Envelope Interaural Time Differences Research Articles

Rate dependent neural responses of interaural-time-difference cues in fine-structure and envelope.

Advancements in cochlear implants (CIs) have led to a significant increase in bilateral CI users, especially among children. Yet, most bilateral CI users do not fully achieve the intended binaural benefit due to potential limitations in signal processing and/or surgical implant positioning. One crucial auditory cue that normal hearing (NH) listeners can benefit from is the interaural time difference (ITD), i.e., the time difference between the arrival of a sound at two ears. The ITD sensitivity is thought to be heavily relying on the effective utilization of temporal fine structure (very rapid oscillations in sound). Unfortunately, most current CIs do not transmit such true fine structure. Nevertheless, bilateral CI users have demonstrated sensitivity to ITD cues delivered through envelope or interaural pulse time differences, i.e., the time gap between the pulses delivered to the two implants. However, their ITD sensitivity is significantly poorer compared to NH individuals, and it further degrades at higher CI stimulation rates, especially when the rate exceeds 300 pulse per second. The overall purpose of this research thread is to improve spatial hearing abilities in bilateral CI users. This study aims to develop electroencephalography (EEG) paradigms that can be used with clinical settings to assess and optimize the delivery of ITD cues, which are crucial for spatial hearing in everyday life. The research objective of this article was to determine the effect of CI stimulation pulse rate on the ITD sensitivity, and to characterize the rate-dependent degradation in ITD perception using EEG measures. To develop protocols for bilateral CI studies, EEG responses were obtained from NH listeners using sinusoidal-amplitude-modulated (SAM) tones and filtered clicks with changes in either fine structure ITD (ITDFS) or envelope ITD (ITDENV). Multiple EEG responses were analyzed, which included the subcortical auditory steady-state responses (ASSRs) and cortical auditory evoked potentials (CAEPs) elicited by stimuli onset, offset, and changes. Results indicated that acoustic change complex (ACC) responses elicited by ITDENV changes were significantly smaller or absent compared to those elicited by ITDFS changes. The ACC morphologies evoked by ITDFS changes were similar to onset and offset CAEPs, although the peak latencies were longest for ACC responses and shortest for offset CAEPs. The high-frequency stimuli clearly elicited subcortical ASSRs, but smaller than those evoked by lower carrier frequency SAM tones. The 40-Hz ASSRs decreased with increasing carrier frequencies. Filtered clicks elicited larger ASSRs compared to high-frequency SAM tones, with the order being 40 > 160 > 80> 320 Hz ASSR for both stimulus types. Wavelet analysis revealed a clear interaction between detectable transient CAEPs and 40-Hz ASSRs in the time-frequency domain for SAM tones with a low carrier frequency.

A comparison of binaural cues transmitted by clinically available cochlear-implant stimulation strategies

Cochlear implants (Cis) effectively restore speech understanding in quiet for many recipients. However, bilateral CI listeners continue to perform poorer than acoustic-hearing listeners on spatial-hearing tasks (e.g., speech understanding in noise and sound localization). One explanation is that current clinically available stimulation strategies and unsynchronized sound processors degrade the fidelity of binaural cues. However, no conclusive comparison of the binaural cues transmitted by different stimulation strategies has been made. Therefore, this study compared binaural cues transmitted by two clinically available cochlear implant stimulation strategies. Two unsynchronized Cochlear Ltd. Nucleus 6 sound processors were placed on a binaural mannequin centered in a horizontal ring of loudspeakers in an anechoic chamber. Acoustic stimuli were presented from the loudspeakers and the resulting electrical pulse trains were recorded for both a peak-picking stimulation strategy and constantly stimulating strategy. Envelope interaural time differences and interaural level differences were extracted from the recordings and compared between stimulation strategies. Compared to the constantly stimulating strategy, results show that the peak-picking strategy led to more variable and less monotonic binaural cues, which could constrain sound-localization abilities. These findings argue for the bilateral synchronization of CIs in multiple domains, including electrode selection in peak-picking stimulation strategies and pulse timing.

Lateralization of binaural envelope cues measured with a mobile cochlear-implant research processora).

Bilateral cochlear implant (BICI) listeners do not have full access to the binaural cues that normal hearing (NH) listeners use for spatial hearing tasks such as localization. When using their unsynchronized everyday processors, BICI listeners demonstrate sensitivity to interaural level differences (ILDs) in the envelopes of sounds, but interaural time differences (ITDs) are less reliably available. It is unclear how BICI listeners use combinations of ILDs and envelope ITDs, and how much each cue contributes to perceived sound location. The CCi-MOBILE is a bilaterally synchronized research processor with the untested potential to provide spatial cues to BICI listeners. In the present study, the CCi-MOBILE was used to measure the ability of BICI listeners to perceive lateralized sound sources when single pairs of electrodes were presented amplitude-modulated stimuli with combinations of ILDs and envelope ITDs. Young NH listeners were also tested using amplitude-modulated high-frequency tones. A cue weighting analysis with six BICI and ten NH listeners revealed that ILDs contributed more than envelope ITDs to lateralization for both groups. Moreover, envelope ITDs contributed to lateralization for NH listeners but had negligible contribution for BICI listeners. These results suggest that the CCi-MOBILE is suitable for binaural testing and developing bilateral processing strategies.

Wave interference at the contralateral ear helps explain non-monotonic envelope interaural time differences as a function of azimuth.

Interaural time differences (ITDs), an important acoustic cue for perceptual sound-source localization, are conventionally modeled as monotonic functions of azimuth. However, recent literature and publicly available databases from binaural manikins demonstrated ITDs conveyed by the envelopes (ENV-ITDs) of high-frequency (≥2 kHz) signals that were non-monotonic functions of azimuth. This study demonstrates using a simple, time-dependent geometric model of an elliptic head that the back-traveling (longer) sound path around the head, delayed and added to the conventionally treated front-traveling path, can account for non-monotonic ENV-ITDs. These findings have implications for spatial-hearing models in acoustic and electric (cochlear-implant) hearing.

Measuring sensitivity to envelope interaural time differences by adapting modulation depth

Listeners are sensitive to interaural time differences carried in the envelope of high-frequency sounds (ITDENV), but the salience of this cue depends on several envelope properties. Making use of the fact that sensitivity to ITDENV varies systematically with the depth of modulation of sinusoidally amplitude-modulated tones, we devised a task in which modulation depth is varied adaptively to measure ITDENV sensitivity. In our task, the stimulus is a 4 kHz tone modulated at rates of 32, 64, or 128 Hz, with a fixed ITD of ±500 μs. Modulation depth is adaptively varied (in dB) and listeners make left-right judgements. The data are used to obtain a psychometric function and threshold value. For listeners with normal hearing, we find that thresholds vary with modulation rate and with sensation level as expected. This task provides us with a convenient means for investigating ITDENV sensitivity in listeners with sensorineural hearing loss. Given that this population shows enhanced sensitivity to amplitude modulation under certain conditions, generally attributed to a loss of cochlear compression, we will test the hypothesis that this translates into superior ITDENV sensitivity under similar conditions.

A creeping-wave elliptic-head model that explains non-monotonic envelope interaural time differences

Interaural time and level differences (ITDs and ILDs) created by interactions of impinging sound with the head facilitate horizontal-plane sound localization. ILDs are known to be non-monotonic functions of azimuth, resulting in predictable mislocalization of tonal sources at some frequencies, while ITDs are typically construed as monotonic functions of azimuth. However, recent measurements using a binaural mannequin revealed envelope ITDs (ENV-ITDs) that were non-monotonic functions of azimuth, with the extent of nonmonotonicity dependent on carrier frequency. This study proposes that delayed sound traveling around the back of the head causes this phenomenon. ENV-ITDs created by a time-dependent model of incident sound waves propagating around the front and back of an elliptical head were compared to ENV-ITDs computed using publicly available HRTF libraries for a broadband speech excitation signal. Results from the model align with data from the HRTF libraries, with nonmonotonicities in the ENV-ITD-azimuth functions occurring at nearly identical azimuths and frequencies. These data suggest the veracity of this phenomenon and may have implications for spatial hearing abilities. They also suggest that time-independent models of ITDs that only consider frequency and incident angle, such as spherical head or circular arc length models, could be updated to include such time dependence.

Simulation of ITD-Dependent Single-Neuron Responses Under Electrical Stimulation and with Amplitude-Modulated Acoustic Stimuli

Interaural time difference (ITD) sensitivity with cochlear implant stimulation is remarkably similar to envelope ITD sensitivity using conventional acoustic stimulation. This holds true for human perception, as well as for neural response rates recorded in the inferior colliculus of several mammalian species. We hypothesize that robust excitatory-inhibitory (EI) interaction is the dominant mechanism. Therefore, we connected the same single EI-model neuron to either a model of the normal acoustic auditory periphery or to a model of the electrically stimulated auditory nerve. The model captured most features of the experimentally obtained response properties with electric stimulation, such as the shape of rate-ITD functions, the dependence on stimulation level, and the pulse rate or modulation-frequency dependence. Rate-ITD functions with high-rate, amplitude-modulated electric stimuli were very similar to their acoustic counterparts. Responses obtained with unmodulated electric pulse trains most resembled acoustic filtered clicks. The fairly rapid decline of ITD sensitivity at rates above 300 pulses or cycles per second is correctly simulated by the 3.1-ms time constant of the inhibitory post-synaptic conductance. As the model accounts for these basic properties, it is expected to help in understanding and quantifying the binaural hearing abilities with electric stimulation when integrated in bigger simulation frameworks.

Sensitivity to Envelope Interaural Time Differences: Modeling Auditory Modulation Filtering.

For amplitude-modulated sound, the envelope interaural time difference (ITDENV) is a potential cue for sound-source location. ITDENV is encoded in the lateral superior olive (LSO) of the auditory brainstem, by excitatory-inhibitory (EI) neurons receiving ipsilateral excitation and contralateral inhibition. Between human listeners, sensitivity to ITDENV varies considerably, but ultimately decreases with increasing stimulus carrier frequency, and decreases more strongly with increasing modulation rate. Mechanisms underlying the variation in behavioral sensitivity remain unclear. Here, with increasing carrier frequency (4-10kHz), as we phenomenologically model the associated decrease in ITDENV sensitivity using arbitrarily fewer neurons consistent across populations, we computationally model the variable sensitivity across human listeners and modulation rates (32-800Hz) as the decreasing range of membrane frequency responses in LSO neurons. Transposed tones stimulate a bilateral auditory-periphery model, driving model EI neurons where electrical membrane impedance filters the frequency content of inputs driven by amplitude-modulated sound, evoking modulation filtering. Calculated from Fisher information in spike-rate functions of ITDENV, for model EI neuronal populations distinctly reflecting the LSO range in membrane frequency responses, just-noticeable differences in ITDENV collectively reproduce the largest variation in ITDENV sensitivity across human listeners. These slow to fast model populations each generally match the best human ITDENV sensitivity at a progressively higher modulation rate, by membrane-filtering and spike-generation properties producing realistically less than Poisson variance. Non-resonant model EI neurons are also sensitive to interaural intensity differences. With peripheral filters centered between carrier frequency and modulation sideband, fast resonant model EI neurons extend ITDENV sensitivity above 500-Hz modulation.

ITD sensitivity provided to bilateral cochlear implant listeners by a mixed-rate, real-time capable processing strategy

Bilateral cochlear implant (CI) listeners have limited access to interaural time differences (ITDs) at low frequencies in part because clinical processors do not coordinate the timing of stimulation across the ears. Further, clinical strategies, which are optimized for good speech reception, operate at stimulation rates that are too high [∼1000 pulses per second (pps)] to enable ITD sensitivity from the timing of pulses. Our newly developed, real-time capable sound coding strategy, delivered using a bilaterally synchronized research processor, enables binaural coordination and pulse-timed ITD encoding. We employ a “mixed-rate” stimulation approach by maintaining the capacity to provide speech or envelope ITDs on high-rate channels while simultaneously providing ITDs in the pulse timing on low-rate (100 pps) channels. We hypothesized that a mixed-rate strategy encoding both high-rate envelope ITDs and pulse ITDs would yield better ITD sensitivity than a strategy encoding pulse ITDs or high-rate envelope ITDs only. This hypothesis was tested by measuring the perceived range of lateralized auditory images in bilateral CI listeners. Preliminary results indicate listeners may have ITD sensitivity with all conditions, though the mixed-rate strategy can provide better ITD sensitivity in some listeners. This demonstrates the potential of the mixed-rate strategy for improving binaural hearing outcomes.

Transmission of Binaural Cues by Bilateral Cochlear Implants: Examining the Impacts of Bilaterally Independent Spectral Peak-Picking, Pulse Timing, and Compression.

Acoustic hearing listeners use binaural cues—interaural time differences (ITDs) and interaural level differences (ILDs)—for localization and segregation of sound sources in the horizontal plane. Cochlear implant users now often receive two implants (bilateral cochlear implants [BiCIs]) rather than one, with the goal to provide access to these cues. However, BiCI listeners often experience difficulty with binaural tasks. Most BiCIs use independent sound processors at each ear; it has often been suggested that such independence may degrade the transmission of binaural cues, particularly ITDs. Here, we report empirical measurements of binaural cue transmission via BiCIs implementing a common “n-of-m” spectral peak-picking stimulation strategy. Measurements were completed for speech and nonspeech stimuli presented to an acoustic manikin “fitted” with BiCI sound processors. Electric outputs from the BiCIs and acoustic outputs from the manikin’s in-ear microphones were recorded simultaneously, enabling comparison of electric and acoustic binaural cues. For source locations away from the midline, BiCI binaural cues, particularly envelope ITD cues, were found to be degraded by asymmetric spectral peak-picking. In addition, pulse amplitude saturation due to nonlinear level mapping yielded smaller ILDs at higher presentation levels. Finally, while individual pulses conveyed a spurious “drifting” ITD, consistent with independent left and right processor clocks, such variation was not evident in transmitted envelope ITDs. Results point to avenues for improvement of BiCI technology and may prove useful in the interpretation of BiCI spatial hearing outcomes reported in prior and future studies.

High frequency sensitivity to interaural onset time differences in the bat inferior colliculus

Many neurons in the auditory midbrain are tuned to binaural cues. Two prominent binaural cues are the interaural level difference (ILD) and the interaural time difference (ITD). The ITD cue can further be subdivided into the ongoing envelope ITD cues and transient onset ITD cues. More is known about the sensitivity of single neurons to ongoing envelope ITDs compared to transient onset ITDs in the mammalian auditory system, particularly in bats. The current study examines the response properties of single neurons in the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus, to onset ITDs in response to high frequency pure tones. Measures of neurons’ dynamic ITD response revealed an average change of 36% of its maximum response within the behaviorally relevant range of ITDs (±50 µs). Across all IC neurons, we measured an average time-intensity trading ratio of 30 µs/dB in the sensitivity of the ITD response function to changing ILDs. Minimum and maximum ITD responses were clustered within a narrow range of ITDs. The average peak in the ITD response function was at 268 µs, a finding that is consistent with other non-echolocating mammals. Some ITD-sensitive neurons also showed weak facilitation of maximum response during binaural stimulation, compared to monaural stimulation. These results suggest that echolocating bats possess the potential to use onset ITD cues to assist in the azimuthal sound localization of ultrasonic frequencies.

Neural rate difference model can account for lateralization of high-frequency stimuli.

Lateralization of complex high-frequency sounds is conveyed by interaural level differences (ILDs) and interaural time differences (ITDs) in the envelope. In this work, the authors constructed an auditory model and simulate data from three previous behavioral studies obtained with, in total, over 1000 different amplitude-modulated stimuli. The authors combine a well-established auditory periphery model with a functional count-comparison model for binaural excitatory-inhibitory (EI) interaction. After parameter optimization of the EI-model stage, the hemispheric rate-difference between pairs of EI-model neurons relates linearly with the extent of laterality in human listeners. If a certain ILD and a certain envelope ITD each cause a similar extent of laterality, they also produce a similar rate difference in the same model neurons. After parameter optimization, the model accounts for 95.7% of the variance in the largest dataset, in which amplitude modulation depth, rate of modulation, modulation exponent, ILD, and envelope ITD were varied. The model also accounts for 83% of the variances in each of the other two datasets using the same EI model parameters.

Neuronal sensitivity to the interaural time difference of the sound envelope in the mouse inferior colliculus

We examined the sensitivity of the neurons in the mouse inferior colliculus (IC) to the interaural time differences (ITD) conveyed in the sound envelope. Utilizing optogenetic methods, we compared the responses to the ITD in the envelope of identified glutamatergic and GABAergic neurons. More than half of both cell types were sensitive to the envelope ITD, and the ITD curves were aligned at their troughs. Within the physiological ITD range of mice (±50 μs), the ITD curves of both cell types had a higher firing rate when the contralateral envelope preceded the ipsilateral envelope. These results show that the circuitry to process ITD persists in the mouse despite its lack of low-frequency hearing. The sensitivity of IC neurons to ITD is most likely to be shaped by the binaural interaction of excitation and inhibition in the lateral superior olive.

Effects of rate and age in processing interaural time and level differences in normal-hearing and bilateral cochlear-implant listeners.

Bilateral cochlear implants (BICIs) provide improved sound localization and speech understanding in noise compared to unilateral CIs. However, normal-hearing (NH) listeners demonstrate superior binaural processing abilities compared to BICI listeners. This investigation sought to understand differences between NH and BICI listeners' processing of interaural time differences (ITDs) and interaural level differences (ILDs) as a function of fine-structure and envelope rate using an intracranial lateralization task. The NH listeners were presented band-limited acoustical pulse trains and sinusoidally amplitude-modulated tones using headphones, and the BICI listeners were presented single-electrode electrical pulse trains using direct stimulation. Lateralization range increased as fine-structure rate increased for ILDs in BICI listeners. Lateralization range decreased for rates above 100 Hz for fine-structure ITDs, but decreased for rates lower or higher than 100 Hz for envelope ITDs in both groups. Lateralization ranges for ITDs were smaller for BICI listeners on average. After controlling for age, older listeners showed smaller lateralization ranges and BICI listeners had a more rapid decline for ITD sensitivity at 300 pulses per second. This work suggests that age confounds comparisons between NH and BICI listeners in temporal processing tasks and that some NH-BICI binaural processing differences persist even when age differences are adequately addressed.

Microsecond sensitivity to envelope interaural time differences in rats.

Currently, there is controversy around whether rats can use interaural time differences (ITDs) to localize sound. Here, naturalistic pulse train stimuli were used to evaluate the rat's sensitivity to onset and ongoing ITDs using a two-alternative forced choice sound lateralization task. Pulse rates between 50 Hz and 4.8 kHz with rectangular or Hanning windows were delivered with ITDs between ±175 μs over a near-field acoustic setup. Similar to other mammals, rats performed with 75% accuracy at ∼50 μs ITD, demonstrating that rats are highly sensitive to envelope ITDs.

Erratum to: A Modeling Study of the Responses of the Lateral Superior Olive to Ipsilateral Sinusoidally Amplitude-Modulated Tones.

The lateral superior olive (LSO) is a brainstem nucleus that is classically understood to encode binaural information in high-frequency sounds. Previous studies have shown that LSO cells are sensitive to envelope interaural time difference in sinusoidally amplitude-modulated (SAM) tones (Joris and Yin, J Neurophysiol 73:1043–1062, 1995; Joris, J Neurophysiol 76:2137–2156, 1996) and that a subpopulation of LSO neurons exhibit low-threshold potassium currents mediated by Kv1 channels (Barnes-Davies et al., Eur J Neurosci 19:325–333, 2004). It has also been shown that in many LSO cells the average response rate to ipsilateral SAM tones decreases with modulation frequency above a few hundred Hertz (Joris and Yin, J Neurophysiol 79:253–269, 1998). This low-pass feature is not directly inherited from the inputs to the LSO since the response rate of these input neurons changes little with increasing modulation frequency. In the current study, an LSO cell model is developed to investigate mechanisms consistent with the responses described above, notably the emergent rate decrease with increasing frequency. The mechanisms explored included the effects of after-hyperpolarization (AHP) channels, the dynamics of low-threshold potassium channels (KLT), and the effects of background inhibition. In the model, AHP channels alone were not sufficient to induce the observed rate decrease at high modulation frequencies. The model also suggests that the background inhibition alone, possibly from the medial nucleus of the trapezoid body, can account for the small rate decrease seen in some LSO neurons, but could not explain the large rate decrease seen in other LSO neurons at high modulation frequencies. In contrast, both the small and large rate decreases were replicated when KLT channels were included in the LSO neuron model. These results support the conclusion that KLT channels may play a major role in the large rate decreases seen in some units and that background inhibition may be a contributing factor, a factor that could be adequate for small decreases.

Influence of envelope waveform on ITD sensitivity of neurons in the auditory midbrain.

Interaural time differences (ITDs) conveyed by the modulated envelopes of high-frequency sounds can serve as a cue for localizing a sound source. Klein-Hennig et al. (J Acoust Soc Am 129: 3856, 2011) demonstrated the envelope attack (the rate at which stimulus energy in the envelope increases) and the duration of the pause (the interval between successive envelope pulses) as important factors affecting sensitivity to envelope ITDs in human listeners. Modulated sounds with rapid attacks and long pauses produce the lowest ITD discrimination thresholds. The duration of the envelope's sustained component (sustain) and the rate at which stimulus energy falls at the offset of the envelope (decay) are only minor factors. We assessed the responses of 71 single neurons, recorded from the midbrains of 15 urethane-anesthetized tri-colored guinea pigs, to envelope shapes in which the four envelope components, i.e., attack, sustain, decay, and pause, were systematically varied. We confirmed the importance of the attack and pause components in generating ITD-sensitive responses. Analysis of neural firing rates demonstrated more neurons (49/71) show ITD sensitivity in response to "damped" stimuli (fast attack and slow decay) compared with "ramped" stimuli (slow attack and fast decay) (14/71). Furthermore, the lowest threshold for the damped stimulus (91 μs) was lower by a factor of 4 than that for the temporally reversed ramped envelope shape (407 μs). The data confirm the importance of fast attacks and optimal pause durations in generating sensitivity to ITDs conveyed in the modulated envelopes of high-frequency sounds and are incompatible with models of ITD processing based on the integration of sound energy over time.NEW & NOTEWORTHY Using single-neuron electrophysiology, we show that the precise shape of a sound's "energy envelope" is a critical factor in determining how well midbrain neurons are able to convey information about auditory spatial cues. Consistent with human behavioral performance, sounds with rapidly rising energy and relatively long intervals between energy bursts are best at conveying spatial information. The data suggest specific sound energy patterns that might best be applied to hearing devices to aid spatial listening.

Envelope contributions to the representation of interaural time difference in the forebrain of barn owls

Birds and mammals use the interaural time difference (ITD) for azimuthal sound localization. While barn owls can use the ITD of the stimulus carrier frequency over nearly their entire hearing range, mammals have to utilize the ITD of the stimulus envelope to extend the upper frequency limit of ITD-based sound localization. ITD is computed and processed in a dedicated neural circuit that consists of two pathways. In the barn owl, ITD representation is more complex in the forebrain than in the midbrain pathway because of the combination of two inputs that represent different ITDs. We speculated that one of the two inputs includes an envelope contribution. To estimate the envelope contribution, we recorded ITD response functions for correlated and anticorrelated noise stimuli in the barn owl's auditory arcopallium. Our findings indicate that barn owls, like mammals, represent both carrier and envelope ITDs of overlapping frequency ranges, supporting the hypothesis that carrier and envelope ITD-based localization are complementary beyond a mere extension of the upper frequency limit.NEW & NOTEWORTHY The results presented in this study show for the first time that the barn owl is able to extract and represent the interaural time difference (ITD) information conveyed by the envelope of a broadband acoustic signal. Like mammals, the barn owl extracts the ITD of the envelope and the carrier of a signal from the same frequency range. These results are of general interest, since they reinforce a trend found in neural signal processing across different species.

Psychophysical and modeling approaches towards determining the cochlear phase response based on interaural time differences.

The cochlear phase response is often estimated by measuring masking of a tonal target by harmonic complexes with various phase curvatures. Maskers yielding most modulated internal envelope representations after passing the cochlear filter are thought to produce minimum masking, with fast-acting cochlear compression as the main contributor to that effect. Thus, in hearing-impaired (HI) listeners, reduced cochlear compression hampers estimation of the phase response using the masking method. This study proposes an alternative approach, based on the effect of the envelope modulation strength on the sensitivity to interaural time differences (ITDs). To evaluate the general approach, ITD thresholds were measured in seven normal-hearing listeners using 300-ms Schroeder-phase harmonic complexes with nine different phase curvatures. ITD thresholds tended to be lowest for phase curvatures roughly similar to those previously shown to produce minimum masking. However, an unexpected ITD threshold peak was consistently observed for a particular negative phase curvature. An auditory-nerve based ITD model predicted the general pattern of ITD thresholds except for the threshold peak, as well as published envelope ITD data. Model predictions simulating outer hair cell loss support the feasibility of the ITD-based approach to estimate the phase response in HI listeners.

A method to enhance the use of interaural time differences for cochlear implants in reverberant environments

The ability of normal-hearing (NH) listeners to exploit interaural time difference (ITD) cues conveyed in the modulated envelopes of high-frequency sounds is poor compared to ITD cues transmitted in the temporal fine structure at low frequencies. Sensitivity to envelope ITDs is further degraded when envelopes become less steep, when modulation depth is reduced, and when envelopes become less similar between the ears, common factors when listening in reverberant environments. The vulnerability of envelope ITDs is particularly problematic for cochlear implant (CI) users, as they rely on information conveyed by slowly varying amplitude envelopes. Here, an approach to improve access to envelope ITDs for CIs is described in which, rather than attempting to reduce reverberation, the perceptual saliency of cues relating to the source is increased by selectively sharpening peaks in the amplitude envelope judged to contain reliable ITDs. Performance of the algorithm with room reverberation was assessed through simulating listening with bilateral CIs in headphone experiments with NH listeners. Relative to simulated standard CI processing, stimuli processed with the algorithm generated lower ITD discrimination thresholds and increased extents of laterality. Depending on parameterization, intelligibility was unchanged or somewhat reduced. The algorithm has the potential to improve spatial listening with CIs.