Sound source localization

Sound source localization is paramount for comfort of life, determining the position of a sound source in 3 dimensions: azimuth, height and distance. It is based on 3 types of cue: 2 binaural (interaural time difference and interaural level difference) and 1 monaural spectral cue (head-related transfer function). These are complementary and vary according to the acoustic characteristics of the incident sound. The objective of this report is to update the current state of knowledge on the physical basis of spatial sound localization.     

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.     

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.     

The advantages of sound localization and speech perception of bilateral electric acoustic stimulation

Conclusion: Bilateral electric acoustic stimulation (EAS) effectively improved speech perception in noise and sound localization in patients with high-frequency hearing loss. Objective: To evaluate bilateral EAS efficacy of sound localization detection and speech perception in noise in two cases of high-frequency hearing loss. Methods: Two female patients, aged 38 and 45 years, respectively, received bilateral EAS sequentially. Pure-tone audiometry was performed preoperatively and postoperatively to evaluate the hearing preservation in the lower frequencies. Speech perception outcomes in quiet and noise and sound localization were assessed with unilateral and bilateral EAS. Results: Residual hearing in the lower frequencies was well preserved after insertion of a FLEX²⁴ electrode (24 mm) using the round window approach. After bilateral EAS, speech perception improved in quiet and even more so in noise. In addition, the sound localization ability of both cases with bilateral EAS improved remarkably.     

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.     

Effects of user training with electronically-modulated sound transmission hearing protectors and the open ear on horizontal localization ability

Abstract

Objective: To determine if training with electronically-modulated hearing protection (EMHP) and the open ear results in auditory learning on a horizontal localization task. Design: Baseline localization testing was conducted in three listening conditions (open-ear, in-the-ear (ITE) EMHP, and over-the-ear (OTE) EMHP). Participants then wore either an ITE or OTE EMHP for 12, almost daily, one-hour training sessions. After training was complete, participants again underwent localization testing in all three listening conditions. A computer with a custom software and hardware interface presented localization sounds and collected participant responses. Study sample: Twelve participants were recruited from the student population at Virginia Tech. Audiometric requirements were 35 dBHL at 500, 1000, and 2000 Hz bilaterally, and 55 dBHL at 4000 Hz in at least one ear. Results: Pre-training localization performance with an ITE or OTE EMHP was worse than open-ear performance. After training with any given listening condition, including open-ear, performance in that listening condition improved, in part from a practice effect. However, post-training localization performance showed near equal performance between the open-ear and training EMHP. Auditory learning occurred for the training EMHP, but not for the non-training EMHP; that is, there was no significant training crossover effect between the ITE and the OTE devices. Conclusion: It is evident from this study that auditory learning (improved horizontal localization performance) occurred with the EMHP for which training was performed. However, performance improvements found with the training EMHP were not realized in the non-training EMHP. Furthermore, localization performance in the open-ear condition also benefitted from training on the task.     

Features of unilateral hearing loss detected by newborn hearing screening programme in different regions of Turkey

Objective

Newborn hearing screening (NHS) works well for babies with bilateral hearing loss. However, for those with unilateral loss, it has yet to be established some standard rules like age of diagnose, risk factors, hearing loss degree. The aim of this study is to identify the demographic characteristics of newborns with unilateral hearing loss to obtain evidence based data in order to see what to be done for children with unilateral hearing loss (UHL).

Method

Newborn hearing screening data of 123 babies with unilateral hearing loss, 71 (57.7%) male and 52 (42.3%) female, were investigated retrospectively. Data provided from the archives of six referral tertiary audiology centers from four regions in Turkey. Data, including type of hearing loss; age of diagnosis; prenatal, natal and postnatal risk factors; familial HL and parental consanguinity was analyzed in all regions and each of the Regions 1–4 separately.

Result

The difference between data obtained in terms of gender and type of hearing loss was detected as statistically significant (p < 0.05). While UHL was significantly higher in females at Region 1, and in males at other Regions of 2–4; SNHL was the most detected type of UHL in all regions with the rate of 82.9–100.0%. There were not significant differences between regions in terms of the degree of hearing loss, presence of risk factors, family history of hearing loss, age at diagnosis and parental consanguinity (p > 0.05). Diagnosis procedure was completed mostly at 3–6 months in Region 4; whereas, in other regions (Regions 1–3), completion of procedure was delayed until 6 months–1 year.

Conclusion

This study indicates that the effect of postnatal risk factors, i.e. curable hyperbilirubinemia, congenital infection and intensive care is relatively high on unilateral hearing loss, precautions should be taken regarding their prevention, as well as physicians and other health personnel should be trained in terms of these risks. For early and timely diagnosis, families will be informed about hearing loss and NHS programme; will be supported, including financial support of diagnosis process. By dissemination of the NHS programme to the total of country by high participation rate, risk factors can be determined better and measures can be increased. Additionally, further studies are needed with more comprehensive standard broad data for more evidence based consensus.     

Findings from multidisciplinary evaluation of children with permanent hearing loss

Objectives

To describe clinical findings from a multidisciplinary program for children with permanent hearing loss (PHL).

Methods

Retrospective chart review at a tertiary care children's hospital. Patients: Two hundred patients charts were selected from the population of 260 children with permanent hearing loss presenting between July 2005 and December 2006. Main outcome measures: PHL etiology; radiographic findings; clinical findings by genetics, ophthalmology, developmental pediatrics, speech pathology, and aural rehabilitation.

Results

Etiology of hearing loss was determined in 60% of subjects. Genetic causes of hearing loss were identified or presumed (positive history of first degree relative with hearing loss) in 27% of the children. Structural ear anomalies were found in 20% of children. Among the 36% of children with CNS imaging, abnormal findings were noted in 32%. There were a high rate of ophthalmological findings (53%) among children seen by ophthalmology (n = 105). Neurodevelopmental evaluations were completed in 58% of subjects and clinically significant findings were noted in 68%. Of the 61% of children who receiving received speech/language evaluations, 77% required intervention. Over half of the 40% of subjects who had an aural rehabilitation evaluation needed therapy. There were not significant differences in rates of findings for children with mild or unilateral hearing loss as compared to children with more severe degrees of hearing loss.

Conclusions

Interdisciplinary medical evaluation of children with PHL allows for the identification and treatment of clinically significant ophthalmologic, neurodevelopmental, genetic, and speech/language disorders. A high rate of CNS and temporal bone abnormalities were identified. These findings provide an understanding of the importance of considering thorough medical and developmental evaluations among children who are deaf/hard of hearing.     

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.     

Management of unilateral hearing loss

ObjectiveA representative sample of literature regarding unilateral hearing loss (UHL) was reviewed to provide evidence of the effects of UHL and the intervention options available for children with UHL. Considerations during the assessment and management of children with UHL are illustrated using case illustrations.MethodResearch articles published from 2013 to 2015 were searched in the PubMed database using the keywords “unilateral hearing loss”. Articles from 1950 to 2013 were included from a previous literature review on minimal hearing loss [1]. A retrospective review of charts of 14 children with UHL was also conducted.ResultsThe evidence indicates that children with UHL are more likely to have structural anomalies of the inner ear; may face challenges in six different domains, and have six intervention options available. Evidence also indicates that although some children appear to exhibit no delays or difficulties, others have significant challenges, some of which continue into adulthood.ConclusionsChildren with UHL have to be treated on a case-by-case basis. Parent education regarding UHL, its effects, and all available management options is critical so they can make informed decisions. Close monitoring and good communication between professionals in different domains is crucial in order to minimize the potential negative effects of UHL.     

A comparison of CIC and BTE hearing aids for three-dimensional localization of speech

Abstract

Three-dimensional sound localization of speech in anechoic space was examined for eleven listeners with sensorineural hearing loss. The listeners were fitted bilaterally with CIC and BTE hearing aids having similar bandwidth capabilities. The goal was to determine whether differences in microphone placement for these two styles (CICs at the ear canal entrance; BTEs above the pinna) would influence the availability of pinna-related spectral cues and hence localization performance. While lateral and polar angle localization was unaffected by the hearing aid style, the rate of front-back reversals was lower with CICs. This pattern persisted after listeners accommodated to each set of aids for a six week period, although the overall rate of reversals declined. Performance on all measures in all conditions was considerably poorer than in a control group of listeners with normal hearing.

Sumario

Se evaluó la localización tridimensional de sonidos del habla en un espacio anecoide en once personas con pérdida auditiva sensorineural. A los once se les adaptaron CIC y BTE bilateralmente con capacidad de ancho de banda similar. El objetivo fue determinar si las diferencias en la colocación del micrófono con ambos tipos (CIC en el ingreso del conducto auditivo; BTE sobre el pabellón auricular) podría influenciar la disponibilidad de claves espectrales relacionadas con el pabellón y por ello, el rendimiento para la localización. Mientras que la localización del ángulo lateral y polar no fue afectada por el tipo de auxiliar, la tasa de inversiones frente-atrás fue menor con los CIC. Este patrón persistió después de que estas personas se acostumbraron a cada set de auxiliares por un período de seis semanas, a pesar de que declinó la tasa global de reversiones. El rendimiento en todas las mediciones y condiciones fue considerablemente más pobre que en el grupo control de personas con audición normal.     

Total ablation of cochlear haircells by perilymphatic perfusion with water

Inner and outer haircells are destroyed within one day following perfusion of the perilymphatic spaces with water, otherwise the structural integrity of the cochlea and the organ of Corti are preserved. This technique is a reliable method to create an acute unilateral deafness model for investigations of auditory function.     

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.     

A case of bilateral enlarged vestibular aqueducts and unilateral hearing loss at birth

Screening of newborn hearing is nowadays widespread, and as many as 3.4% of all screened infants are found to have unilateral hearing loss. However, we, as clinicians and parents, usually dismiss the severity of unilateral hearing loss in pediatric patients. Recently, we experienced the case of unilateral hearing loss since birth in a 2-year-old girl who was finally found to have bilateral enlarged vestibular aqueducts when the disorder progressed to bilateral hearing loss. Proper evaluation of the cause of hearing loss, including imaging studies of the inner ear, and consecutive hearing evaluations should be mandatory in children with unilateral sensorineural hearing loss, due to the possibility of progression of hearing loss bilaterally.     

Safe and reliable sound threshold measures with direct vibration of the ossicular chain

OBJECTIVES: The aim of the study was to evaluate the safety and feasibility of piezoelectric malleus vibration audiometer (MVA), which presents micromechanical vibrations to the umbo membranae tympani. STUDY DESIGN: Phase I study performed in a tertiary referral center (University Hospital). METHODS: The coupling rod of the MVA was moved slowly through the outer ear canal toward the eardrum with a micromanipulator. Coupling was completed when the rod tip touched the umbo membranae tympani. Basic audiologic measures of sound threshold obtained with direct stimulation of the malleus are presented. We used MANOVA (multivariate repeated measures ANOVA) to investigate the repeatability of MVA thresholds from one day to the other and when decoupling and retracting the coupling rod 2 mm off the umbo. We also selected the MANOVA to test for unwanted bone-conduction threshold shifts after MVA application. We assessed normality of the data by quantile-quantile plots of the residuals. RESULTS: Twenty-eight male and 10 female subjects with normal hearing, 22.2 to 34.6 years old (median age, 27.2 yr) underwent an examination. Thirty-six subjects underwent MVA, because 2 of the 38 subjects who volunteered for the study have not undergone the procedure due to the external auditory canal anatomy preventing application of the MVA. The results show that it is possible to safely and reliably measure thresholds of direct vibration of the ossicular chain. Using pure tone audiograms, no pure tone bone- and/or air-conduction threshold shifts occurred after the procedure. None of the subjects reported any other ear-related symptoms such as vertigo, tinnitus, or dizziness. Geometric mean vibratory displacements at threshold ranged from 0.55 nm at 250 Hz to 0.03 nm at 6 kHz. MANOVA demonstrated a repeatability of MVA thresholds. CONCLUSION: Malleus vibration audiometry will not allow exact linkage of actual implantable hearing aid. But the present study demonstrates that MVA can provide an audiometric tool for assessing ossicular function and integrity prior to implantation of an electronic hearing amplifier.     

Responses of young and aged Fischer 344 rat inferior colliculus neurons to binaural tonal stimuli

The inferior colliculus (IC) is one nucleus of the central auditory system which displays age-related changes. Inputs to the IC use primarily the amino acid neurotransmitters glutamate and γ-aminobutryic acid (GABA). Neurochemical and anatomical studies of the Fischer 344 (F344) rat IC have shown decreases in GABA and GABA receptor levels (see Caspary et al., 1995 for review). GABA neurotransmission affects binaural response properties in the IC (Faingold et al., 1991a,b; Vater et al., 1992a; Park and Pollak, 1993, 1994). We hypothesized that aged F344 rats would show alterations in binaural IC neuronal response properties due to an imbalance in the relative levels of inhibition and excitation.

Extracellular recordings from 189 single units localized to the IC of anesthetized aged (24 month) F344 rats were compared to those obtained from 221 IC units in young adult (3 month) animals. Quantitative analyses were performed to determine the distribution of ipsilateral and binaural rate/intensity functions (RIFs) in the central nucleus of the IC and external cortex of the IC units.

The majority of IC units in both young and aged F344 rats were not responsive to monaural ipsilateral characteristic frequency tone bursts. Although there was some shift in the distribution of binaural RIF shapes with age, it was not statistically significant. The shift included a reduction in the percentage of units classified as E/I (excited by contralateral stimulation/ipsilaterally inhibited during binaural stimulation), but an increase with age in the percentage of units classified as E/f (excited by contralateral stimulation/ further facilitated by the addition of low intensity ipsilateral stimulation, but inhibited by higher intensity ipsilateral stimulation). Despite the role of inhibitory neurotransmission in binaural processing in the IC, age-related neurochemical deficits in the IC do not appear to result in a major deficit in the processing of simple binaural stimuli in F344 rats.     

Noise levels required to mask stimuli used in auditory brainstem response testing

Abstract

The levels of noise necessary to effectively mask the stimuli commonly used in auditory brainstem response (ABR) tests were determined. The relative masking level (RLM) of twenty normally-hearing volunteers was measured behaviourally using ipsilaterally presented unfiltered noise in the presence of ABR stimuli. The upper limit for RLM was found to be typically 30 dB above the stimulus level when the stimulus was calibrated in dBnHL to ISO 389-6 (2007) and the noise was measured in dBSPL. This value is recommended when calculating the level of noise necessary to prevent cross-hearing during ABR testing.

Sumario

Se determinaron los niveles de ruido necesarios para enmascarar efectivamente los estímulos comúnmente utilizados en las pruebas de respuestas auditivas del tallo cerebral (ABR). Se midió en forma conductual el nivel de enmascaramiento relativo (RLM) de veinte voluntarios con audición normal, usando ruido no filtrado presentado ipsilateralmente en presencia de estímulos para ABR. Se encontró que el límite superior del RLM típicamente era de 30 dB por encima de nivel del estímulo cuando el estímulo estaba calibrado en dB nHL para ISO 389-6 (2007) y el ruido era medido en dB SPL. Este valor es recomendado cuando se calcula el nivel de ruido necesario para prevenir audición cruzada durante la evaluación con ABR.