Horizontal Sound Localization Research Articles

Bonebridge implants versus atresiaplasty in children with unilateral congenital aural atresia: A comparison study of audiological outcomes

ObjectivesTo evaluate and compare audiological outcomes of atresiaplasty and Bonebridge (BB) implantation in patients with unilateral congenital aural atresia (UCAA), to guide clinical decision-making. MethodsTwenty-seven subjects diagnosed with UCAA were included in the study. Thirteen were implanted with the BB, while 14 undergone atresiaplasty. All patients underwent pre-and post-surgery examinations, including pure-tone audiometry, sound field threshold (SFT), speech reception threshold (SRT), word recognition score (WRS), and horizontal sound source localization tests. Results(1) Postoperatively, the average SFT decreased by 11.79 ± 5.93 dB HL in the atresiaplasty group and by 24.46 ± 9.36 dB HL in the BB group, with a significantly greater decrease in the BB group compared to the atresiaplasty group (P < 0.05). (2) Both groups demonstrated a significant improvement in average disyllabic WRS postoperatively under normal ear-masking conditions, with the BB group showing a significantly higher improvement than the atresiaplasty group. (3) When the speech signal was presented from the CAA side with noise from the normal hearing side, both surgical groups exhibited a significant decrease in postoperative signal-to-noise ratio compared to preoperative levels, with improvements of 2.14 ± 2.95 dB SNR in the atresiaplasty group and 4.92 ± 5.83 dB SNR in the BB group (P < 0.05). (4) The average minimum audible angle preoperative in the atresiaplasty group was 29.71 ± 18.42°, which decreased to 18.1 ± 10.07° at 6 months postoperatively, showing a statistically significant improvement (P < 0.05). ConclusionWe concluded that both atresiaplasty and Bonebridge implantation can significantly improve speech perception under both quiet and noisy conditions in children with UCAA. BoneBridge implantation appears to provide better audiological outcomes than atresiaplasty. Atresiaplasty can significantly improve the accuracy of sound localization. No significant improvement in sound localization accuracy was observed in the short period after Bonebridge implantation. Further research should be conducted with a larger sample size and longer follow-up time.

Horizontal Sound Localization and Spatial Short-Term Memory Span in Hearing-Impaired Listeners and Listeners With Simulated Hearing Loss.

Localization of a sound source in the horizontal plane depends on the listener's interaural comparison of arrival time and level. Hearing loss (HL) can reduce access to these binaural cues, possibly disrupting the localization and memory of spatial information. Thus, this study aimed to investigate the horizontal sound localization performance and the spatial short-term memory in listeners with actual and simulated HL. Seventeen listeners with bilateral symmetric HL and 17 listeners with normal hearing (NH) participated in the study. The hearing thresholds of NH listeners were elevated by a spectrally shaped masking noise for the simulations of unilateral hearing loss (UHL) and bilateral hearing loss (BHL). The localization accuracy and errors as well as the spatial short-term memory span were measured in the free field using a set of 11 loudspeakers arrayed over a 150° arc. The localization abilities and spatial short-term memory span did not significantly differ between actual BHL listeners and BHL-simulated NH listeners. Overall, the localization performance with the UHL simulation was approximately twofold worse than that with the BHL simulation, and the hearing asymmetry led to a detrimental effect on spatial memory. The mean localization score as a function of stimulus location in the UHL simulation was less than 30% even for the front (0° azimuth) stimuli and much worse on the side closer to the simulated ear. In the UHL simulation, the localization responses were biased toward the side of the intact ear even when sounds were coming from the front. Hearing asymmetry induced by the UHL simulation substantially disrupted the localization performance and recall abilities of spatial positions encoded and stored in the memory, due to fewer chances to learn strategies to improve localization. The marked effect of hearing asymmetry on sound localization highlights the need for clinical assessments of spatial hearing in addition to conventional hearing tests.

Horizontal sound localization in presence of noise in normal-hearing young adults

Objective: This study investigates the effect of signal-to-noise ratio (SNR), frequency, and bandwidth on horizontal sound localization accuracy in normal-hearing young adults. Methods: From August 2022 to December 2022, a total of 20 normal-hearing young adults, including 7 males and 13 females, with an age range of 20 to 35 years and a mean age of 25.4 years, were selected to participate in horizontal azimuth recognition tests under both quiet and noisy conditions. Six narrowband filtered noise stimuli were used with central frequencies (CF) of 250, 2 000, and 4 000 Hz and bandwidths of 1/6 and 1 octave. Continuous broadband white noise was used as the background masker, and the signal-to-noise ratio (SNR) was 0, -3, and -12 dB. The root-mean-square error (RMS error) was used to measure sound localization accuracy, with smaller values indicating higher accuracy. Friedman test was used to compare the effects of SNR and CF on sound localization accuracy, and Wilcoxon signed-rank test was used to compare the impact of the two bandwidths on sound localization accuracy in noise. Results: In a quiet environment, the RMS error in horizontal azimuth in normal-hearing young adults ranged from 4.3 to 8.1 degrees. Sound localization accuracy decreased with decreasing SNR: at 0 dB SNR (range: 5.3-12.9 degrees), the difference from the quiet condition was not significant (P>0.05); however, at -3 dB (range: 7.3-16.8 degrees) and -12 dB SNR (range: 9.4-41.2 degrees), sound localization accuracy significantly decreased compared to the quiet condition (all P<0.01). Under noisy conditions, there were differences in sound localization accuracy among stimuli with different frequencies and bandwidths, with higher frequencies performing the worst, followed by middle frequencies, and lower frequencies performing the best, with significant differences (all P<0.01). Sound localization accuracy for 1/6 octave stimuli was more susceptible to noise interference than 1 octave stimuli (all P<0.01). Conclusions: The ability of normal-hearing young adults to localize sound in the horizontal plane in the presence of noise is influenced by SNR, CF, and bandwidth. Noise with SNRs of ≥-3 dB can lead to decreased accuracy in narrowband sound localization. Higher CF signals and narrower bandwidths are more susceptible to noise interference.

Further investigation of horizontal sound localization in noise

The accuracy of sound localization suffers significantly in a noisy environment. Noise can have an impact on the binaural localization cues such as interaural time difference (ITD) and interaural level difference (ILD). This impact is not only related to the signal-to-noise ratio (SNR), but also to the noise level. Therefore, it's crucial to understand how to describe binaural localization cues in noise. This study investigates the effect of noise level (ranging from 80 dB to 95 dB) and SNR (ranging from -15 dB to -5 dB) on the localization of horizontal sound sources at different azimuths. By analyzing the interaction between different binaural cues and localization accuracy in noise, the results demonstrate that there is a strong association between localization accuracy and interaural partial loudness difference. A follow-up experiment with modified ILD confirmed that the interaural partial loudness difference might more accurately reflect the auditory localization properties of the horizontal sound source in noise.

Effects of vowel coloration on perceived directional offsets between single-syllable speech sounds

The ability of normal hearing listeners to spatially localize single-syllable speech sounds was examined via a two-interval, two alternative forced choice (2AFC) spatial offset discrimination task. To examine directional biases in horizontal sound localization, this task required listeners to observe the direction of an unseen second talker’s voice immediately after hearing a first talker’s voice in an alternative direction. Listeners reported only the relative frontward versus rearward spatial offset between the two voices heard during each trial. Headphone-based virtual acoustic simulation enabled blind listening tests that were completed without feedback to the listener, revealing the ease with which listeners judged the relative spatial offset of the second speech sound, although those judgments were strongly biased by differences in the source spectra. The single-syllable speech-sound stimuli were processed using individually measured head-related transfer functions (HRTFs) of 15 listeners for whom individually measured earphone transfer functions (ETFs) were also employed. While analysis of the 2AFC response data via receiver operating characteristic (ROC) curves showed a generally high level of directional sensitivity, systematic biases were observed that could be explained via the differences in vowel coloration between the first and second source presented during each trial.

Time scales of adaptation to context in horizontal sound localization.

Psychophysical experiments explored how the repeated presentation of a context, consisting of an adaptor and a target, induces plasticity in the localization of an identical target presented alone on interleaved trials. The plasticity, and its time course, was examined both in a classroom and in an anechoic chamber. Adaptors and targets were 2 ms noise clicks and listeners were tasked with localizing the targets while ignoring the adaptors (when present). The context was either simple, consisting of a single-click adaptor and a target, or complex, containing either a single-click or an eight-click adaptor that varied from trial to trial. The adaptor was presented either from a frontal or a lateral location, fixed within a run. The presence of context caused responses to the isolated targets to be displaced up to 14° away from the adaptor location. This effect was stronger and slower if the context was complex, growing over the 5 min duration of the runs. Additionally, the simple context buildup had a slower onset in the classroom. Overall, the results illustrate that sound localization is subject to slow adaptive processes that depend on the spatial and temporal structure of the context and on the level of reverberation in the environment.

A pilot study on spatial hearing in children with congenital unilateral aural atresia.

Despite normal hearing in one ear, individuals with congenital unilateral aural atresia may perceive difficulties in everyday listening conditions typically containing multiple sound sources. While previous work shows that intervention with bone conduction devices may aid spatial hearing for some children, testing conditions are often arranged to maximize any benefit and are not very similar to daily life. The benefit from amplification on spatial tasks has been found to vary between individuals, for reasons not entirely clear. This study has sought to expand on the limited knowledge on how children with unilateral aural atresia recognize speech masked by competing speech, and how horizontal sound localization accuracy is affected by the degree of unilateral hearing loss and by amplification using unilateral bone conduction devices when fitted before 3 years of age. In a within-subject, repeated measures design, including 11 children (mean age = 7.9 years), bone conduction hearing device (BCD) amplification did not negatively affect horizontal sound localization accuracy. The effect on speech recognition scores showed greater inter-individual variability. No benefit from amplification on a group level was found. There was no association between age at fitting and the benefit of the BCD. For children with poor unaided sound localization accuracy, there was a greater BCD benefit. Unaided localization accuracy increased as a function of decreasing hearing thresholds in the atretic ear. While it is possible that low sound levels in the atretic ear provided access to interaural localization cues for the children with the lowest hearing thresholds, the association has to be further investigated in a larger sample of children.

Development of Sound Localization in Infants and Young Children with Cochlear Implants.

Cochlear implantation as a treatment for severe-to-profound hearing loss allows children to develop hearing, speech, and language in many cases. However, cochlear implants are generally provided beyond the infant period and outcomes are assessed after years of implant use, making comparison with normal development difficult. The aim was to study whether the rate of improvement of horizontal localization accuracy in children with bilateral implants is similar to children with normal hearing. A convenience sample of 20 children with a median age at simultaneous bilateral implantation = 0.58 years (0.42-2.3 years) participated in this cohort study. Longitudinal follow-up of sound localization accuracy for an average of ≈1 year generated 42 observations at a mean age = 1.5 years (0.58-3.6 years). The rate of development was compared to historical control groups including children with normal hearing and with relatively late bilateral implantation (≈4 years of age). There was a significant main effect of time with bilateral implants on localization accuracy (slope = 0.21/year, R2 = 0.25, F = 13.6, p &lt; 0.001, n = 42). No differences between slopes (F = 0.30, p = 0.58) or correlation coefficients (Cohen's q = 0.28, p = 0.45) existed when comparing children with implants and normal hearing (slope = 0.16/year since birth, p = 0.015, n = 12). The rate of development was identical to children implanted late. Results suggest that early bilateral implantation in children with severe-to-profound hearing loss allows development of sound localization at a similar age to children with normal hearing. Similar rates in children with early and late implantation and normal hearing suggest an intrinsic mechanism for the development of horizontal sound localization abilities.

Sensitivity to interaural level and time differences in individuals with autism spectrum disorder

Individuals with autism spectrum disorders (ASD) are reported to exhibit degraded performance in sound localization. This study investigated whether the sensitivity to the interaural level differences (ILDs) and interaural time differences (ITDs), major cues for horizontal sound localization, are affected in ASD. Thresholds for discriminating the ILD and ITD were measured for adults with ASD and age- and IQ-matched controls in a lateralization experiment. Results show that the ASD group exhibited higher ILD and ITD thresholds than the control group. Moreover, there was a significant diversity of ITD sensitivity in the ASD group, and it contained a larger proportion of participants with poor ITD sensitivity than the control group. The current study suggests that deficits in relatively low-level processes in the auditory pathway are implicated in degraded performance of sound localization in individuals with ASD. The results are consistent with the structural abnormalities and great variability in the morphology in the brainstem reported by neuroanatomical studies of ASD.

Differing Bilateral Benefits for Spatial Release From Masking and Sound Localization Accuracy Using Bone Conduction Devices.

Objectives:Normal binaural hearing facilitates spatial hearing and therefore many everyday listening tasks, such as understanding speech against a backdrop of competing sounds originating from various locations, and localization of sounds. For stimulation with bone conduction hearing devices (BCD), used to alleviate conductive hearing losses, limited transcranial attenuation results in cross-stimulation so that both cochleae are stimulated from the position of the bone conduction transducer. As such, interaural time and level differences, hallmarks of binaural hearing, are unpredictable at the level of the inner ears. The aim of this study was to compare spatial hearing by unilateral and bilateral BCD stimulation in normal-hearing listeners with simulated bilateral conductive hearing loss.Design:Bilateral conductive hearing loss was reversibly induced in 25 subjects (mean age = 28.5 years) with air conduction and bone conduction (BC) pure-tone averages across 0.5, 1, 2, and 4 kHz (PTA4) <5 dB HL. The mean (SD) PTA4 for the simulated conductive hearing loss was 48.2 dB (3.8 dB). Subjects participated in a speech-in-speech task and a horizontal sound localization task in a within-subject repeated measures design (unilateral and bilateral bone conduction stimulation) using Baha 5 clinical sound processors on a softband. For the speech-in-speech task, the main outcome measure was the threshold for 40% correct speech recognition when masking speech and target speech were both colocated (0°) and spatially and symmetrically separated (target 0°, maskers ±30° and ±150°). Spatial release from masking was quantified as the difference between colocated and separated masking and target speech thresholds. For the localization task, the main outcome measure was the overall variance in localization accuracy quantified as an error index (0.0 = perfect performance; 1.0 = random performance). Four stimuli providing various spatial cues were used in the sound localization task.Results:The bilateral BCD benefit for recognition thresholds of speech in competing speech was statistically significant but small regardless if the masking speech signals were colocated with, or spatially and symmetrically separated from, the target speech. Spatial release from masking was identical for unilateral and bilateral conditions, and significantly different from zero. A distinct bilateral BCD sound localization benefit existed but varied in magnitude across stimuli. The smallest benefit occurred for a low-frequency stimulus (octave-filtered noise, CF = 0.5 kHz), and the largest benefit occurred for unmodulated broadband and narrowband (octave-filtered noise, CF = 4.0 kHz) stimuli. Sound localization by unilateral BCD was poor across stimuli.Conclusions:Results suggest that the well-known transcranial transmission of BC sound affects bilateral BCD benefits for spatial processing of sound in differing ways. Results further suggest that patients with bilateral conductive hearing loss and BC thresholds within the normal range may benefit from a bilateral fitting of BCD, particularly for horizontal localization of sounds.

Auditory Spatial Saliency and Its Effects on Perceptual Noisiness

Environmental noise affects significantly our health and quality of life. Simple techniques, such as A-weighted decibels, have been applied commonly to the real issues of environmental noise. More elaborate techniques, considering mechanisms in the central nervous system, have also been developed continuously and approved by the international organization of standardization (e.g., ISO 532-3; hereafter, ISO loudness model). These techniques have advanced our knowledge of perceptual noisiness, but still have some limitations to account for a variety of psychophysical phenomena and our empirical experiences in acoustic engineering. Here, we propose that perceptual noisiness can be explained better by considering auditory attention. Attention driven by sensory input has been modeled originally as “saliency” in vision. This algorithm has also been applied to capture spectral-temporal dynamics of auditory attention (hereafter, spectral saliency). It has been suggested that the central auditory system contains two pathways identifying what and where a sound source is. The above spectral saliency corresponds only to the what-pathway. We therefore created a new auditory spatial saliency model to capture attentional effects along the where-pathway based on an algorithm of horizontal sound localization. We found that our spatial saliency model accounted for perceptual phenomena that cannot be explained by the ISO loudness model. Furthermore, the prediction of perceptual noisiness of environmental sounds (driving sounds of passenger cars) was improved significantly by integrating spatial saliency with ISO loudness. We conclude that spatial saliency can be used to capture sound features affecting perceptual noisiness in everyday life.

The Development of Sound Localization Latency in Infants and Young Children with Normal Hearing.

With the advances in eye tracking, saccadic reflexes towards auditory stimuli have become an easily accessible behavioral response. The present study investigated the development of horizontal sound localization latency quantified by saccadic reflexes in infants and young children with normal hearing (0.55 to 5.6 years, n = 22). The subject was seated in front of an array of 12 loudspeaker/display-pairs arranged equidistantly in an arc from −55 to + 55° azimuth. An ongoing auditory-visual stimulus was presented at 63 dB SPL and shifted to another randomly selected pair at 24 occasions. At each shift, the visual part of the stimulus was blanked for 1.6 s providing auditory-only localization cues. A sigmoid model was fitted to the gaze samples following the azimuthal sound shifts. The overall sound localization latency (SLL) for a subject was defined as the mean of the latencies for all trials included by objective criteria. The SLL was assessed in 21 of 22 children with a mean of 6.1 valid trials. The SLL ranged 400 to 1400 ms (mean = 860 ms). An inverse model demonstrated a significant relationship between SLL and age (R2 = 0.79, p < 0.001), reflecting a distinct reduction of latency with increasing age. No partial correlation between SLL and sound localization accuracy was found when controlling for age (p = 0.5), suggesting that localization latency may provide diagnostic value beyond accuracy.

Amount of Frequency Compression in Bimodal Cochlear Implant Users Is a Poor Predictor for Audibility and Spatial Hearing.

Speech understanding in noise and horizontal sound localization is poor in most cochlear implant (CI) users with a hearing aid (bimodal stimulation). This study investigated the effect of static and less-extreme adaptive frequency compression in hearing aids on spatial hearing. By means of frequency compression, we aimed to restore high-frequency audibility, and thus improve sound localization and spatial speech recognition. Sound-detection thresholds, sound localization, and spatial speech recognition were measured in eight bimodal CI users, with and without frequency compression. We tested two compression algorithms: a static algorithm, which compressed frequencies beyond the compression knee point (160 or 480 Hz), and an adaptive algorithm, which aimed to compress only consonants leaving vowels unaffected (adaptive knee-point frequencies from 736 to 2946 Hz). Compression yielded a strong audibility benefit (high-frequency thresholds improved by 40 and 24 dB for static and adaptive compression, respectively), no meaningful improvement in localization performance (errors remained > 30 deg), and spatial speech recognition across all participants. Localization biases without compression (toward the hearing-aid and implant side for low- and high-frequency sounds, respectively) disappeared or reversed with compression. The audibility benefits provided to each bimodal user partially explained any individual improvements in localization performance; shifts in bias; and, for six out of eight participants, benefits in spatial speech recognition. We speculate that limiting factors such as a persistent hearing asymmetry and mismatch in spectral overlap prevent compression in bimodal users from improving sound localization. Therefore, the benefit in spatial release from masking by compression is likely due to a shift of attention to the ear with the better signal-to-noise ratio facilitated by compression, rather than an improved spatial selectivity. Supplemental Material https://doi.org/10.23641/asha.16869485.

Predicting the effect of hearing-protection devices on horizontal-plane sound localization

Hearing-protection devices (HPD) are critical to hearing health in hazardous noise environments, but can be detrimental to job effectiveness and safety when degrading a wearer’s sound localization accuracy. Understanding specifically how localization is disrupted by HPDs is critical for mitigating the deleterious effects of HPDs on situation awareness. This study attempts to better quantify the effects of the degradations by predicting horizontal sound localization. A 360 deg horizontal array with 10 deg between-speaker spacing is used to collect head-related transfer functions (HRTFs) and to measure localization [ANSI/ASA S3.71] for the different conditions (open-ear, passive plugs, and both passive and active muffs). Our model attempts to select the response location that was most likely to have elicited the observed stimulus for the trial, by comparing narrow-band statistics between prior knowledge (“templates”) and trial observations. Bone conduction related noise is added, and the degree to which the priors are “device-related” (i.e., based on HRTFs captured for the device) or “open ear” related is varied. Results are discussed in terms of two potential mechanisms: (1) the wrong location is chosen due to a prior/template mismatch in percent device-related and (2) multiple response locations are viable because cue variability is reduced between locations compared to within.

Adults with unilateral congenital ear canal atresia – sound localization ability and recognition of speech in competing speech in unaided condition

Background Individuals with unilateral hearing loss show poor spatial hearing, but individual variability is high. Aims/objectives To investigate if the degree of hearing loss in unilateral ear canal atresia affects horizontal sound localization and speech recognition. Materials and methods Twelve subjects with unilateral ear canal atresia without childhood hearing intervention. Previously published data from eight normal-hearing subjects in normal binaural as well as experimentally induced unilateral hearing loss served as a reference. Horizontal sound localization and recognition of speech in spatially separate competing speech were assessed. Results Linear regression analysis demonstrated a relationship between sound localization accuracy (SLA) and the air conduction pure tone average of the atretic ear (r = 0.85, p=.007). The large proportion of variability in SLA (72%) explained by the degree of hearing loss of the atretic ear indicates that binaural processing is possible. SLA was worse than for normal hearing individuals (p<.0001), and comparable to moderate simulated unilateral hearing loss (p=.13). Speech discrimination was significantly worse than normal (p<.0001) and not dependent on degree of hearing loss of the atretic ear. Conclusions and significance Individuals with congenital unilateral ear canal atresia show impaired horizontal SLA and recognition of speech in competing speech.

A mobile sound localization setup

In this paper, a mobile sound localization setup is described that can be used to measure a persons’ localization performance in a sophisticated way. With this mobile setup, researchers can travel to subjects, and studies are not limited by the willingness of participants to visit the clinic. In the setup, sounds are presented within a partial sphere in both the horizontal (−70° to 70° azimuth) and vertical (−35° to 40° elevation) plane. Participants are asked to indicate the perceived sound origin by pointing with a head-mounted LED. Head movements are recorded and instantly visualized (i.e. online target response plots). Depending on the research question, the setup can be adjusted for more advanced or simplified measurements, making the setup suitable for a wide range of research questions. The rationale for building this mobile setup was to test horizontal sound localization abilities (binaural hearing) and vertical sound localization abilities (monaural hearing) of children and patients who were otherwise not accessible for testing. In this setup loudspeakers are not visible and subjects are asked to indicate the perceived sound direction by a natural head-pointing response towards the perceived location. An advantage of the implemented pointing-method is the playful manner in which children are tested. They are ‘shooting’ at the perceived sound target location with a head-mounted LED and have fun while performing the test.•We present a mobile sound localization setup suitable for measuring horizontal and vertical sound localization in children and adult patients in the convenience of their own environment.

Auditory Space Perception in the Blind: Horizontal Sound Localization in Acoustically Simple and Complex Situations.

Even though it is widely believed that cross-modal neuroplasticity in blindness results in enhanced auditory spatial abilities, a growing number of studies also indicate disadvantages of blind persons for specific, more demanding audiospatial tasks. Here, the effects of blindness on horizontal sound localization were compared for simple localization of target sounds presented in isolation and target localization in an acoustically complex ( cocktail-party) situation with multiple distractor sounds. While performances of blind and sighted subjects were similar in the cocktail-party setting, a significant disadvantage of blind subjects was found for simple localization of sound sources presented in the center of the frontal space, with the mean absolute error being more than twice that of controls for targets in the median plane. The latter result could be due to the lack of visual calibration of auditory space, which may, in sighted persons, be most effective in the region of maximum visual acuity. On the other hand, the normal performance of blind persons in cocktail-party localization could be explained by the fact that this task, in addition to genuine spatial analysis, critically involved spectrotemporal analysis of the auditory scene to separate different sources for which superior performance is known from previous research.

Influence of Audiovisual Training on Horizontal Sound Localization and Its Related ERP Response.

The objective was to investigate the influence of audiovisual training on horizontal sound localization and the underlying neurological mechanisms using a combination of psychoacoustic and electrophysiological (i.e., event-related potential, ERP) measurements on sound localization. Audiovisual stimuli were used in the training group, whilst the control group was trained using auditory stimuli only. Training sessions were undertaken once per day for three consecutive days. Sound localization accuracy was evaluated daily after training, using psychoacoustic tests. ERP responses were measured on the first and last day of tasks. Sound localization was significantly improved in the audiovisual training group when compared to the control group. Moreover, a significantly greater reduction in front-back confusion ratio for both trained and untrained angles was found between pre- and post-test in the audiovisual training group. ERP measurement showed a decrease in N1 amplitude and an increase in P2 amplitude in both groups. However, changes in late components were only found in the audiovisual training group, with an increase in P400 amplitude and decrease in N500 amplitude. These results suggest that the interactive effect of audiovisual localization training is likely to be mediated at a relatively late cognitive processing stage.

Characteristics of horizontal sound localization of elderly people and analysis of its potential influential factors

To investigate the performance of horizontal sound localization of elderly people, authors conducted a test using 19 elderly participants (10 males, 9 females; ages 65-85; mean 71.5 years old.) The stimuli were presented through headphones to their right and left ears with 4 interaural time differences (ITDs) (0.2, 0.4, 0.6, and 0.8 ms) and 4 interaural level differences (ILDs) (3, 6, 9, and 12 dB(A)) besides the same condition to both ears. Concerning the frequency, pure tones of 1.0 and 2.0 kHz, and composite sound of 0.5 + 2.0 kHz were presented, which lasted one second for each trial. The participants answered “Right” or “Left” after each trial according to their own judgment. The results of ITD condition showed that in case of the pure tones a right-sided inferiority was recognized, which was poorer than the results of the younger participants. Meanwhile, no right/left difference was recognized with the composite sound. Concerning the ILD condition, no right/left asymmetricity was shown. Furthermore, no significant differences were recognized regarding the relationships between their sound localization performance and potential influential factors. The factors were Trail Making Test (TMT) results, their preferred ear, and their hearing loss difference between right/left ears.

Right/left asymmetricity of horizontal sound localization of younger people

To investigate the performance of horizontal sound localization of younger people, authors conducted a test using 12 young participants (11 males, 1 females; ages 21–24.) The stimuli were presented through headphones to their right and left ears with 4 interaural time differences (ITDs) (0.2, 0.4, 0.6, and 0.8 ms) and 4 interaural level differences (ILDs) (3, 6, 9, and 12 dB(A)) besides the same condition to both ears. Concerning the frequency, pure tones of 0.5, 1.0, and 2.0 kHz, and composite sound of 0.5 + 2.0 kHz were presented, which lasted one second for each trial. The participants answered “Right” or “Left” after each trial according to their own judgment. The results of ITD condition showed that a right-sided inferiority was recognized in case of 1.0 and 2.0 kHz. Concerning the ILD condition, no right/left asymmetricity was shown. This asymmetricity may be caused by the improper setting of the experiment apparatus. Therefore, we switched right/left of the headphones to carry out an additional test in the same way. The same tendency as the original test was obtained, which means that the asymmetricity is likely due to the native characteristics of the participants.