Six Degrees of Auditory Spatial Separation

Spatial rehabilitation using virtual auditory space training paradigm in individuals with sensorineural hearing impairment

Purpose

The present study aimed to quantify the effects of spatial training using virtual sources on a battery of spatial acuity measures in listeners with sensorineural hearing impairment (SNHI).

Methods

An intervention-based time-series comparison design involving 82 participants divided into three groups was adopted. Group I (n = 27, SNHI-spatially trained) and group II (n = 25, SNHI-untrained) consisted of SNHI listeners, while group III (n = 30) had listeners with normal hearing (NH). The study was conducted in three phases. In the pre-training phase, all the participants underwent a comprehensive assessment of their spatial processing abilities using a battery of tests including spatial acuity in free-field and closed-field scenarios, tests for binaural processing abilities (interaural time threshold [ITD] and level difference threshold [ILD]), and subjective ratings. While spatial acuity in the free field was assessed using a loudspeaker-based localization test, the closed-field source identification test was performed using virtual stimuli delivered through headphones. The ITD and ILD thresholds were obtained using a MATLAB psychoacoustic toolbox, while the participant ratings on the spatial subsection of speech, spatial, and qualities questionnaire in Kannada were used for the subjective ratings. Group I listeners underwent virtual auditory spatial training (VAST), following pre-evaluation assessments. All tests were re-administered on the group I listeners halfway through training (mid-training evaluation phase) and after training completion (post-training evaluation phase), whereas group II underwent these tests without any training at the same time intervals.

Results and discussion

Statistical analysis showed the main effect of groups in all tests at the pre-training evaluation phase, with post hoc comparisons that revealed group equivalency in spatial performance of both SNHI groups (groups I and II). The effect of VAST in group I was evident on all the tests, with the localization test showing the highest predictive power for capturing VAST-related changes on Fischer discriminant analysis (FDA). In contrast, group II demonstrated no changes in spatial acuity across timelines of measurements. FDA revealed increased errors in the categorization of NH as SNHI-trained at post-training evaluation compared to pre-training evaluation, as the spatial performance of the latter improved with VAST in the post-training phase.

Conclusion

The study demonstrated positive outcomes of spatial training using VAST in listeners with SNHI. The utility of this training program can be extended to other clinical population with spatial auditory processing deficits such as auditory neuropathy spectrum disorder, cochlear implants, central auditory processing disorders etc.

Effects of interaural decoherence on sensitivity to interaural level differences across frequency

The interaural level difference (ILD) is a robust indicator of sound source azimuth, and human ILD sensitivity persists under conditions that degrade normally-dominant interaural time difference (ITD) cues. Nonetheless, ILD sensitivity varies somewhat with both stimulus frequency and interaural correlation (coherence). To further investigate the combined binaural perceptual influence of these variables, the present study assessed ILD sensitivity at frequencies 250-4000 Hz using stimuli of varied interaural correlation. In the first of two experiments, ILD discrimination thresholds were modestly elevated, and subjective lateralization slightly reduced, for both half-correlated and uncorrelated narrowband noise tokens relative to correlated tokens. Different from thresholds in the correlated condition, which were worst at 1000 Hz [Grantham, D.W. (1984). J. Acoust. Soc. Am. 75, 1191-1194], thresholds in the decorrelated conditions were independent of frequency. However, intrinsic envelope fluctuations in narrowband stimuli caused moment-to-moment variation of the nominal ILD, complicating interpretation of measured thresholds. Thus, a second experiment employed low-fluctuation noise tokens, revealing a clear effect of interaural decoherence per se that was strongly frequency-dependent, decreasing in magnitude from low to high frequencies. Measurements are consistent with known integration times in ILD-sensitive neurons and also suggest persistent influences of covert ITD cues in putative "ILD" tasks.

Usability of Individualized Head-Related Transfer Functions in Virtual Reality: Empirical Study With Perceptual Attributes in Sagittal Plane Sound Localization

BACKGROUND

In order to present virtual sound sources via headphones spatially, head-related transfer functions (HRTFs) can be applied to audio signals. In this so-called binaural virtual acoustics, the spatial perception may be degraded if the HRTFs deviate from the true HRTFs of the listener.

OBJECTIVE

In this study, participants wearing virtual reality (VR) headsets performed a listening test on the 3D audio perception of virtual audiovisual scenes, thus enabling us to investigate the necessity and influence of the individualization of HRTFs. Two hypotheses were investigated: first, general HRTFs lead to limitations of 3D audio perception in VR and second, the localization model for stationary localization errors is transferable to nonindividualized HRTFs in more complex environments such as VR.

METHODS

For the evaluation, 39 subjects rated individualized and nonindividualized HRTFs in an audiovisual virtual scene on the basis of 5 perceptual qualities: localizability, front-back position, externalization, tone color, and realism. The VR listening experiment consisted of 2 tests: in the first test, subjects evaluated their own and the general HRTF from the Massachusetts Institute of Technology Knowles Electronics Manikin for Acoustic Research database and in the second test, their own and 2 other nonindividualized HRTFs from the Acoustics Research Institute HRTF database. For the experiment, 2 subject-specific, nonindividualized HRTFs with a minimal and maximal localization error deviation were selected according to the localization model in sagittal planes.

RESULTS

With the Wilcoxon signed-rank test for the first test, analysis of variance for the second test, and a sample size of 78, the results were significant in all perceptual qualities, except for the front-back position between own and minimal deviant nonindividualized HRTF (P=.06).

CONCLUSIONS

Both hypotheses have been accepted. Sounds filtered by individualized HRTFs are considered easier to localize, easier to externalize, more natural in timbre, and thus more realistic compared to sounds filtered by nonindividualized HRTFs.

Neurons in primary auditory cortex represent sound source location in a cue-invariant manner

Auditory cortex is required for sound localisation, but how neural firing in auditory cortex underlies our perception of sound sources in space remains unclear. Specifically, whether neurons in auditory cortex represent spatial cues or an integrated representation of auditory space across cues is not known. Here, we measured the spatial receptive fields of neurons in primary auditory cortex (A1) while ferrets performed a relative localisation task. Manipulating the availability of binaural and spectral localisation cues had little impact on ferrets’ performance, or on neural spatial tuning. A subpopulation of neurons encoded spatial position consistently across localisation cue type. Furthermore, neural firing pattern decoders outperformed two-channel model decoders using population activity. Together, these observations suggest that A1 encodes the location of sound sources, as opposed to spatial cue values.

The brain's auditory cortex is involved not just in detection of sounds, but also in localizing them. Here, the authors show that neurons in ferret primary auditory cortex (A1) encode the location of sound sources, as opposed to merely reflecting spatial cues.

Spatial variation in signal and sensory precision both constrain auditory acuity at high frequencies

Sensory performance is constrained by the information in the stimulus and the precision of the involved sensory system(s). Auditory spatial acuity is robust across a broad range of sound frequencies and source locations, but declines at eccentric lateral angles. The basis of such variation is not fully understood. Low-frequency auditory spatial acuity is mediated by sensitivity to interaural time difference (ITD) cues. While low-frequency spatial acuity varies across azimuth and some physiological models predict strong medial bias in the precision of ITD sensitivity, human psychophysical ITD sensitivity appears to vary only slightly with reference ITD magnitude. Correspondingly, recent analyses suggest that spatial variation in human low-frequency acuity is well-accounted for by acoustic factors alone. Here we examine the matter of high-frequency auditory acuity, which is mediated by sensitivity to interaural level difference (ILD) cues. Using two different psychophysical tasks in human subjects, we demonstrate decreasing ILD acuity with increasing ILD magnitude. We then demonstrate that the multiplicative combination of spatially variant sensory precision and physical cue information (local slope of the ILD cue) provides improved prediction of classic high-frequency spatial acuity data. Finally, we consider correlates of magnitude dependent acuity in neurons that are sensitive to ILDs.

Multisensory stimuli improve relative localisation judgments compared to unisensory auditory or visual stimuli

Observers performed a relative localisation task in which they reported whether the second of two sequentially presented signals occurred to the left or right of the first. Stimuli were detectability-matched auditory, visual, or auditory-visual signals and the goal was to compare changes in performance with eccentricity across modalities. Visual performance was superior to auditory at the midline, but inferior in the periphery, while auditory-visual performance exceeded both at all locations. No such advantage was seen when performance for auditory-only trials was contrasted with trials in which the first stimulus was auditory-visual and the second auditory only.

Adaptive spatial filtering improves speech reception in noise while preserving binaural cues

Hearing loss greatly reduces an individual's ability to comprehend speech in the presence of background noise. Over the past decades, numerous signal-processing algorithms have been developed to improve speech reception in these situations for cochlear implant and hearing aid users. One challenge is to reduce background noise while not introducing interaural distortion that would degrade binaural hearing. The present study evaluates a noise reduction algorithm, referred to as binaural Fennec, that was designed to improve speech reception in background noise while preserving binaural cues. Speech reception thresholds were measured for normal-hearing listeners in a simulated environment with target speech generated in front of the listener and background noise originating 90° to the right of the listener. Lateralization thresholds were also measured in the presence of background noise. These measures were conducted in anechoic and reverberant environments. Results indicate that the algorithm improved speech reception thresholds, even in highly reverberant environments. Results indicate that the algorithm also improved lateralization thresholds for the anechoic environment while not affecting lateralization thresholds for the reverberant environments. These results provide clear evidence that this algorithm can improve speech reception in background noise while preserving binaural cues used to lateralize sound.