials Evoked by Monaural Interruptions of a Binaural Click Train: Connection to Sound Lateralization Based on Interaural Intensity Differences

In 9 subjects with normal hearing, monaural offset (MO) responses in the long-latency range were recorded with and without an ongoing sound (click train at a rate of 250/s) at the opposite ear. In the latter case MOs were perceived simply as termination of a sound. In the former case, however, the abrupt transition from binaural to monaural (BM) stimulation was perceived as a shift of the fused image from the center to either side. Therefore, the fairly large difference potential obtained by subtracting the MO response from the BM response was evaluated as the cortical response to stimulation of the sound lateralization mechanism based on interaural intensity differences. These center-to-side responses, which could be characterized by an N1-P2 wave sequence at latencies of 120 and 220 ms, respectively, were compared with the auditory onset responses also recorded from the same subjects by means of a sequential stimulation paradigm. The scalp topography of the N1 components in all these responses recorded simultaneously from frontocentral, parietocentral and two superior temporal electrodes with a neck reference is discussed.     

Sound Localisation in Children with a Severe Unilateral Hearing Loss

The ability of 44 children with a severe unilateral sensorineural hearing loss to localise sound in the horizontal plane, has been compared with that of 40 subjects with normal hearing. It was found that the normally hearing group had no difficulty localising sounds in contrast to the majority of hearing- impaired children. None of the children with a hearing loss localised a 500-Hz pure tone as well as the normally hearing group, but 1 was able to localise a low-frequency noise stimulus and 9 a high-pass noise as accurately as those with normal hearing. Covering the pinna in 22 of the hearing-impaired children affected the ability of those who localised the high-pass noise normally more than the remainder of this group. This suggests that better use of pinna information was an important factor in their superior performance.     

Sound localization measured by eye-tracking

Objective: To introduce a new method of measuring sound localization ability based on eye-tracking and to test this method by analysing the influence of mild induced conductive hearing loss on sound localization. Design: Sound signals were presented from different angles, and the participant's responses were measured using an eye-tracking device. For validation, a comparison of responses to visual stimuli was performed. To test the clinical application of this method, a mild conductive hearing loss was simulated, and the impact of this change on sound localization was measured. Study sample: Fifteen participants. Results: The system provided repeatable measurements, and there was a good correlation of sound and visual signals. A large number of trials could be completed fairly rapidly. Following the induced conductive hearing loss, a decline of 5.5° in the accuracy of sound localization in the horizontal plane was found towards the side of the non-impaired ear for frontal presentations. Conclusions: Quantifying sound localization by eye-tracking was found to be feasible, fast and accurate. A mild conductive hearing loss caused a slight degradation of sound localization accuracy within the 30° frontal sector, which is in good agreement with results found using methods requiring more extensive instrumentation.     

Spatial audio through a bone conduction interface

Headphones are the standard presentation device for radio communication in the military. Although bone conduction devices possess several advantages over headphones for some military applications, they are generally considered inappropriate for inclusion in a multi-channel system. The current study tested the feasibility of a multi-channel bone conduction system by measuring the localizability of spatialized auditory stimuli presented through a pair of bone conduction vibrators. Listeners localized a Gaussian noise stimulus spatialized with individualized head-related transfer functions (HRTFs). The sounds were presented from eight virtual locations on the horizontal plane (0, ±45, ±90, ±135, and 180°) through either stereo headphones or a stereo bone conduction system. Localization performance was found to be nearly identical for both audio systems, indicating that bone conduction systems can be effectively used for displaying spatial information.     

Behavioral Sensitivity to Broadband Binaural Localization Cues in the Ferret

Although the ferret has become an important model species for studying both fundamental and clinical aspects of spatial hearing, previous behavioral work has focused on studies of sound localization and spatial release from masking in the free field. This makes it difficult to tease apart the role played by different spatial cues. In humans and other species, interaural time differences (ITDs) and interaural level differences (ILDs) play a critical role in sound localization in the azimuthal plane and also facilitate sound source separation in noisy environments. In this study, we used a range of broadband noise stimuli presented via customized earphones to measure ITD and ILD sensitivity in the ferret. Our behavioral data show that ferrets are extremely sensitive to changes in either binaural cue, with levels of performance approximating that found in humans. The measured thresholds were relatively stable despite extensive and prolonged (>16 weeks) testing on ITD and ILD tasks with broadband stimuli. For both cues, sensitivity was reduced at shorter durations. In addition, subtle effects of changing the stimulus envelope were observed on ITD, but not ILD, thresholds. Sensitivity to these cues also differed in other ways. Whereas ILD sensitivity was unaffected by changes in average binaural level or interaural correlation, the same manipulations produced much larger effects on ITD sensitivity, with thresholds declining when either of these parameters was reduced. The binaural sensitivity measured in this study can largely account for the ability of ferrets to localize broadband stimuli in the azimuthal plane. Our results are also broadly consistent with data from humans and confirm the ferret as an excellent experimental model for studying spatial hearing.     

Effect of extreme adaptive frequency compression in bimodal listeners on sound localization and speech perception

Objectives: This study aimed to improve access to high-frequency interaural level differences (ILD), by applying extreme frequency compression (FC) in the hearing aid (HA) of 13 bimodal listeners, using a cochlear implant (CI) and conventional HA in opposite ears.

Design: An experimental signal-adaptive frequency-lowering algorithm was tested, compressing frequencies above 160?Hz into the individual audible range of residual hearing, but only for consonants (adaptive FC), thus protecting vowel formants, with the aim to preserve speech perception. In a cross-over design with at least 5 weeks of acclimatization between sessions, bimodal performance with and without adaptive FC was compared for horizontal sound localization, speech understanding in quiet and in noise, and vowel, consonant and voice-pitch perception.

Results: On average, adaptive FC did not significantly affect any of the test results. Yet, two subjects who were fitted with a relatively weak frequency compression ratio, showed improved horizontal sound localization. After the study, four subjects preferred adaptive FC, four preferred standard frequency mapping, and four had no preference. Noteworthy, the subjects preferring adaptive FC were those with best performance on all tasks, both with and without adaptive FC.

Conclusion: On a group level, extreme adaptive FC did not change sound localization and speech understanding in bimodal listeners. Possible reasons are too strong compression ratios, insufficient residual hearing or that the adaptive switching, although preserving vowel perception, may have been ineffective to produce consistent ILD cues. Individual results suggested that two subjects were able to integrate the frequency-compressed HA input with that of the CI, and benefitted from enhanced binaural cues for horizontal sound localization.     

听障儿童双耳双模式配戴的声源定位能力初探*

目的探讨双耳双模式配戴能否帮助听障儿童改善声源定位能力以及哪些因素可能影响双耳双模式声源定位优势的发挥。方法采用强迫二选一任务,比较16名听障儿童在不同助听模式及不同声源角度下的声源定位能力。结果声源位于90°/270°时,38%的被试体现出双耳双模式的声源定位优势;位于45°/315°时,优势比例下降到25%。进一步分析表明,人工耳蜗麦克风的位置以及术前双耳配戴助听器的经验与双耳双模式声源定位优势的发挥密切相关。结论声源定位能力是听障儿童日常生活中的难点,即使双耳双模式配戴,也仅在声源位于左右方时,比单侧耳蜗状态体现出一定优势,随着声源角度的减小,双耳强度差和时间差线索减弱,双耳双模式的声源定位优势也随之减小。人工耳蜗的麦克风须放置在正确位置,否则会影响声源定位能力。此外,术前助听器配戴经验可能会影响听障儿童利用双耳线索的能力。     

Improvement of sound source localization abilities in patients bilaterally supplied with active middle ear implants

Conclusions: Patients, who are bilaterally supplied with active middle ear implants, perform slightly better in sound localization tasks than when unilaterally aided or unaided. Objectives: To investigate the impact of bilateral use of active middle ear implants on sound source localization in the horizontal plane in patients with a sloping moderate-to-severe hearing loss. Methods: Ten adults supplied with Med-EL Vibrant Soundbridge systems (VSB) in both ears participated in the study. Four listening conditions were tested: unaided, aided with VSB on left or right ear and on both sides. In each condition the subjects had to judge the direction of broadband noises delivered randomly across a semicircular array of 11 loudspeakers arranged in an anechoic chamber. Results: When unaided or bilaterally aided, the subjects localized on average 40% of the stimuli correct; when unilaterally aided (left or right), this rate dropped to 20–30% in either condition. Precision of sound localization was highest when bilaterally aided, i.e. the mean RMS angular error was 10°, and lowest when unilaterally aided, i.e. 15°. This is in line with bilateral hearing aid users, who show similar performance in sound localization tasks.     

Interaural Phase and the Release from Masking

In previous presentations to this congress we have reported an increase in intelligibility of isolated words at poor signal-to-noise ratios when the phase angle differences between the ears were adjusted to certain values. Question has arisen as to whether this phenomenon is one of a peculiar property of speech or one of that kind usually called release from masking. Therefore, a study was done to compare normal voiced speech with whispered speech at a signal-to-noise ratio of 0 dB and subject to various phase angle differences between the ears. Data are presented which suggest that the apparent release from masking may be due in part to properties of voice, but not entirely so.     

4岁健康儿童水平方位声源定位测试方法的初步探讨

目的 探索一种适合检测4岁正常儿童水平方位声源定位能力的方法,并对测试结果进行分析.方法 标准测听室内,利用自制的声源定位测试仪,采用儿童角度辨别阈方法测试10名4岁正常儿童水平01°±45°、±90°、±135°、180°八个方位的角度辨别阈值(minimum audible angle,MAA).结果 MAA(0°)=(3.80±0.71)°,MAA(-45°)=(7.70±1.27)°,MAA(45°)=(7.10±1.39)°,MAA(-90°)=(8.15±2.38)°,MAA(90°):(7.61±2.47)°,MAA(-135°)=(8.85±2.70)°,MAA(135°)=(8.30±1.42)°,MAA(180°)=(5.20±1.27)°.受试儿童水平8个方位的MAA均小于10°,且正前方辨别阈值最小,正后方次之.结论 儿童角度辨别阈测试方法可用来评估4岁正常儿童声源定位能力.     

Sound localization in wild Norway rats (Rattus norvegicus)

The ability of three wild Norway rats to localize sound was determined for single clicks and 100-ms white noise bursts. Chance level localization thresholds were 12 degrees for clicks and 9.7 degrees for white noise. A comparison of these results with published localization thresholds for the domestic albino rat yielded no significant differences. It appears that the combined effects of domestication and albinism have not affected the ability of the laboratory rat to localize sound. Instead, the relatively poor localization acuity of these rats appears to be part of the normal variation in sound localization acuity found among different species of mammals.     

Auditory Processing of Spectral Cues for Sound Localization in the Inferior Colliculus

The head-related transfer function (HRTF) of the cat adds directionally dependent energy minima to the amplitude spectrum of complex sounds. These spectral notches are a principal cue for the localization of sound source elevation. Physiological evidence suggests that the dorsal cochlear nucleus (DCN) plays a critical role in the brainstem processing of this directional feature. Type O units in the central nucleus of the inferior colliculus (ICC) are a primary target of ascending DCN projections and, therefore, may represent midbrain specializations for the auditory processing of spectral cues for sound localization. Behavioral studies confirm a loss of sound orientation accuracy when DCN projections to the inferior colliculus are surgically lesioned. This study used simple analogs of HRTF notches to characterize single-unit response patterns in the ICC of decerebrate cats that may contribute to the directional sensitivity of the brain's spectral processing pathways. Manipulations of notch frequency and bandwidth demonstrated frequency-specific excitatory responses that have the capacity to encode HRTF-based cues for sound source location. These response patterns were limited to type O units in the ICC and have not been observed for the projection neurons of the DCN. The unique spectral integration properties of type O units suggest that DCN influences are transformed into a more selective representation of sound source location by a local convergence of wideband excitatory and frequency-tuned inhibitory inputs.     

1~3岁正常儿童水平方位声源定位能力测试方法的初步探讨

目的 探索检测1~3岁正常儿童水平方位声源定位能力的方法,获得1~3岁正常儿童水平方位0°角度辨别阈值(minimum audible angle, MAA),即MAA(0°).方法 在标准测听室内,利用声源定位测试仪和Conera听力计,采用视觉强化测听或游戏测听和角度辨别阈测试相结合的方法,对37例1~7岁儿童进行水平方位500、1 000及4 000 Hz的0°角度辨别阈值[MAA(0°)]测试.结果 37例受试儿童都完成了1 000 Hz的测试,35例完成了500 Hz的测试,30例完成了4 000 Hz的测试(1~3岁、4~7岁各15例).1~3岁组500 、 1 000 、4 000 Hz MAA(0°) 分别为 5.03°±2.96°、3.57°±1.35°、 5.40°±2.86°,4~7岁组分别为3.50°±2.17°、3.56°±1.48°、 6.13°±4.09°,两组间各频率MAA(0°)差异无统计学意义(P>0.05);1~3岁组内三个频率间MAA(0°)差异无统计学意义;4~7岁组内500 Hz与1 000 Hz MAA(0°)差异无统计学意义(P>0.05),4 000 Hz MAA(0°)大于1 000 Hz及500 Hz,差异均有统计学意义(P<0.05).结论 采用视觉强化测听或游戏测听和角度辨别阈测试相结合的方法可评估1~3岁正常儿童水平方位声源定位能力;刺激声频率对儿童声源定位测试结果有影响,1~3岁与4~7岁正常儿童水平方位声源定位能力差异无统计学意义.