The Rapid Decline in Interaural-Time-Difference Sensitivity for Pure Tones Can Be Explained by Peripheral Filtering.

PURPOSE: The interaural time difference (ITD) is a primary horizontal-plane sound localization cue computed in the auditory brainstem. ITDs are accessible in the temporal fine structure of pure tones with a frequency of no higher than about 1400 Hz. How listeners' ITD sensitivity transitions from very best sensitivity near 700 Hz to impossible to detect within 1 octave currently lacks a fully compelling physiological explanation. Here, it was hypothesized that the rapid decline in ITD sensitivity is dictated not by a central neural limitation but by initial peripheral sound encoding, specifically, the low-frequency (apical) portion of the cochlear excitation pattern produced by a pure tone. METHODS: ITD sensitivity was measured in 16 normal-hearing listeners as a joint function of frequency (900-1500 Hz) and level (10-50 dB sensation level). RESULTS: Performance decreased with increasing frequency and decreasing sound level. The slope of performance decline was 90 dB/octave, consistent with the low-frequency slope of the cochlear excitation pattern. CONCLUSION: Fine-structure ITD sensitivity near 1400 Hz may be conveyed primarily by "off-frequency" activation of neurons tuned to lower frequencies near 700 Hz. Physiologically, this could be realized by having neurons sensitive to fine-structure ITD up to only about 700 Hz. A more extreme model would have only a single narrow channel near 700 Hz that conveys fine-structure ITDs. Such a model is a major simplification and departure from the classic formulation of the binaural display, which consists of a matrix of neurons tuned to a wide range of relevant frequencies and ITDs.

Effects of Stimulus Type on 16-kHz Detection Thresholds.

OBJECTIVES: Audiometric testing typically does not include frequencies above 8 kHz. However, recent research suggests that extended high-frequency (EHF) sensitivity could affect hearing in natural communication environments. Clinical assessment of hearing often employs pure tones and frequency-modulated (FM) tones interchangeably regardless of frequency. The present study was designed to evaluate how the stimulus chosen to measure EHF thresholds affects estimates of hearing sensitivity. DESIGN: The first experiment used standard audiometric procedures to measure 8- and 16-kHz thresholds for 5- to 28-year olds with normal hearing in the standard audiometric range (250 to 8000 Hz). Stimuli were steady tones, pulsed tones, and FM tones. The second experiment tested 18- to 28-year olds with normal hearing in the standard audiometric range using psychophysical procedures to evaluate how changes in sensitivity as a function of frequency affect detection of stimuli that differ with respect to bandwidth, including bands of noise. Thresholds were measured using steady tones, pulsed tones, FM tones, narrow bands of noise, and one-third-octave bands of noise at a range of center frequencies in one ear. RESULTS: In experiment 1, thresholds improved with increasing age at 8 kHz and worsened with increasing age at 16 kHz. Thresholds for individual participants were relatively similar for steady, pulsed, and FM tones at 8 kHz. At 16 kHz, mean thresholds were approximately 5 dB lower for FM tones than for steady or pulsed tones. This stimulus effect did not differ as a function of age. Experiment 2 replicated this greater stimulus effect at 16 kHz than at 8 kHz and showed that the slope of the audibility curve accounted for these effects. CONCLUSIONS: Contrary to prior expectations, there was no evidence that the choice of stimulus type affected school-age children more than adults. For individual participants, audiometric thresholds at 16 kHz were as much as 20 dB lower for FM tones than for steady tones. Threshold differences across stimuli at 16 kHz were predicted by differences in audibility across frequency, which can vary markedly between listeners. These results highlight the importance of considering spectral width of the stimulus used to evaluate EHF thresholds.

The rapid decline in interaural-time-difference sensitivity for pure tones can be explained by peripheral filtering.

Purpose: The interaural time difference (ITD) is a primary horizontal-plane sound localization cue computed in the auditory brainstem. ITDs are accessible in the temporal fine structure of pure tones with a frequency of no higher than about 1400 Hz. Explaining how listeners' ITD sensitivity transitions from very best sensitivity near 700 Hz to impossible to detect within 1 octave currently lacks a fully compelling physiological explanation. Here, it was hypothesized that the rapid decline in ITD sensitivity is dictated not by a central neural limitation but by initial peripheral sound encoding, specifically, the low-frequency (apical) edge of the cochlear excitation pattern produced by a pure tone. Methods: ITD sensitivity was measured in 16 normal-hearing listeners as a joint function of frequency (900-1500 Hz) and level (10-50 dB sensation level). Results: Performance decreased with increasing frequency and decreasing sound level. The slope of performance decline was 90 dB/octave, consistent with the low-frequency slope of the cochlear excitation pattern. Conclusion: Fine-structure ITD sensitivity near 1400 Hz may be conveyed primarily by "off-frequency" activation of neurons tuned to lower frequencies near 700 Hz. Physiologically, this could be realized by having neurons sensitive to fine-structure ITD up to only about 700 Hz. A more extreme model would have only a single narrow channel near 700 Hz that conveys fine-structure ITDs. Such a model is a major simplification and departure from the classic formulation of the binaural display, which consists of a matrix of neurons tuned to a wide range of relevant frequencies and ITDs.

Spectral weighting functions for localization of complex sound. II. The effect of competing noise.

Spectral weighting of sound localization cues was measured in the presence of three levels of competing noise presented in the free field. Target stimuli were complex tones containing seven tonal components, presented from an ∼120° range of frontal azimuths. Competitors were two independent Gaussian noises presented from 90° left and right azimuth at one of three levels yielding +9, 0, and -6 dB signal-to-noise ratio. Results revealed the greatest perceptual weight for components within the interaural time difference (ITD) "dominance region," which was found previously to peak around the 800-Hz component in quiet [Folkerts and Stecker (2022) J. Acoust. Soc. Am. 151, 3409-3425]. Here, peak weights were shifted toward lower-frequency components (i.e., 400 Hz) in all competing noise conditions. These results contradict the hypothesis of a shift in the peak weights toward higher frequencies based on previous behavioral localization performance in competing noise but are consistent with binaural cue sensitivity, availability, and reliability; measured low-frequency ITD cues within the dominance region were least disrupted by the presence of competing noise.

Sound Level Changes the Auditory Cortical Activation Detected with Functional Near-Infrared Spectroscopy.

BACKGROUND: Functional near-infrared spectroscopy (fNIRS) is a viable non-invasive technique for functional neuroimaging in the cochlear implant (CI) population; however, the effects of acoustic stimulus features on the fNIRS signal have not been thoroughly examined. This study examined the effect of stimulus level on fNIRS responses in adults with normal hearing or bilateral CIs. We hypothesized that fNIRS responses would correlate with both stimulus level and subjective loudness ratings, but that the correlation would be weaker with CIs due to the compression of acoustic input to electric output. METHODS: Thirteen adults with bilateral CIs and 16 with normal hearing (NH) completed the study. Signal-correlated noise, a speech-shaped noise modulated by the temporal envelope of speech stimuli, was used to determine the effect of stimulus level in an unintelligible speech-like stimulus between the range of soft to loud speech. Cortical activity in the left hemisphere was recorded. RESULTS: Results indicated a positive correlation of cortical activation in the left superior temporal gyrus with stimulus level in both NH and CI listeners with an additional correlation between cortical activity and perceived loudness for the CI group. The results are consistent with the literature and our hypothesis. CONCLUSIONS: These results support the potential of fNIRS to examine auditory stimulus level effects at a group level and the importance of controlling for stimulus level and loudness in speech recognition studies. Further research is needed to better understand cortical activation patterns for speech recognition as a function of both stimulus presentation level and perceived loudness.

Effects of Gamification on Assessment of Spatial Release From Masking.

PURPOSE: Difficulty understanding speech in noise is a common communication problem. Clinical tests of speech in noise differ considerably from real-world listening and offer patients limited intrinsic motivation to perform well. In order to design a test that captures motivational aspects of real-world communication, this study investigated effects of gamification, or the inclusion of game elements, on a laboratory spatial release from masking test. METHOD: Fifty-four younger adults with normal hearing completed a traditional laboratory and a gamified test of spatial release from masking in counterbalanced order. Masker level adapted based on performance, with the traditional test ending after 10 reversals and the gamified test ending when participants solved a visual puzzle. Target-to-masker ratio thresholds (TMRs) with colocated maskers, separated maskers, and estimates of spatial release were calculated after the 10th reversal for both tests and from the last six reversals of the adaptive track from the gamified test. RESULTS: Thresholds calculated from the 10th reversal indicated no significant differences between the traditional and gamified tests. A learning effect was observed with spatially separated maskers, such that TMRs were better for the second test than the first, regardless of test order. Thresholds calculated from the last six reversals of the gamified test indicated better TMRs in the separated condition compared to the traditional test. CONCLUSIONS: Adding gamified elements to a traditional test of spatial release from masking did not negatively affect test validity or estimates of spatial release. Participants were willing to continue playing the gamified test for an average of 30.2 reversals of the adaptive track. For some listeners, performance in the separated condition continued to improve after the 10th reversal, leading to better TMRs and greater spatial release from masking at the end of the gamified test compared to the traditional test. SUPPLEMENTAL MATERIAL: https://doi.org/10.23641/asha.22028789.

FORUM: Remote testing for psychological and physiological acoustics.

Acoustics research involving human participants typically takes place in specialized laboratory settings. Listening studies, for example, may present controlled sounds using calibrated transducers in sound-attenuating or anechoic chambers. In contrast, remote testing takes place outside of the laboratory in everyday settings (e.g., participants' homes). Remote testing could provide greater access to participants, larger sample sizes, and opportunities to characterize performance in typical listening environments at the cost of reduced control of environmental conditions, less precise calibration, and inconsistency in attentional state and/or response behaviors from relatively smaller sample sizes and unintuitive experimental tasks. The Acoustical Society of America Technical Committee on Psychological and Physiological Acoustics launched the Task Force on Remote Testing (https://tcppasa.org/remotetesting/) in May 2020 with goals of surveying approaches and platforms available to support remote testing and identifying challenges and considerations for prospective investigators. The results of this task force survey were made available online in the form of a set of Wiki pages and summarized in this report. This report outlines the state-of-the-art of remote testing in auditory-related research as of August 2021, which is based on the Wiki and a literature search of papers published in this area since 2020, and provides three case studies to demonstrate feasibility during practice.

Spectral weighting functions for lateralization and localization of complex sound.

Perceptual weighting of sound localization cues across spectral components was measured over headphones [experiment (expt.) 1] and in the free field (expt. 2) and quantified in the form of spectral weighting functions (SWFs). Stimuli consisted of five complex sounds (conditions), each containing seven frequency components. Participants judged the spatial position of the stimuli with spatial cues varying across frequency components. In separate experiments, free-field stimuli were presented anechoically (expt. 2), in the presence of simulated reverberation (expt. 3), or with stimuli varying in level either corrected for equal loudness (expt. 4.1) or sloped by ±6 dB per component (expt. 4.2). Overall results revealed greatest weight in the vicinity of 800 Hz, for both localization and interaural time difference (ITD)-based lateralization, although specific features of the SWFs did vary across stimulus conditions. The shape of the SWF follows the pattern of ITD sensitivity across frequency and is consistent with previous descriptions of an ITD "dominance region" peaking around 600-800 Hz. The close similarity of free field and ITD-based SWFs is further consistent with the hypothesized dominant role of low-frequency ITD in localization of broadband sounds. Other conditions revealed relatively modest effects of reverberation and component level.

Auditory cortex: Representing sound locations during active sensing.

Active exploration changes how the brain processes auditory space. A new study reveals the dynamic properties of neural activity that encode combinations of sound identity and location during active sensing.

Combined Electric and Acoustic Stimulation (EAS) in Children: Investigating Benefit Afforded by Bilateral Versus Unilateral Acoustic Hearing.

PURPOSE: Electric and acoustic stimulation (EAS) with preserved hearing in the implanted ear provides benefit for speech understanding, spatial hearing, and quality of life in adults. However, there is limited research on EAS outcomes in children. The aims of this study were to estimate the magnitude of EAS-related benefit on speech understanding in children with preserved acoustic hearing and to determine what role acoustic interaural time difference (ITD) sensitivity may have on said EAS benefit. METHODS: Six children with acoustic hearing preservation and 20 children with normal hearing (NH) were recruited to participate. Speech recognition was assessed via an eight-loudspeaker array with speech presented from one loudspeaker at 0 degree and restaurant noise from all other loudspeakers (45-315 degrees). ITD thresholds were measured for a 250-Hz signal presented acoustically via insert earphones. RESULTS: Only one EAS listener demonstrated significant benefit from bilateral acoustic hearing as compared with acoustic hearing from a single ear. ITD thresholds were poor in the range of 302 to 1000+ ms and were considerably poorer than ITD thresholds for the NH group. CONCLUSION: These data suggest that children with acoustic hearing preservation may not exhibit initial EAS benefit for speech recognition in semi-diffuse noise; however, because none exhibited a decrement in performance with bilateral acoustic stimulation, EAS fittings are recommended to provide binaural acoustic access allowing for EAS adaptation to binaural cues over time. Future research should address the emergence of EAS benefit, binaural cue sensitivity, and the role of EAS experience in children and adults.

Binaural spatial adaptation as a mechanism for asymmetric trading of interaural time and level differences.

A classic paradigm used to quantify the perceptual weighting of binaural spatial cues requires a listener to adjust the value of one cue, while the complementary cue is held constant. Adjustments are made until the auditory percept appears centered in the head, and the values of both cues are recorded as a trading relation (TR), most commonly in µs interaural time difference per dB interaural level difference. Interestingly, existing literature has shown that TRs differ according to the cue being adjusted. The current study investigated whether cue-specific adaptation, which might arise due to the continuous, alternating presentation of signals during adjustment tasks, could account for this poorly understood phenomenon. Three experiments measured TRs via adjustment and via lateralization of single targets in virtual reality (VR). Targets were 500 Hz pure tones preceded by silence or by adapting trains that held one of the cues constant. VR removed visual anchors and provided an intuitive response technique during lateralization. The pattern of results suggests that adaptation can account for cue-dependent TRs. In addition, VR seems to be a viable tool for psychophysical tasks.

Binaural cue sensitivity in cochlear implant recipients with acoustic hearing preservation.

Bilateral acoustic hearing in cochlear implant (CI) recipients with hearing preservation may allow access to binaural cues. Sensitivity to acoustic binaural cues has been shown in some listeners combining electric and acoustic stimulation (EAS), yet remains poorly understood and may be subject to limitations imposed by the electrical stimulation and/or amplification asymmetries. The purpose of this study was to investigate the effect of stimulus level, frequency-dependent gain, and the addition of unilateral electrical stimulation on sensitivity to low-frequency binaural cues. Thresholds were measured for interaural time and level differences (ITD and ILD) carried by a low-frequency, bandpass noise (100-800 Hz). 16 adult CI EAS listeners (mean age = 50.2 years) each participated in three listening conditions: acoustic hearing only at 90 dB SPL, acoustic hearing only at 60 dB SPL with frequency-dependent gain, and acoustic hearing plus unilateral CI at 60 dB SPL with frequency-dependent gain applied to the acoustic channels only. Results revealed thresholds within the ecologically relevant ITD and/or ILD range for most EAS listeners. No significant effects of presentation level, frequency-dependent gain, or the addition of unilateral electrical stimulation on the resultant thresholds for ITDs or ILDs were observed at the group level. Correlational analyses related ITD and ILD thresholds to the degree of EAS benefit (i.e., advantage of acoustic hearing in the implanted ear) for speech recognition in diffuse noise. There was a significant relationship between EAS benefit and ITD thresholds, but no statistically significant relationship between EAS benefit and ILD thresholds. In summary, the results of this study are not consistent with our previous data obtained with simulated EAS in normal-hearing listeners, which showed significant binaural interference by a unilateral electrical "distractor" (Van Ginkel et al., 2019). The difference between studies suggests that chronic exposure to unilateral electrical stimulation combined with bilateral acoustic stimulation may reduce interference effects, perhaps because listeners adapt to the presence of the constant but binaurally incongruous CI stimulus. These results are consistent with past studies that demonstrated no interference in spatial hearing tasks due to the addition of a unilateral CI in adult EAS listeners.

Binaural interference with simulated electric acoustic stimulation.

Preserved low-frequency acoustic hearing in cochlear implant (CI) recipients affords combined electric-acoustic stimulation (EAS) that could improve access to low-frequency acoustic binaural cues and enhance spatial hearing. Such benefits, however, could be undermined by interactions between electrical and acoustical inputs to adjacent (spectral overlap) or distant (binaural interference) cochlear places in EAS. This study simulated EAS in normal-hearing listeners, measuring interaural time difference (ITD) and interaural level difference (ILD) discrimination thresholds for a low-frequency noise (simulated acoustic target) in the presence or absence of a pulsatile high-frequency complex presented monotically or diotically (simulated unilateral or bilateral electric distractor). Unilateral distractors impaired thresholds for both cue types, suggesting influences of both binaural interference (which appeared more consistently for ITD than ILD) and physical spectral overlap (for both cue types). Reducing spectral overlap with an EAS gap between 1 and 3 kHz consistently improved binaural sensitivity. Finally, listeners displayed significantly lower thresholds with simulated bilateral versus unilateral electric stimulation. The combined effects revealed similar or better thresholds in bilateral full spectral overlap than in unilateral EAS gap conditions, suggesting that bilateral implantation with bilateral acoustic hearing preservation could allow for higher tolerance of spectral overlap in CI users and improved binaural sensitivity over unilateral EAS.

Reverberation enhances onset dominance in sound localization.

Temporal variation in sensitivity to sound-localization cues was measured in anechoic conditions and in simulated reverberation using the temporal weighting function (TWF) paradigm [Stecker and Hafter (2002). J. Acoust. Soc. Am. 112, 1046-1057]. Listeners judged the locations of Gabor click trains (4 kHz center frequency, 5-ms interclick interval) presented from an array of loudspeakers spanning 360° azimuth. Targets ranged ±56.25° across trials. Individual clicks within each train varied by an additional ±11.25° to allow TWF calculation by multiple regression. In separate conditions, sounds were presented directly or in the presence of simulated reverberation: 13 orders of lateral reflection were computed for a 10 m × 10 m room ( RT60≊300 ms) and mapped to the appropriate locations in the loudspeaker array. Results reveal a marked increase in perceptual weight applied to the initial click in reverberation, along with a reduction in the impact of late-arriving sound. In a second experiment, target stimuli were preceded by trains of "conditioner" sounds with or without reverberation. Effects were modest and limited to the first few clicks in a train, suggesting that impacts of reverberant pre-exposure on localization may be limited to the processing of information from early reflections.

Temporal weighting functions for interaural time and level differences. V. Modulated noise carriers.

Sound onsets dominate spatial judgments of many types of periodic sound. Conversely, ongoing cues often dominate in spatial judgments of aperiodic noise. This study quantified onset dominance as a function of both the bandwidth and the temporal regularity of stimuli by measuring temporal weighting functions (TWF) from Stecker, Ostreicher, and Brown [(2013) J. Acoust. Soc. Am. 134, 1242-1252] for lateralization of periodic and aperiodic noise-burst trains. Stimuli consisted of 16 noise bursts (1 ms each) repeating at an interval of 2 or 5 ms. TWFs were calculated by multiple regression of lateralization judgments onto interaural time and level differences, which varied independently ( ±100 µs, ±2 dB) across bursts. Noise tokens were either refreshed on each burst (aperiodic) or repeated across sets of 2, 4, 8, or 16 bursts. TWFs revealed strong onset dominance for periodic noise-burst trains (16 repeats per token), which was markedly reduced in aperiodic trains. A second experiment measured TWFs for periodic but sinusoidally amplitude-modulated noise burst trains, revealing greater weight on the earliest and least intense bursts of the rising envelope slope. The results support the view that envelope fluctuations drive access to binaural information in both periodic and aperiodic sounds.

Temporal binding of auditory spatial information across dynamic binaural events.

Simultaneity judgments were used to measure temporal binding windows (TBW) for brief binaural events (changes in interaural time and/or level differences [ITD and ILD]) and test the hypothesis that ITD and ILD contribute to perception via separate sensory dimensions subject to binding via slow (100+ ms)-presumably cortical-mechanisms as in multisensory TBW. Stimuli were continuous low-frequency noises that included two brief shifts of either type (ITD or ILD), both of which are heard as lateral position changes. TBW for judgments within a single cue dimension were narrower for ITD (mean = 444 ms) than ILD (807 ms). TBW for judgments across cue dimensions (i.e., one ITD shift and one ILD shift) were similar to within-cue ILD (778 ms). The results contradict the original hypothesis, in that cross-cue comparisons were no slower than within-cue ILD comparisons. Rather, the wide TBW values-consistent with previous estimates of multisensory TBW-suggest slow integrative processing for both types of judgments. Narrower TBW for ITD than ILD judgments suggests important cue-specific differences in the neural mechanisms or the perceptual correlates of integration across binaural-cue dimensions.

Evidence for cue-independent spatial representation in the human auditory cortex during active listening.

Few auditory functions are as important or as universal as the capacity for auditory spatial awareness (e.g., sound localization). That ability relies on sensitivity to acoustical cues-particularly interaural time and level differences (ITD and ILD)-that correlate with sound-source locations. Under nonspatial listening conditions, cortical sensitivity to ITD and ILD takes the form of broad contralaterally dominated response functions. It is unknown, however, whether that sensitivity reflects representations of the specific physical cues or a higher-order representation of auditory space (i.e., integrated cue processing), nor is it known whether responses to spatial cues are modulated by active spatial listening. To investigate, sensitivity to parametrically varied ITD or ILD cues was measured using fMRI during spatial and nonspatial listening tasks. Task type varied across blocks where targets were presented in one of three dimensions: auditory location, pitch, or visual brightness. Task effects were localized primarily to lateral posterior superior temporal gyrus (pSTG) and modulated binaural-cue response functions differently in the two hemispheres. Active spatial listening (location tasks) enhanced both contralateral and ipsilateral responses in the right hemisphere but maintained or enhanced contralateral dominance in the left hemisphere. Two observations suggest integrated processing of ITD and ILD. First, overlapping regions in medial pSTG exhibited significant sensitivity to both cues. Second, successful classification of multivoxel patterns was observed for both cue types and-critically-for cross-cue classification. Together, these results suggest a higher-order representation of auditory space in the human auditory cortex that at least partly integrates the specific underlying cues.

Sensitivity to an Illusion of Sound Location in Human Auditory Cortex.

Human listeners place greater weight on the beginning of a sound compared to the middle or end when determining sound location, creating an auditory illusion known as the Franssen effect. Here, we exploited that effect to test whether human auditory cortex (AC) represents the physical vs. perceived spatial features of a sound. We used functional magnetic resonance imaging (fMRI) to measure AC responses to sounds that varied in perceived location due to interaural level differences (ILD) applied to sound onsets or to the full sound duration. Analysis of hemodynamic responses in AC revealed sensitivity to ILD in both full-cue (veridical) and onset-only (illusory) lateralized stimuli. Classification analysis revealed regional differences in the sensitivity to onset-only ILDs, where better classification was observed in posterior compared to primary AC. That is, restricting the ILD to sound onset-which alters the physical but not the perceptual nature of the spatial cue-did not eliminate cortical sensitivity to that cue. These results suggest that perceptual representations of auditory space emerge or are refined in higher-order AC regions, supporting the stable perception of auditory space in noisy or reverberant environments and forming the basis of illusions such as the Franssen effect.

Spectrotemporal weighting of binaural cues: Effects of a diotic interferer on discrimination of dynamic interaural differences.

Spatial judgments are often dominated by low-frequency binaural cues and onset cues when binaural cues vary across the spectrum and duration, respectively, of a brief sound. This study combined these dimensions to assess the spectrotemporal weighting of binaural information. Listeners discriminated target interaural time difference (ITD) and interaural level difference (ILD) carried by the onset, offset, or full duration of a 4-kHz Gabor click train with a 2-ms period in the presence or absence of a diotic 500-Hz interferer tone. ITD and ILD thresholds were significantly elevated by the interferer in all conditions and by a similar amount to previous reports for static cues. Binaural interference was dramatically greater for ITD targets lacking onset cues compared to onset and full-duration conditions. Binaural interference for ILD targets was similar across dynamic-cue conditions. These effects mirror the baseline discriminability of dynamic ITD and ILD cues [Stecker and Brown. (2010). J. Acoust. Soc. Am. 127, 3092-3103], consistent with stronger interference for less-robust/higher-variance cues. The results support the view that binaural cue integration occurs simultaneously across multiple variance-weighted dimensions, including time and frequency.