Frontal Hemifield Research Articles

Effects of spatial configuration and fundamental frequency on speech intelligibility in multiple-talker conditions in the ipsilateral horizontal plane and median planea).

Spatial separation and fundamental frequency (F0) separation are effective cues for improving the intelligibility of target speech in multi-talker scenarios. Previous studies predominantly focused on spatial configurations within the frontal hemifield, overlooking the ipsilateral side and the entire median plane, where localization confusion often occurs. This study investigated the impact of spatial and F0 separation on intelligibility under the above-mentioned underexplored spatial configurations. The speech reception thresholds were measured through three experiments for scenarios involving two to four talkers, either in the ipsilateral horizontal plane or in the entire median plane, utilizing monotonized speech with varying F0s as stimuli. The results revealed that spatial separation in symmetrical positions (front-back symmetry in the ipsilateral horizontal plane or front-back, up-down symmetry in the median plane) contributes positively to intelligibility. Both target direction and relative target-masker separation influence the masking release attributed to spatial separation. As the number of talkers exceeds two, the masking release from spatial separation diminishes. Nevertheless, F0 separation remains as a remarkably effective cue and could even facilitate spatial separation in improving intelligibility. Further analysis indicated that current intelligibility models encounter difficulties in accurately predicting intelligibility in scenarios explored in this study.

Exploring attentive listening in noise through the just-follow conversation task

Listening to a conversation demands comprehension, attention and prediction. To better understand the impact of these demands, as well as how hearing aids might alleviate them, a conversational listening test should be realistic, repeatable and relatable (i.e., have interpretable units). A test that potentially satisfies these needs is the just-follow conversation (JFC) task, where the listener adjusts the signal level to where they can understand, with effort, the gist of what is being said in a background of noise. Fifty-four participants sat in the centre of a circular loudspeaker array and adjusted the overall level of one monologue, one dialogue, two monologues or two dialogues presented in the front hemifield to where they could just follow the speech four times per trial. Signals were presented in surrounding fixed-level café and same-spectrum noise backgrounds. Bilateral hearing-aid users adjusted aided and unaided; non-users repeated each condition to evaluate reliability. Results showed an increase in JFC for dialogues re monologues. Individual JFC SNRs correlated with SSQ12 speech subscale scores as well as pure-tone threshold averages. JFC reliability was comparable to more objective speech understanding measures, but it may not be suitable to capture perceived conversational benefits for more subtle changes in hearing-aid processing. [Work supported by the UK Medical Research Council Grant No. MR/X003620/1 & WS Audiology.]

Localizing concurrent sound sources with binaural microphones: A simulation study

The human auditory system can localize multiple sound sources using time, intensity, and frequency cues in the sound received by the two ears. Being able to spatially segregate the sources helps perception in a challenging condition when multiple sounds coexist. This study used model simulations to explore an algorithm for localizing multiple sources in azimuth with binaural (i.e., two) microphones. The algorithm relies on the “sparseness” property of daily signals in the time-frequency domain, and sound coming from different locations carrying unique spatial features will form clusters. Based on an interaural normalization procedure, the model generated spiral patterns for sound sources in the frontal hemifield. The model itself was created using broadband noise for better accuracy, because speech typically has sporadic energy at high frequencies. The model at an arbitrary frequency can be used to predict locations of speech and music that occurred alone or concurrently, and a classification algorithm was applied to measure the localization error. Under anechoic conditions, averaged errors in azimuth increased from 4.5° to 19° with RMS errors ranging from 6.4° to 26.7° as model frequency increased from 300 to 3000 Hz. The low-frequency model performance using short speech sound was notably better than the generalized cross-correlation model. Two types of room reverberations were then introduced to simulate difficult listening conditions. Model performance under reverberation was more resilient at low frequencies than at high frequencies. Overall, our study presented a spiral model for rapidly predicting horizontal locations of concurrent sound that is suitable for real-world scenarios.

The pinna enhances angular discrimination in the frontal hemifield.

Human sound localization in the horizontal dimension is thought to be dominated by binaural cues, particularly interaural time delays, because monaural localization in this dimension is relatively poor. Remaining ambiguities of front versus back and up versus down are distinguished by high-frequency spectral cues generated by the pinna. The experiments in this study show that this account is incomplete. Using binaural listening throughout, the pinna substantially enhanced horizontal discrimination in the frontal hemifield, making discrimination in front better than discrimination at the rear, particularly for directions away from the median plane. Eliminating acoustic effects of the pinna by acoustically bypassing them or low-pass filtering abolished the advantage at the front without affecting the rear. Acoustic measurements revealed a pinna-induced spectral prominence that shifts smoothly in frequency as sounds move from 0° to 90° azimuth. The improved performance is discussed in terms of the monaural and binaural changes induced by the pinna.

Effects of Bilateral Automatic Gain Control Synchronization in Cochlear Implants With and Without Head Movements: Sound Source Localization in the Frontal Hemifield.

Purpose For bilaterally implanted patients, the automatic gain control (AGC) in both left and right cochlear implant (CI) processors is usually neither linked nor synchronized. At high AGC compression ratios, this lack of coordination between the two processors can distort interaural level differences, the only useful interaural difference cue available to CI patients. This study assessed the improvement, if any, in the utility of interaural level differences for sound source localization in the frontal hemifield when AGCs were synchronized versus independent and when listeners were stationary versus allowed to move their heads. Method Sound source identification of broadband noise stimuli was tested for seven bilateral CI patients using 13 loudspeakers in the frontal hemifield, under conditions where AGCs were linked and unlinked. For half the conditions, patients remained stationary; in the other half, they were encouraged to rotate or reorient their heads within a range of approximately ± 30° during sound presentation. Results In general, those listeners who already localized reasonably well with independent AGCs gained the least from AGC synchronization, perhaps because there was less room for improvement. Those listeners who performed worst with independent AGCs gained the most from synchronization. All listeners performed as well or better with synchronization than without; however, intersubject variability was high. Head movements had little impact on the effectiveness of synchronization of AGCs. Conclusion Synchronization of AGCs offers one promising strategy for improving localization performance in the frontal hemifield for bilaterally implanted CI patients. Supplemental Material https://doi.org/10.23641/asha.14681412.

Application of Wireless Contralateral Routing of Signal Technology in Unilateral Cochlear Implant Users with Bilateral Profound Hearing Loss.

The aim of the study was to determine if contralateral routing of signal (CROS) technology results in improved hearing outcomes in unilateral cochlear implant (CI) patients and provides similar gains in speech perception in noise to traditional monaural listeners (MLs). The study is a prospective, within-subject repeated-measures experiment. Adult, English-speaking patients with bilateral severe-profound sensorineural hearing loss using an Advanced Bionics CI (n = 12) in one ear were enrolled for the study. Hearing performance in the monaural listening condition (CI only) was compared with the CROS-aided (unilateral CI + CROS) condition. Participants were tested for speech-in-noise performance using the Bamford-Kowal-Bench Speech-in-Noise™ test materials in the speech front/noise front (0 degrees/0 degrees azimuth), speech front/noise back (0 degrees/180 degrees azimuth), speech deaf ear/noise monaural ear (90 degrees/270 degrees azimuth), and speech monaural ear/noise deaf ear (90 degrees/270 degrees azimuth) configurations. Localization error was assessed using three custom stimuli consisting of 1/3 octave narrowband noises centered at 500 and 4000 Hz and a broadband speech stimulus. Localization stimuli were presented at random in the front hemifield by 19 speakers spatially separated by 10 degrees. Outcomes were compared with a previously described group of traditional MLs in the CROS-aided condition (normal hearing ear + CROS). All participants were tested acutely with no adaptation to the CROS device. Statistical analyses were performed using Wilcoxon signed rank tests for nonparametric data and paired sample. Statistical significance was set to p < 0.00625 after Bonferroni adjustment for eight tests. Significant benefit was observed from unaided to the CI + CROS-aided condition for listening in noise across most listening conditions with the greatest benefit observed in the speech deaf ear/noise monaural ear (90 degrees/270 degrees azimuth) condition (p < 0.0005). When compared with traditional MLs, no significant difference in decibel gain from the unaided to CROS-aided conditions was observed between participant groups. There was no improvement in localization ability in the CROS-aided condition for either participant group and no significant difference in performance between traditional MLs and unilateral CI listeners. These findings support that unilateral CI users are capable of achieving similar gains in speech perception to that of traditional MLs with wireless CROS. These results indicate that the use of wireless CROS stimulation in unilateral CI recipients provides increased benefit and an additional rehabilitative option for this population when bilateral implantation is not possible. The results suggest that noninvasive CROS solutions can successfully rehabilitate certain monaural listening deficits, provide improved hearing outcomes, and expand the reach of treatment in this population.

Measured localization performance at low stimulus levels with an adaptive tracking task

Sabin et al. (2005) measured localization accuracy for 250-ms broadband noises that varied in location along a spherical surface, and varied in stimulus level, from 0 to 60 dB above the detection threshold. They found that, for lower stimulus levels, responses tended to be biased towards lower elevations and to the front, and were more accurate at higher stimulus levels. Whereas Sabin et al. (2005) randomized level and location between trials, the current study fixed location and increased level until a correct response was given (or the 80 dB SPL limit was reached). Our initial level was 12.5 dB SPL, and our step size 2.5 dB SPL. Responses tended to be biased towards lower vertical elevations and to the front at low stimulus levels, and be more accurate at higher levels, consistent with Sabin et al. The percentage of front/back errors was generally greater for locations in the rear hemifield relative to those in the frontal hemifield, and for locations at higher elevations relative to locations at lower elevations. Audibility analysis, performed using a KEMAR manikin, showed that localization errors tended to decrease the most when audible bandwidth increased at the acoustically better ear.

Exploring the relationship between sound localization and individual use of spectral and temporal cues among hearing-impaired listeners

When locating a sound source, listeners are expected to rely more on spectral cues for elevation and temporal cues for azimuth. This work investigates whether sound localization ability can be predicted by individually-measured use of spectral and temporal cues. Participants were older adults with sensorineural hearing loss. Sound sources for the localization task were created in a virtual room, using a mix of virtual and physical loudspeakers in the front hemifield of the listener. Sources were distributed evenly between ±90 deg azimuth and ±20 deg elevation. The signal was a 1 s broadband 4-Hz amplitude-modulated noise. To assess the relationship between localization ability and use of spectral and temporal cues, listeners performed cue weighting and cue discrimination tasks. In the cue weighting task, listeners identified synthetic speech sounds that varied spectro-temporally. The cue discrimination task measured the smallest detectable difference in either spectral or temporal information among the same set of ambiguous speech sounds. Together, these tasks form a cue profile which identifies whether the listener relies to a greater extent on temporal or spectral cues. Older hearing-impaired listeners varied in their ability to localize sounds. Localization results will be discussed in the context of cue profiles and audiometry. [Work supported by NIH.]

Sound localization behavior in Drosophilamelanogaster depends on inter-antenna vibration amplitude comparisons.

Drosophila melanogaster hear with their antennae: sound evokes vibration of the distal antennal segment, and this vibration is transduced by specialized mechanoreceptor cells. The left and right antennae vibrate preferentially in response to sounds arising from different azimuthal angles. Therefore, by comparing signals from the two antennae, it should be possible to obtain information about the azimuthal angle of a sound source. However, behavioral evidence of sound localization has not been reported in Drosophila Here, we show that walking D. melanogaster do indeed turn in response to lateralized sounds. We confirm that this behavior is evoked by vibrations of the distal antennal segment. The rule for turning is different for sounds arriving from different locations: flies turn toward sounds in their front hemifield, but they turn away from sounds in their rear hemifield, and they do not turn at all in response to sounds from 90 or -90deg. All of these findings can be explained by a simple rule: the fly steers away from the antenna with the larger vibration amplitude. Finally, we show that these behaviors generalize to sound stimuli with diverse spectro-temporal features, and that these behaviors are found in both sexes. Our findings demonstrate the behavioral relevance of the antenna's directional tuning properties. They also pave the way for investigating the neural implementation of sound localization, as well as the potential roles of sound-guided steering in courtship and exploration.

Learning to localise weakly-informative sound spectra with and without feedback

How the human auditory system learns to map complex pinna-induced spectral-shape cues onto veridical estimates of sound-source elevation in the median plane is still unclear. Earlier studies demonstrated considerable sound-localisation plasticity after applying pinna moulds, and to altered vision. Several factors may contribute to auditory spatial learning, like visual or motor feedback, or updated priors. We here induced perceptual learning for sounds with degraded spectral content, having weak, but consistent, elevation-dependent cues, as demonstrated by low-gain stimulus-response relations. During training, we provided visual feedback for only six targets in the midsagittal plane, to which listeners gradually improved their response accuracy. Interestingly, listeners’ performance also improved without visual feedback, albeit less strongly. Post-training results showed generalised improved response behaviour, also to non-trained locations and acoustic spectra, presented throughout the two-dimensional frontal hemifield. We argue that the auditory system learns to reweigh contributions from low-informative spectral bands to update its prior elevation estimates, and explain our results with a neuro-computational model.

Exploiting Deep Neural Networks and Head Movements for Robust Binaural Localization of Multiple Sources in Reverberant Environments

This paper presents a novel machine-hearing system that exploits deep neural networks (DNNs) and head movements for robust binaural localisation of multiple sources in reverberant environments. DNNs are used to learn the relationship between the source azimuth and binaural cues, consisting of the complete cross-correlation function (CCF) and interaural level differences (ILDs). In contrast to many previous binaural hearing systems, the proposed approach is not restricted to localisation of sound sources in the frontal hemifield. Due to the similarity of binaural cues in the frontal and rear hemifields, front-back confusions often occur. To address this, a head movement strategy is incorporated in the localisation model to help reduce the front-back errors. The proposed DNN system is compared to a Gaussian mixture model (GMM) based system that employs interaural time differences (ITDs) and ILDs as localisation features. Our experiments show that the DNN is able to exploit information in the CCF that is not available in the ITD cue, which together with head movements substantially improves localisation accuracies under challenging acoustic scenarios in which multiple talkers and room reverberation are present.

Robust Speech Dereverberation With a Neural Network-Based Post-Filter That Exploits Multi-Conditional Training of Binaural Cues

This study presents an algorithm for binaural speech dereverberation based on the supervised learning of short-term binaural cues. The proposed system combined a delay-and-sum beamformer with a neural network-based post-filter that attenuated reverberant components in individual time-frequency units. A multi-conditional training procedure was used to simulate the uncertainties of short-term binaural cues in response to room reverberation by mixing the direct part of head related impulse responses (HRIRs) with diffuse noise. Despite being trained with only anechoic HRIRs, the proposed dereverberation algorithm was tested in a variety of reverberant environments and achieved considerable improvements relative to a coherence-based approach in terms of three objective metrics reflecting speech quality and speech intelligibility. Moreover, a systematic evaluation showed that the proposed system generalized very well to a wide range of acoustic conditions, including various measured binaural room impulse responses reflecting different reverberation times, azimuth positions spanning the entire frontal hemifield, various source-receiver distances as well as different artificial heads.

Behavioural benefits of multisensory processing in ferrets.

Enhanced detection and discrimination, along with faster reaction times, are the most typical behavioural manifestations of the brain's capacity to integrate multisensory signals arising from the same object. In this study, we examined whether multisensory behavioural gains are observable across different components of the localization response that are potentially under the command of distinct brain regions. We measured the ability of ferrets to localize unisensory (auditory or visual) and spatiotemporally coincident auditory–visual stimuli of different durations that were presented from one of seven locations spanning the frontal hemifield. During the localization task, we recorded the head movements made following stimulus presentation, as a metric for assessing the initial orienting response of the ferrets, as well as the subsequent choice of which target location to approach to receive a reward. Head‐orienting responses to auditory–visual stimuli were more accurate and faster than those made to visual but not auditory targets, suggesting that these movements were guided principally by sound alone. In contrast, approach‐to‐target localization responses were more accurate and faster to spatially congruent auditory–visual stimuli throughout the frontal hemifield than to either visual or auditory stimuli alone. Race model inequality analysis of head‐orienting reaction times and approach‐to‐target response times indicates that different processes, probability summation and neural integration, respectively, are likely to be responsible for the effects of multisensory stimulation on these two measures of localization behaviour.

Localization-in-noise and binaural medial olivocochlear functioning in children and young adults.

Children as young as 5 yr old localize sounds as accurately as adults in quiet in the frontal hemifield. However, children's ability to localize in noise and in the front/back (F/B) dimension are scantily studied. To address this, the first part of this study investigated localization-in-noise ability of children vs young adults in two maskers: broadband noise (BBN) and speech-babble (SB) at three signal-to-noise ratios: -12, -6, and 0 dB. In the second part, relationship between binaural medial olivocochlear system (MOC) function and localization-in-noise was investigated. In both studies, 21 children and 21 young adults participated. Results indicate, while children are able to differentiate sounds arriving in the F/B dimension on par with adults in quiet and in BBN, larger differences were found for SB. Accuracy of children's localization in noise (for both maskers) in the lateral plane was also poorer than adults'. Significant differences in binaural MOC interaction (mBIC; the difference between the sum of two monaural- and binaural-MOC strength) between adults and children were also found. For reasons which are not clear, adult F/B localization in BBN correlates better with mBIC while children's F/B localization in SB correlated better with binaural MOC strength.

Behavior and modeling of two-dimensional precedence effect in head-unrestrained cats.

The precedence effect (PE) is an auditory illusion that occurs when listeners localize nearly coincident and similar sounds from different spatial locations, such as a direct sound and its echo. It has mostly been studied in humans and animals with immobile heads in the horizontal plane; speaker pairs were often symmetrically located in the frontal hemifield. The present study examined the PE in head-unrestrained cats for a variety of paired-sound conditions along the horizontal, vertical, and diagonal axes. Cats were trained with operant conditioning to direct their gaze to the perceived sound location. Stereotypical PE-like behaviors were observed for speaker pairs placed in azimuth or diagonally in the frontal hemifield as the interstimulus delay was varied. For speaker pairs in the median sagittal plane, no clear PE-like behavior occurred. Interestingly, when speakers were placed diagonally in front of the cat, certain PE-like behavior emerged along the vertical dimension. However, PE-like behavior was not observed when both speakers were located in the left hemifield. A Hodgkin-Huxley model was used to simulate responses of neurons in the medial superior olive (MSO) to sound pairs in azimuth. The novel simulation incorporated a low-threshold potassium current and frequency mismatches to generate internal delays. The model exhibited distinct PE-like behavior, such as summing localization and localization dominance. The simulation indicated that certain encoding of the PE could have occurred before information reaches the inferior colliculus, and MSO neurons with binaural inputs having mismatched characteristic frequencies may play an important role.

Single-sided deafness and directional hearing: contribution of spectral cues and high-frequency hearing loss in the hearing ear.

Direction-specific interactions of sound waves with the head, torso, and pinna provide unique spectral-shape cues that are used for the localization of sounds in the vertical plane, whereas horizontal sound localization is based primarily on the processing of binaural acoustic differences in arrival time (interaural time differences, or ITDs) and sound level (interaural level differences, or ILDs). Because the binaural sound-localization cues are absent in listeners with total single-sided deafness (SSD), their ability to localize sound is heavily impaired. However, some studies have reported that SSD listeners are able, to some extent, to localize sound sources in azimuth, although the underlying mechanisms used for localization are unclear. To investigate whether SSD listeners rely on monaural pinna-induced spectral-shape cues of their hearing ear for directional hearing, we investigated localization performance for low-pass filtered (LP, <1.5 kHz), high-pass filtered (HP, >3kHz), and broadband (BB, 0.5–20 kHz) noises in the two-dimensional frontal hemifield. We tested whether localization performance of SSD listeners further deteriorated when the pinna cavities of their hearing ear were filled with a mold that disrupted their spectral-shape cues. To remove the potential use of perceived sound level as an invalid azimuth cue, we randomly varied stimulus presentation levels over a broad range (45–65 dB SPL). Several listeners with SSD could localize HP and BB sound sources in the horizontal plane, but inter-subject variability was considerable. Localization performance of these listeners strongly reduced after diminishing of their spectral pinna-cues. We further show that inter-subject variability of SSD can be explained to a large extent by the severity of high-frequency hearing loss in their hearing ear.

Effects of forward masking on sound localization in cats: basic findings with broadband maskers

Forward masking is traditionally measured with a detection task in which the addition of a preceding masking sound results in an increased signal-detection threshold. Little is known about the influence of forward masking on localization of free-field sound for human or animal subjects. Here we recorded gaze shifts of two head-unrestrained cats during localization using a search-coil technique. A broadband (BB) noise masker was presented straight ahead. A brief signal could come from 1 of the 17 speaker locations in the frontal hemifield. The signal was either a BB or a band-limited (BL) noise. For BB targets, the presence of the forward masker reduced localization accuracy at almost all target levels (20 to 80 dB SPL) along both horizontal and vertical dimensions. Temporal decay of masking was observed when a 15-ms interstimulus gap was added between the end of the masker and the beginning of the target. A large effect of forward masking was also observed for BL targets with low (0.2-2 kHz) and mid (2-7 kHz) frequencies, indicating that the interaural timing cue is susceptible to forward masking. Except at low sound levels, a small or little effect was observed for high-frequency (7-15 kHz) targets, indicating that the interaural level and the spectral cues in that frequency range remained relatively robust. Our findings suggest that different localization mechanisms can operate independently in a complex listening environment.

Sound source localization of filtered noises by listeners with normal hearing: A statistical analysis

Several measures of sound source localization performance of 45 listeners with normal hearing were obtained when loudspeakers were in the front hemifield. Localization performance was not statistically affected by filtering the 200-ms, 2-octave or wider noise bursts (125 to 500, 1500 to 6000, and 125 to 6000 Hz wide noise bursts). This implies that sound source localization performance for noise stimuli is not differentially affected by which interaural cue (interaural time or level difference) a listener with normal hearing uses for sound source localization, at least for relatively broadband signals. This sound source localization task suggests that listeners with normal hearing perform with high reliability/repeatability, little response bias, and with performance measures that are normally distributed with a mean root-mean-square error of 6.2° and a standard deviation of 1.79°.

Age-related Hearing Loss and Ear Morphology Affect Vertical but not Horizontal Sound-Localization Performance

Several studies have attributed deterioration of sound localization in the horizontal (azimuth) and vertical (elevation) planes to an age-related decline in binaural processing and high-frequency hearing loss (HFHL). The latter might underlie decreased elevation performance of older adults. However, as the pinnae keep growing throughout life, we hypothesized that larger ears might enable older adults to localize sounds in elevation on the basis of lower frequencies, thus (partially) compensating their HFHL. In addition, it is not clear whether sound localization has already matured at a very young age, when the body is still growing, and the binaural and monaural sound-localization cues change accordingly. The present study investigated sound-localization performance of children (7-11 years), young adults (20-34 years), and older adults (63-80 years) under open-loop conditions in the two-dimensional frontal hemifield. We studied the effect of age-related hearing loss and ear size on localization responses to brief broadband sound bursts with different bandwidths. We found similar localization abilities in azimuth for all listeners, including the older adults with HFHL. Sound localization in elevation for the children and young adult listeners with smaller ears improved when stimuli contained frequencies above 7 kHz. Subjects with larger ears could also judge the elevation of sound sources restricted to lower frequency content. Despite increasing ear size, sound localization in elevation deteriorated in older adults with HFHL. We conclude that the binaural localization cues are successfully used well into later stages of life, but that pinna growth cannot compensate the more profound HFHL with age.

A cocktail party model of spatial release from masking by both noise and speech interferers

A mathematical formula for estimating spatial release from masking (SRM) in a cocktail party environment would be useful as a simpler alternative to computationally intensive algorithms and may enhance understanding of underlying mechanisms. The experiment presented herein was designed to provide a strong test of a model that divides SRM into contributions of asymmetry and angular separation [Bronkhorst (2000). Acustica 86, 117-128] and to examine whether that model can be extended to include speech maskers. Across masker types the contribution to SRM of angular separation of maskers from the target was found to grow at a diminishing rate as angular separation increased within the frontal hemifield, contrary to predictions of the model. Speech maskers differed from noise maskers in the overall magnitude of SRM and in the contribution of angular separation (both greater for speech). These results were used to develop a modified model that achieved good fits to data for noise maskers (ρ=0.93) and for speech maskers (ρ=0.94) while using the same functions to describe separation and asymmetry components of SRM for both masker types. These findings suggest that this approach can be used to accurately model SRM for speech maskers in addition to primarily "energetic" noise maskers.