Minimum Audible Angle Research Articles

Comparison of horizontal plane auditory spatial discrimination abilities and testing methods in patients with symmetrical sensorineural hearing loss

Objective: To evaluate auditory spatial discrimination capabilities in patients with mild to moderately severe symmetrical sensorineural hearing loss (SNHL) and to compare the impact of different psychophysical testing methods on Minimum Audible Angle (MAA) and test duration. Methods: A total of 105 symmetrical SNHL patients aged from 18 to 60 years old were enrolled from April to July 2023, including 56 males and 49 females. They were divided into three groups based on PTA: mild, moderate, and moderately severe hearing loss, with 35 individuals in each group. Additionally, a control group of 35 individuals with normal hearing was tested, including 18 males and 17 females. Participants underwent four distinct psychophysical discrimination tests: the block up-down, 1-up/1-down, 1-up/2-down, and 1-up/3-down procedures. We recorded the MAA and test duration for each. We employed repeated measures of ANOVA to compare the MAA and test duration across different methods and groups, and Pearson's correlation to assess the relationship between MAA and degree of hearing loss. Results: MAA of sound localization in patients with symmetrical SNHL was significantly positively correlated with the degree of hearing loss (r=0.59, P<0.01). Significant deterioration in MAA was observed as hearing loss progressed to the moderate level (PTA≥35 dBHL, P<0.01). The testing methods significantly influenced MAA and testing duration (F=24.02, P<0.01; F=75.56, P<0.01) and the 1-up/1-down method was the quickest, averaging only (0.69±0.32) mins. Conclusions: The horizontal plane auditory spatial discrimination abilities in patients with symmetrical SNHL is impaired progressively with increasing hearing loss, notably beyond moderate hearing loss levels. Different psychophysical methods influence both MAA and test duration, the quicker 1-up/1-down method is recommended for assessing MAA in symmetrical SNHL patients.

The acoustic change complex evoked by location-change sounds in the elderly

Objective We have shown that the acoustic change complex (ACC) can be elicited by changing the horizontal sound location in young individuals. In this study, we aimed to evaluate the application of ACC within the elderly and its relationship with behavioural results. Design The minimum audible angle (MAA), as well as onset cortical auditory evoked potentials (onset-CAEPs) and ACC elicited by the stimuli of location-change white noise (±45 to ±2 degrees) were recorded. Latencies and amplitudes were analysed using repeated-measures ANOVA. Pearson correlation analysis was conducted to examine the relationship between ACC and MAA. Study sample Ten older adults with normal hearing (NH) and twenty with presbycusis. Results The ACC was effectively elicited with angular variations in elderly participants. The onset-CAEP N1 latency, ACC N1’-P2’ amplitude, and N1’ latency were all associated with the angle shifts, with the N1’ latency being the most predictive factor for angle discrimination. The consistency between MAA and ACC made them complementary for the clinical evaluation of sound localisation. Conclusions The utilisation of ACC, evoked by location-change sounds, presented a promising clinical objective measure for evaluating sound localisation abilities in the elderly.

Degradation in Binaural and Spatial Hearing and Auditory Temporal Processing Abilities as a Function of Aging.

Sensorineural hearing loss is common with advancing age, but even with normal or near normal hearing in older persons, performance deficits are often seen for suprathreshold listening tasks such as understanding speech in background noise or localizing sound direction. This suggests there is also a more central source of the problem. Objectives of this study were to examine as a function of age (young adult to septuagenarian) performance on: 1) a spatial acuity task examining lateralization ability, and a spatial speech-in-noise (SSIN) recognition task, both measured in a hemi-anechoic sound field using a circular horizontal-plane loudspeaker array, and 2) a suprathreshold auditory temporal processing task and a spectro-temporal processing task, both measured under headphones. Further, we examined any correlations between the measures.DesignSubjects were 48 adults, aged 21 to 78, with either normal hearing or only a mild sensorineural hearing loss through 4000 Hz. The lateralization task measured minimum audible angle (MAA) for 500 and 4000 Hz narrowband noise (NBN) bursts in diffuse background noise for both an on-axis (subject facing 0°) and off-axis (facing 45°) listening condition at signal-to-noise ratios (SNRs) of -3, -6, -9, and -12 dB. For 42 of the subjects, SSIN testing was also completed for key word recognition in sentences in multi-talker babble noise; specifically, the separation between speech and noise loudspeakers was adaptively varied to determine the difference needed for 40% and 80% correct performance levels. Finally, auditory temporal processing ability was examined using the Temporal Fine Structure test (44 subjects), and the Spectro-Temporal Modulation test (46 subjects). Mean lateralization performances were poorer (larger MAAs) in older compared to younger subjects, particularly in the more adverse listening conditions (4000 Hz, off-axis, and poorer SNRs). Performance variability was notably higher for older subjects than for young adults. The 4000 Hz NBN bursts produced larger MAAs than did 500 Hz NBN bursts. The SSIN data also showed declining mean performance with age at both criterion levels, with greater variability again found for older subjects. Spearman rho analyses revealed some low to moderate, but significant correlation coefficients for age versus MAA and age versus SSIN results. A low but significant correlation was also observed between the most adverse MAA and SSIN conditions. Results from both the TFS and STM assessments showed decreased mean performance with aging, and revealed moderate, significant correlations, with the strongest relationship shown with the TFS test. Finally, of note, extended-high-frequency (EHF) hearing loss (measured between 9000 and 16,000 Hz) was found in older but not young subjects, and correlated with decreasing performance on several tasks. Particularly for more adverse listening conditions, age-related deficits were found on both of the spatial hearing tasks and in temporal and spectro-temporal processing abilities. It may be that deficits in temporal processing ability contribute to poorer spatial hearing performance in older subjects due to inaccurate coding of binaural/interaural timing information sent from the periphery to the brainstem. In addition, EHF hearing loss may be a coexisting factor in the reduced performance in older subjects.

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.

Static and moving minimum audible angle: Independent contributions of reverberation and position.

Two measures of auditory spatial resolution, the minimum audible angle and the minimum audible movement angle, have been obtained in a simulated acoustic environment using Ambisonics sound field reproduction. Trajectories were designed to provide no reliable cues for the spatial discrimination task. Larger threshold angles were found in reverberant compared to anechoic conditions, for stimuli on the side compared to the front, and for moving compared to static stimuli. The effect of reverberation appeared to be independent of the position of the sound source (same relative threshold increase) and was independently present for static and moving sound sources.

Comparison of 2D and 3D multichannel audio rendering methods for hearing research applications using technical and perceptual measures

Virtual reality with multichannel audio playback is increasingly used in hearing aid research. The purpose of this study is to compare horizontal (2D) and periphonic (3D) rendering methods in terms of localization, minimum audible angle, and perceptual ratings related to spatial quality. Higher Order Ambisonics, Vector-Base Amplitude Panning, and Nearest Speaker Selection were used, with playback through 16, 29 and 45 speakers. The results show that an improvement in vertical localization can be obtained by using periphonic rendering instead of horizontal rendering. The perceptual advantage of periphonic rendering depends on the spatial complexity of the scene; it disappears in complex acoustic environments. Scenes with low acoustic complexity, such as a single primary sound source in a room, benefit from Nearest Speaker Selection rendering. For more complex scenes with multiple sound sources, such as a symphony orchestra in a concert hall with many primary sources, or traffic on a road with moving sources, horizontal rendering methods such as 2D Higher Order Ambisonics will provide similar or better performance.

Auditory Spatial Discrimination and Sound Localization in Single-Sided Deaf Participants Provided with a Cochlear Implant

Introduction: Spatial hearing is most accurate using both ears, but accuracy decreases in persons with asymmetrical hearing between ears. In participants with deafness in one ear but normal hearing in the other ear (single-sided deafness [SSD]), this difference can be compensated by a unilateral cochlear implant (CI). It has been shown that a CI can restore sound localization performance, but it is still unclear to what extent auditory spatial discrimination can be improved. Methods: The present study investigated auditory spatial discrimination using minimum audible angles (MAAs) in 18 CI-SSD participants. Results were compared to 120 age-matched normal-hearing (NH) listeners. Low-frequency (LF) and high-frequency (HF) noise bursts were presented from 4°, 30°, and 60° azimuth on the CI side and on the NH side. MAA thresholds were tested for correlation with localization performance in the same participants. Results: There were eight good performers and ten poor performers. There were more poor performers for LF signals than for HF signals. Performance on the CI side was comparable to performance on the NH side. Most difficulties occurred at 4° and at 30°. Eight of the good performers in the localization task were also good performers in the MAA task. Only the localization ability at 4° on the CI side was positively correlated with the MAA at that location. Conclusion: Our data suggest that a CI can restore localization ability but not necessarily auditory spatial discrimination at the same time. The ability to discriminate between adjacent locations may be trainable during rehabilitation to enhance important auditory skills.

Temporal integration of sound motion: Motion-onset response and perception

The purpose of our study was to estimate the time interval required for integrating the acoustical changes related to sound motion using both psychophysical and EEG measures. Healthy listeners performed direction identification tasks under dichotic conditions in the delayed-motion paradigm. Minimal audible movement angle (MAMA) has been measured over the range of velocities from 60 to 360 deg/s. We also measured minimal duration of motion, at which the listeners could identify its direction. EEG was recorded in the same group of subjects during passive listening. Motion onset responses (MOR) were analyzed. MAMA increased linearly with motion velocity. Minimum audible angle (MAA) calculated from this linear function was about 2 deg. For higher velocities of the delayed motion, we found 2- to 3-fold better spatial resolution than the one previously reported for motion starting at the sound onset. The time required for optimal discrimination of motion direction was about 34 ms.The main finding of our study was that both direction identification time obtained in the behavioral task and cN1 latency behaved like hyperbolic functions of the sound's velocity. Direction identification time decreased asymptotically to 8 ms, which was considered minimal integration time for the instantaneous shift detection. Peak latency of cN1 also decreased with increasing velocity and asymptotically approached 137 ms. This limit corresponded to the latency of response to the instantaneous sound shift and was 37 ms later than the latency of the sound-onset response. The direction discrimination time (34 ms) was of the same magnitude as the additional time required for motion processing to be reflected in the MOR potential. Thus, MOR latency can be viewed as a neurophysiological index of temporal integration. Based on the findings obtained, we may assume that no measurable MOR would be evoked by slowly moving stimuli as they would reach their MAMAs in a time longer than the optimal integration time.

The effect of angular disparity between sound source sequences on the perceptual organization of auditory objects

Within auditory scene analysis is a well-known streaming phenomenon which is observed when listeners group several sounds into “auditory objects” in complex auditory scenes. When two auditory sequences consist of similar sounds, these sequences tend to be perceived as a single stream, but can form segregated streams when the sounds are presented from disparate angles. This study investigated the tolerance thresholds for the angular disparity at which the sound sources would segregate rather than group together into single auditory objects, when 50-ms one-third octave bands of noise were presented in the horizontal plane with changing azimuth angle. Listeners were asked to judge whether the presented noise sequences were perceived as a single stream or not. The results revealed that listeners tend to perceive the presented sequences as a single stream when the angular disparity between noises was small; however, these tolerance thresholds depended on the spatial region within which the sequences were presented. Tolerance thresholds became large when the noise sequences were presented from the side, in comparison to the situation when sequences were presented from in front of the listeners. Results were consistent with the minimum audible angle on the horizontal plane.

Aging leads to impairment of spatial hearing abilities in the Mongolian Gerbil

Aging in humans is known to affect spatial hearing and speech in noise recognition, often even when hearing thresholds are normal. The Mongolian Gerbil (Meriones unguiculatus) can provide a model organism to investigate these changes. We performed auditory brainstem responses (ABRs), prepulse inhibition of the acoustic startle (PPI) behavior, and cochlear histology to assess age-related changes of the gerbil auditory brainstem. Aging (&amp;gt;33month) gerbils showed reduced ABR waveform amplitudes compared to young gerbils (2-10 months). Aging gerbils also show a reduced amplitude of the binaural interaction component (BIC), a biomarker for spatial hearing. However, ABR thresholds were not significantly different between cohorts. Aging gerbils showed impaired performance in our behavior tasks: young gerbils detected shorter gaps in noise and smaller minimum audible angles than aging animals, indicating that aging impairs temporal and spatial hearing abilities. Cochlear histology revealed an increase in cochlear synaptopathy in aging animals, revealing a potential mechanism for age-related dysfunction in the auditory pathway. Taken together, these data reveal structural and physiological changes in the auditory brainstem that underlie spatial hearing deficits in the gerbil. [Work supported by R01-DC017924.]

Electrophysiological and behavioral assessment of binaural processing: Effects of age

Spatial hearing deteriorates with age, but the basis(es) of this age-dependency is not clear and could reflect both bottom-up and top-down factors. Since binaural processing depends in part on interaural time difference cues, this study assessed binaural temporal acuity using both objective and behavioral measures. Three tasks incorporating interaural time/phase differences were tested: (1) in-phase and out-of-phase frequency modulation (FM) detection as a function of FM rate; (2) upper frequency limit of in-phase versus out-of-phase tone discrimination; and (3) minimum audible angle for low frequency tones. Each task incorporated parallel behavioral and electrophysiological measures: For behavior, forced-choice adaptive methods were employed; for electrophysiology, the acoustic change complex (ACC) was recorded. Additional measures of peripheral auditory function, as well as cognitive status, were also collected. Young and middle-aged adults (n = 20 per age group) with normal audiometric hearing were tested. Differences in behavioral measures of binaural performance were noted between the two age groups and these were reflected to some extent in the ACC results. However, group differences were also noted in terms of cochlear function and in terms of cognitive abilities. This pattern of results suggests that age-related deterioration in spatial perception is likely to be multi-faceted.

Hearing in helmeted guineafowl (Numida meleagris): audiogram from 2Hz to 10kHz and localization acuity for brief noise bursts.

Behavioral hearing thresholds and noise localization acuity were determined using a conditioned avoidance/suppression procedure for three Helmeted guineafowl (Numida meleagris). The guineafowl responded to frequencies as low as 2Hz at 82.5dB SPL, and as high as 8kHz at 84.5dB SPL.At a level of 60dB SPL, their hearing range spanned 8.12 octaves (24.6Hz-6.86kHz). Like most birds, they do not hear sounds above 8kHz. However, the guineafowl demonstrated good low-frequency hearing (frequencies below 32Hz), showing thresholds that are more sensitive than both the peafowl and pigeon, both of which hear infrasound. It thus appears that infrasound perception may be more common than previously thought and may have implications for species that inhabit areas with wind energy facilities. The guineafowls' minimum audible angle for a 100-ms broadband noise burst was 13.8 °, at the median for birds and near the mean for mammals. Unlike in mammals, the small sample of bird species and limited representation of lifestyles do not yet allow for meaningful interpretations of the selective pressures or mechanisms that underlie their abilities to locate sound sources.

Sound localization acuity of the common marmoset (Callithrix jacchus)

The common marmoset (Callithrix jacchus) is a small arboreal New World primate which has emerged as a promising model in auditory neuroscience. One potentially useful application of this model system is in the study of the neural mechanism underlying spatial hearing in primate species, as the marmosets need to localize sounds to orient their head to events of interest and identify their vocalizing conspecifics that are not visible. However, interpretation of neurophysiological data on sound localization requires an understanding of perceptual abilities, and the sound localization behavior of marmosets has not been well studied. The present experiment measured sound localization acuity using an operant conditioning procedure in which marmosets were trained to discriminate changes in sound location in the horizontal (azimuth) or vertical (elevation) dimension. Our results showed that the minimum audible angle (MAA) for horizontal and vertical discrimination was 13.17° and 12.53°, respectively, for 2 to 32 kHz Gaussian noise. Removing the monaural spectral cues tended to increase the horizontal localization acuity (11.31°). Marmosets have larger horizontal MAA (15.54°) in the rear than the front. Removing the high-frequency (> 26 kHz) region of the head-related transfer function (HRTF) affected vertical acuity mildly (15.76°), but removing the first notch (12–26 kHz) region of HRTF substantially reduced the vertical acuity (89.01°). In summary, our findings indicate that marmosets’ spatial acuity is on par with other species of similar head size and field of best vision, and they do not appear to use monaural spectral cues for horizontal discrimination but rely heavily on first notch region of HRTF for vertical discrimination.

Azimuthal sound localization in the chicken.

Sound localization is crucial for the survival and reproduction of animals, including non-auditory specialist animals such as the majority of avian species. The chicken (Gallus gallus) is a well-suited representative of a non-auditory specialist bird and several aspects of its auditory system have been well studied in the last decades. We conducted a behavioral experiment where 3 roosters performed a sound localization task with broad-band noise, using a 2-alternative forced choice paradigm. We determined the minimum audible angle (MAA) as measure for localization acuity. In general, our results compare to previous MAA measurements with hens in Go/NoGo tasks. The chicken has high localization acuity compared to other auditory generalist bird species tested so far. We found that chickens were better at localizing broadband noise with long duration (1 s; MAA = 16°) compared to brief duration (0.1 s; MAA = 26°). Moreover, the interaural difference in time of arrival and level (ITD and ILD, respectively) at these MAAs are comparable to what measured in other non-auditory specialist bird species, indicating that they might be sufficiently broad to be informative for azimuthal sound localization.

Sound Localization Ability in Dogs.

The minimum audible angle (MAA), defined as the smallest detectable difference between the azimuths of two identical sources of sound, is a standard measure of spatial auditory acuity in animals. Few studies have explored the MAA of dogs, using methods that do not allow potential improvement throughout the assessment, and with a very small number of dog(s) assessed. To overcome these limits, we adopted a staircase method on 10 dogs, using a two-forced choice procedure with two sound sources, testing angles of separation from 60° to 1°. The staircase method permits the level of difficulty for each dog to be continuously adapted and allows for the observation of improvement over time. The dogs' average MAA was 7.6°, although with a large interindividual variability, ranging from 1.3° to 13.2°. A global improvement was observed across the procedure, substantiated by a gradual lowering of the MAA and of choice latency across sessions. The results indicate that the staircase method is feasible and reliable in the assessment of auditory spatial localization in dogs, highlighting the importance of using an appropriate method in a sensory discrimination task, so as to allow improvement over time. The results also reveal that the MAA of dogs is more variable than previously reported, potentially reaching values lower than 2°. Although no clear patterns of association emerged between MAA and dogs' characteristics such as ear shape, head shape or age, the results suggest the value of conducting larger-scale studies to determine whether these or other factors influence sound localization abilities in dogs.

Hearing in African pygmy hedgehogs (Atelerix albiventris): audiogram, sound localization, and ear anatomy.

The behavioral audiogram and sound localization performance, together with the middle and inner ear anatomy, were examined in African pygmy hedgehogs Atelerix albiventris. Their auditory sensitivity at 60dB SPL extended from 2 to 46kHz, revealing a relatively narrow hearing range of 4.6 octaves, with a best sensitivity of 21dB at 8kHz. Their noise-localization acuity around the midline (minimum audible angle) was 14°, matching the mean of terrestrial mammals. The African pygmy hedgehog was not able to localize low-frequency pure tones or a 3-kHz amplitude-modulated tone when forced to rely on the interaural phase-difference cue, a trait shared by at least nine other mammals. The middle ear of Atelerix has a configuration including an ectotympanic which is not fused to the surrounding bones, a substantial pars flaccida, a synostosed malleo-ectotympanic articulation and a 'microtype' malleus. The hearing and sound localization of A. albiventris is compared to that of a broad range of other mammals. It is shown that a malleus morphology like that of Atelerix, including a stiff articulation with the ectotympanic, is a consistent feature of other mammals that do not hear frequencies below 400Hz.

Acoustic Change Complex Evoked by Horizontal Sound Location Change in Young Adults With Normal Hearing.

Acoustic change complex (ACC) is a cortical auditory-evoked potential induced by a change of continuous sound stimulation. This study aimed to explore: (1) whether the change of horizontal sound location can elicit ACC; (2) the relationship between the change of sound location and the amplitude or latency of ACC; (3) the relationship between the behavioral measure of localization, minimum audible angle (MAA), and ACC. A total of 36 normal-hearing adults participated in this study. A 180° horizontal arc-shaped bracket with a 1.2 m radius was set in a sound field where participants sat at the center. MAA was measured in a two-alternative forced-choice setting. The objective electroencephalography recording of ACC was conducted with the location changed at four sets of positions, ±45°, ±15°, ±5°, and ±2°. The test stimulus was a 125–6,000 Hz broadband noise of 1 s at 60 ± 2 dB SPL with a 2 s interval. The N1′–P2′ amplitudes, N1′ latencies, and P2′ latencies of ACC under four positions were evaluated. The influence of electrode sites and the direction of sound position change on ACC waveform was analyzed with analysis of variance. Results suggested that (1) ACC can be elicited successfully by changing the horizontal sound location position. The elicitation rate of ACC increased with the increase of location change. (2) N1′–P2′ amplitude increased and N1′ and P2′ latencies decreased as the change of sound location increased. The effects of test angles on N1′–P2′ amplitude [F(1.91,238.1) = 97.172, p < 0.001], N1′ latency [F(1.78,221.90) = 96.96, p < 0.001], and P2′ latency [F(1.87,233.11) = 79.97, p < 0.001] showed a statistical significance. (3) The direction of sound location change had no significant effect on any of the ACC peak amplitudes or latencies. (4) Sound location discrimination threshold by the ACC test (97.0% elicitation rate at ±5°) was higher than MAA threshold (2.08 ± 0.5°). The current study results show that though the ACC thresholds are higher than the behavioral thresholds on MAA task, ACC can be used as an objective method to evaluate sound localization ability. This article discusses the implications of this research for clinical practice and evaluation of localization skills, especially for children.

Spatial acuity of the bottlenose dolphin (Tursiops truncatus) biosonar system with a bat and human comparison.

Horizontal angular resolution was measured in two bottlenose dolphins using a two-alternative forced-choice, biosonar target discrimination paradigm. The task required a stationary dolphin positioned in a hoop to discriminate two physical targets at a range of 4 m. The angle separating the targets was manipulated to estimate an angular discrimination threshold of 1.5°. In a second experiment, a similar two-target biosonar discrimination task was conducted with one free-swimming dolphin, to test whether its emission beam was a critical factor in discriminating the targets. The spatial separation between two targets was manipulated to measure a discrimination threshold of 6.7 cm. There was a relationship between differences in acoustic signals received at each target and the dolphin's performance. The results of the angular resolution experiment were in good agreement with measures of the minimum audible angle of both dolphins and humans and remarkably similar to measures of angular difference discrimination in echolocating dolphins, bats, and humans. The results suggest that horizontal auditory spatial acuity may be a common feature of the mammalian auditory system rather than a specialized feature exclusive to echolocating auditory predators.

Chickens have excellent sound localization ability.

The mechanisms of sound localization are actively debated, especially which cues are predominately used and why. Our study provides behavioural data in chickens (Gallus gallus) and relates these to estimates of the perceived physical cues. Sound localization acuity was quantified as the minimum audible angle (MAA) in azimuth. Pure-tone MAA was 12.3, 9.3, 8.9 and 14.5deg for frequencies of 500, 1000, 2000 and 4000 Hz, respectively. Broadband-noise MAA was 12.2deg, which indicates excellent behavioural acuity. We determined 'external cues' from head-related transfer functions of chickens. These were used to derive 'internal cues', taking into account published data on the effect of the coupled middle ears. Our estimates of the internal cues indicate that chickens likely relied on interaural time difference cues alone at low frequencies of 500 and 1000 Hz, whereas at 2000 and 4000 Hz, interaural level differences may be the dominant cue.

Investigating Bilateral Cochlear Implant Users' Localization of Amplitude- and Time-Panned Stimuli Produced Over a Limited Loudspeaker Arrangement.

The objective of this study was to investigate the localization ability of bilateral cochlear implant (BiCI) users for virtual sound sources produced over a limited loudspeaker arrangement. Ten BiCI users and 10 normal-hearing subjects participated in listening tests in which amplitude- and time-panned virtual sound sources were produced over a limited loudspeaker setup with varying azimuth angles. Three stimuli were utilized: speech, bandpassed pink noise between 20 Hz and 1 kHz, and bandpassed pink noise between 1 kHz and 8 kHz. The data were collected via a two-alternative forced-choice procedure and used to calculate the minimum audible angle (MAA) of each subject, which was subsequently compared to the results of previous studies in which real sound sources were employed. The median MAAs of the amplitude-panned speech, low-frequency pink noise, and high-frequency pink noise stimuli for the BiCI group were calculated to be 20°, 38°, and 12°, respectively. For the time-panned stimuli, the MAAs of the BiCI group for all three stimuli were calculated to be close to the upper limit of the listening test. The computed MAAs of the BiCI group for amplitude-panned speech were marginally larger than BiCI users' previously reported MAAs for real sound sources, whereas their computed MAAs for the time-panned stimuli were significantly larger. Subsequent statistical analysis indicated a statistically significant difference in the performances of the BiCI group in localizing the amplitude-panned sources and the time-panned sources. It follows that time-panning over limited loudspeaker arrangements may not be a useful clinical tool, whereas amplitude-panning utilizing such a setup may be further explored as such. Additionally, a comparison with the patient demographics indicated correlations between the results and the patients' age at time of diagnoses and the time passed between date of diagnosis and their implant surgeries.