Interaural Time Differences Sensitivity Research Articles

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

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 1400Hz. How listeners' ITD sensitivity transitions from very best sensitivity near 700Hz 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. ITD sensitivity was measured in 16 normal-hearing listeners as a joint function of frequency (900-1500Hz) and level (10-50dB sensation level). Performance decreased with increasing frequency and decreasing sound level. The slope of performance decline was 90dB/octave, consistent with the low-frequency slope of the cochlear excitation pattern. Fine-structure ITD sensitivity near 1400Hz may be conveyed primarily by "off-frequency" activation of neurons tuned to lower frequencies near 700Hz. Physiologically, this could be realized by having neurons sensitive to fine-structure ITD up to only about 700Hz. A more extreme model would have only a single narrow channel near 700Hz 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.

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.

Objective measure of binaural processing: acoustic change complex in response to interaural phase differences

Combining cochlear implants with binaural acoustic hearing via preserved hearing in the implanted ear(s) if commonly referred to as combined electric and acoustic stimulation (EAS). EAS fittings can provide patients with significant benefit for speech recognition in complex noise, perceived listening difficulty, and horizontal-plane localization as compared to traditional bimodal hearing conditions with contralateral and monaural acoustic hearing. However, EAS benefit varies across patients and the degree of benefit is not reliably related to the underlying audiogram. Previous research has indicated that EAS benefit for speech recognition in complex listening scenarios and localization is significantly correlated with the patients’ binaural cue sensitivity, namely interaural time differences (ITD). In the context of pure tones, interaural phase differences (IPD) and ITD can be understood as two perspectives on the same phenomenon. Through simple mathematical conversion, one can be transformed into the other, illustrating their inherent interrelation for spatial hearing abilities. However, assessing binaural cue sensitivity is not part of a clinical assessment battery as psychophysical tasks are time consuming, require training to achieve performance asymptote, and specialized programming and software all of which render this clinically unfeasible. In this study, we investigated the possibility of using an objective measure of binaural cue sensitivity by the acoustic change complex (ACC) via imposition of an IPD of varying degrees at stimulus midpoint. Ten adult listeners with normal hearing were assessed on tasks of behavioral and objective binaural cue sensitivity for carrier frequencies of 250 and 1000 Hz. Results suggest that 1) ACC amplitude increases with IPD; 2) ACC-based IPD sensitivity for 250 Hz is significantly correlated with behavioral ITD sensitivity; 3) Participants were more sensitive to IPDs at 250 Hz as compared to 1000 Hz. Thus, this objective measure of IPD sensitivity may hold clinical application for pre- and post-operative assessment for individuals meeting candidacy indications for cochlear implantation with low-frequency acoustic hearing preservation as this relatively quick and objective measure may provide clinicians with information identifying patients most likely to derive benefit from EAS technology.

Interaural frequency mismatch jointly modulates neural brainstem binaural interaction and behavioral interaural time difference sensitivity in humans

The binaural interaction component (BIC) of the auditory brainstem response (ABR) is the difference obtained after subtracting the sum of right and left ear ABRs from binaurally evoked ABRs. The BIC has attracted interest as a biomarker of binaural processing abilities. Best binaural processing is presumed to require spectrally-matched inputs at the two ears, but peripheral pathology and/or impacts of hearing devices can lead to mismatched inputs. Such mismatching can degrade behavioral sensitivity to interaural time difference (ITD) cues, but might be detected using the BIC. Here, we examine the effect of interaural frequency mismatch (IFM) on BIC and behavioral ITD sensitivity in audiometrically normal adult human subjects (both sexes). Binaural and monaural ABRs were recorded and BICs computed from subjects in response to narrowband tones. Left ear stimuli were fixed at 4000 Hz while right ear stimuli varied over a ∼2-octave range (re: 4000 Hz). Separately, subjects performed psychophysical lateralization tasks using the same stimuli to determine ITD discrimination thresholds jointly as a function of IFM and sound level. Results demonstrated significant effects of IFM on BIC amplitudes, with lower amplitudes in mismatched conditions than frequency-matched. Behavioral ITD discrimination thresholds were elevated at mismatched frequencies and lower sound levels, but also more sharply modulated by IFM at lower sound levels. Combinations of ITD, IFM and overall sound level that resulted in fused and lateralized percepts were bound by the empirically-measured BIC, and also by model predictions simulated using an established computational model of the brainstem circuit thought to generate the BIC.

Pitch discrimination in electric hearing with inconsistent and consistent amplitude-modulation and inter-pulse rate cues.

Users of cochlear implants (CIs) struggle in situations that require selective hearing to focus on a target source while ignoring other sources. One major reason for that is the limited access to timing cues such as temporal pitch or interaural time differences (ITDs). Various approaches to improve timing-cue sensitivity while maintaining speech understanding have been proposed, among them inserting extra pulses with short inter-pulse intervals (SIPIs) into amplitude-modulated (AM) high-rate pulse trains. Indeed, SIPI rates matching the naturally occurring AM rates improve pitch discrimination. For ITD, however, low SIPI rates are required, potentially mismatching the naturally occurring AM rates and thus creating unknown pitch effects. In this study, we investigated the perceptual contribution of AM and SIPI rate to pitch discrimination in five CI listeners and with two AM depths (0.1 and 0.5). Our results show that the SIPI-rate cue generally dominated the percept for both consistent and inconsistent cues. When tested with inconsistent cues, also the AM rate contributed, however, at the large AM depth only. These findings have implications when aiming at jointly improving temporal-pitch and ITD sensitivity in a future mixed-rate stimulation approach.

Interaural time difference sensitivity under binaural cochlear implant stimulation persists at high pulse rates up to 900 pps

Spatial hearing remains one of the major challenges for bilateral cochlear implant (biCI) users, and early deaf patients in particular are often completely insensitive to interaural time differences (ITDs) delivered through biCIs. One popular hypothesis is that this may be due to a lack of early binaural experience. However, we have recently shown that neonatally deafened rats fitted with biCIs in adulthood quickly learn to discriminate ITDs as well as their normal hearing litter mates, and perform an order of magnitude better than human biCI users. Our unique behaving biCI rat model allows us to investigate other possible limiting factors of prosthetic binaural hearing, such as the effect of stimulus pulse rate and envelope shape. Previous work has indicated that ITD sensitivity may decline substantially at the high pulse rates often used in clinical practice. We therefore measured behavioral ITD thresholds in neonatally deafened, adult implanted biCI rats to pulse trains of 50, 300, 900 and 1800 pulses per second (pps), with either rectangular or Hanning window envelopes. Our rats exhibited very high sensitivity to ITDs at pulse rates up to 900 pps for both envelope shapes, similar to those in common clinical use. However, ITD sensitivity declined to near zero at 1800 pps, for both Hanning and rectangular windowed pulse trains. Current clinical cochlear implant (CI) processors are often set to pulse rates ≥ 900 pps, but ITD sensitivity in human CI listeners has been reported to decline sharply above ~ 300 pps. Our results suggest that the relatively poor ITD sensitivity seen at > 300 pps in human CI users may not reflect the hard upper limit of biCI ITD performance in the mammalian auditory pathway. Perhaps with training or better CI strategies good binaural hearing may be achievable at pulse rates high enough to allow good sampling of speech envelopes while delivering usable ITDs.

Is the rapid decline in interaural time difference sensitivity above 700 Hz explained by downward spread of excitation into the frequency dominant region?

Low-frequency temporal-fine-structure interaural time differences (ITDs) are so important that they almost completely dominate localization of broadband sounds. This “Dominant Region” occurs at ∼700 Hz and also coincides with best ITD discrimination sensitivity. Above this frequency range, ITD sensitivity worsens, until ∼1400 Hz where it rapidly becomes impossible. Explaining how listeners’ ITD sensitivity transitions from best sensitivity to impossible to detect within ∼1 octave challenges models of ITD processing. One possible explanation is that listeners continue to use binaural information from the Dominant Region even for higher-frequency pure tones. Within this explanation, the steep decline in ITD sensitivity reflects the high-frequency limb of peripheral frequency tuning for neurons within the Dominant Region (i.e., downward spread of excitation). To test this hypothesis, we measured the upper frequency limit of ITD processing as a function of stimulus level. Normal-hearing listeners were tested on a left-right ITD discrimination task. Stimuli were pure tones with 500–1500Hz frequencies and 10–50 dB sensation levels. It was hypothesized that the upper frequency limit of ITD processing would decrease in frequency with decreasing level, which would indicate that the downward spread of excitation into the Dominant Region at least partially explains the rapid decline in ITD sensitivity.

Temporal hyper-precision of brainstem neurons alters spatial sensitivity of binaural auditory processing with cochlear implants.

The ability to localize a sound source in complex environments is essential for communication and navigation. Spatial hearing relies predominantly on the comparison of differences in the arrival time of sound between the two ears, the interaural time differences (ITDs). Hearing impairments are highly detrimental to sound localization. While cochlear implants (CIs) have been successful in restoring many crucial hearing capabilities, sound localization via ITD detection with bilateral CIs remains poor. The underlying reasons are not well understood. Neuronally, ITD sensitivity is generated by coincidence detection between excitatory and inhibitory inputs from the two ears performed by specialized brainstem neurons. Due to the lack of electrophysiological brainstem recordings during CI stimulation, it is unclear to what extent the apparent deficits are caused by the binaural comparator neurons or arise already on the input level. Here, we use a bottom-up approach to compare response features between electric and acoustic stimulation in an animal model of CI hearing. Conducting extracellular single neuron recordings in gerbils, we find severe hyper-precision and moderate hyper-entrainment of both the excitatory and inhibitory brainstem inputs to the binaural comparator neurons during electrical pulse-train stimulation. This finding establishes conclusively that the binaural processing stage must cope with highly altered input statistics during CI stimulation. To estimate the consequences of these effects on ITD sensitivity, we used a computational model of the auditory brainstem. After tuning the model parameters to match its response properties to our physiological data during either stimulation type, the model predicted that ITD sensitivity to electrical pulses is maintained even for the hyper-precise inputs. However, the model exhibits severely altered spatial sensitivity during electrical stimulation compared to acoustic: while resolution of ITDs near midline was increased, more lateralized adjacent source locations became inseparable. These results directly resemble recent findings in rodent and human CI listeners. Notably, decreasing the phase-locking precision of inputs during electrical stimulation recovered a wider range of separable ITDs. Together, our findings suggest that a central problem underlying the diminished ITD sensitivity in CI users might be the temporal hyper-precision of inputs to the binaural comparator stage induced by electrical stimulation.

Neonatal Deafening Selectively Degrades the Sensitivity to Interaural Time Differences of Electrical Stimuli in Low-Frequency Pathways in Rats.

We examined the effect of neonatal deafening on frequency-specific pathways for processing of interaural time differences (ITDs) in cochlear-implant stimuli. Animal studies have demonstrated differences in neural ITD sensitivity in the inferior colliculus (IC) depending on the intracochlear location of intracochlear stimulating electrodes. We used neonatally deafened (ND) rats of both sexes and recorded the responses of single neurons in the IC to electrical stimuli with ITDs delivered to the apical or basal cochlea and compared them with acutely deafened (AD) rats of both sexes with normal hearing (NH) during development. We found that neonatal deafness significantly impacted the ITD sensitivity and the ITD tuning patterns restricted to apically driven IC neurons. In ND rats, the ITD sensitivity of apically driven neurons is reduced to values similar to basally driven neurons. The prevalence of ITD-sensitive apical neurons with a peak-shaped ITD tuning curve, which may reflect predominant input from the medial superior olivary (MSO) complex, in ND rats was diminished compared with that in AD rats (67%, AD vs 40%, ND). Conversely, monotonic-type responses rarely occurred in AD rats (14%) but were approximately equally as prevalent as peak-type tuning curves in ND rats (42%). Nevertheless, in ND rats, the ITD at the maximum slope of the ITD tuning curve was still more concentrated within the physiological ITD range in apically driven than in basally driven neurons. These results indicate that the development of high ITD sensitivity processed by low-frequency pathways depends on normal auditory experience and associated biases in ITD tuning strategies.

Sensitivity to interaural level and time differences in individuals with autism spectrum disorder

Individuals with autism spectrum disorders (ASD) are reported to exhibit degraded performance in sound localization. This study investigated whether the sensitivity to the interaural level differences (ILDs) and interaural time differences (ITDs), major cues for horizontal sound localization, are affected in ASD. Thresholds for discriminating the ILD and ITD were measured for adults with ASD and age- and IQ-matched controls in a lateralization experiment. Results show that the ASD group exhibited higher ILD and ITD thresholds than the control group. Moreover, there was a significant diversity of ITD sensitivity in the ASD group, and it contained a larger proportion of participants with poor ITD sensitivity than the control group. The current study suggests that deficits in relatively low-level processes in the auditory pathway are implicated in degraded performance of sound localization in individuals with ASD. The results are consistent with the structural abnormalities and great variability in the morphology in the brainstem reported by neuroanatomical studies of ASD.

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.

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.

Predicting the Influence of Axon Myelination on Sound Localization Precision Using a Spiking Neural Network Model of Auditory Brainstem.

Spatial hearing allows animals to rapidly detect and localize auditory events in the surrounding environment. The auditory brainstem plays a central role in processing and extracting binaural spatial cues through microsecond-precise binaural integration, especially for detecting interaural time differences (ITDs) of low-frequency sounds at the medial superior olive (MSO). A series of mechanisms exist in the underlying neural circuits for preserving accurate action potential timing across multiple fibers, synapses and nuclei along this pathway. One of these is the myelination of afferent fibers that ensures reliable and temporally precise action potential propagation in the axon. There are several reports of fine-tuned myelination patterns in the MSO circuit, but how specifically myelination influences the precision of sound localization remains incompletely understood. Here we present a spiking neural network (SNN) model of the Mongolian gerbil auditory brainstem with myelinated axons to investigate whether different axon myelination thicknesses alter the sound localization process. Our model demonstrates that axon myelin thickness along the contralateral pathways can substantially modulate ITD detection. Furthermore, optimal ITD sensitivity is reached when the MSO receives contralateral inhibition via thicker myelinated axons compared to contralateral excitation, a result that is consistent with previously reported experimental observations. Our results suggest specific roles of axon myelination for extracting temporal dynamics in ITD decoding, especially in the pathway of the contralateral inhibition.

The Relationship Between Interaural Insertion-Depth Differences, Scalar Location, and Interaural Time-Difference Processing in Adult Bilateral Cochlear-Implant Listeners.

Sensitivity to interaural time differences (ITDs) in acoustic hearing involves comparison of interaurally frequency-matched inputs. Bilateral cochlear-implant arrays are, however, only approximately aligned in angular insertion depth and scalar location across the cochleae. Interaural place-of-stimulation mismatch therefore has the potential to impact binaural perception. ITD left-right discrimination thresholds were examined in 23 postlingually-deafened adult bilateral cochlear-implant listeners, using low-rate constant-amplitude pulse trains presented via direct stimulation to single electrodes in each ear. Angular insertion depth and scalar location measured from computed-tomography (CT) scans were used to quantify interaural mismatch, and their association with binaural performance was assessed. Number-matched electrodes displayed a median interaural insertion-depth mismatch of 18° and generally yielded best or near-best ITD discrimination thresholds. Two listeners whose discrimination thresholds did not show this pattern were confirmed via CT to have atypical array placement. Listeners with more number-matched electrode pairs located in the scala tympani displayed better thresholds than listeners with fewer such pairs. ITD tuning curves as a function of interaural electrode separation were broad; bandwidths at twice the threshold minimum averaged 10.5 electrodes (equivalent to 5.9 mm for a Cochlear-brand pre-curved array). Larger angular insertion-depth differences were associated with wider bandwidths. Wide ITD tuning curve bandwidths appear to be a product of both monopolar stimulation and angular insertion-depth mismatch. Cases of good ITD sensitivity with very wide bandwidths suggest that precise matching of insertion depth is not critical for discrimination thresholds. Further prioritizing scala tympani location at implantation should, however, benefit ITD sensitivity.

Effects of Hearing Loss on Interaural Time Difference Sensitivity at Low and High Frequencies.

While many studies have reported a loss of sensitivity to interaural time differences (ITDs) carried in the fine structure of low-frequency signals for listeners with hearing loss, relatively few data are available on the perception of ITDs carried in the envelope of high-frequency signals in this population. The relevant studies found stronger effects of hearing loss at high frequencies than at low frequencies in most cases, but small subject numbers and several confounding effects prevented strong conclusions from being drawn. In the present study, we revisited this question while addressing some of the issues identified in previous studies. Participants were ten young adults with normal hearing (NH) and twenty adults with sensorineural hearing impairment (HI) spanning a range of ages. ITD discrimination thresholds were measured for octave-band-wide “rustle” stimuli centered at 500 Hz or 4000 Hz, which were presented at 20 or 40 dB sensation level. Broadband rustle stimuli and 500-Hz pure-tone stimuli were also tested. Thresholds were poorer on average for the HI group than the NH group. The ITD deficit, relative to the NH group, was similar at low and high frequencies for most HI participants. For a small number of participants, however, the deficit was strongly frequency-dependent. These results provide new insights into the binaural perception of complex sounds and may inform binaural models that incorporate effects of hearing loss.

Interaural-time-difference thresholds for broad band-limited pulses are affected by relative bandwidth not temporal envelope sharpness.

Humans are sensitive to interaural time differences (ITDs) conveyed by slow modulations on high-frequency carrier signals. Sensitivity appears to be affected by temporal envelope sharpness, but it is unclear if there is a limit to which sharpness affects sensitivity. Pulse trains were varied in relative bandwidth (re: critical bandwidths) and center frequency. ITD sensitivity increased with increasing bandwidth. There was no effect of center frequency when relative bandwidths were analyzed, suggesting that the temporal envelope sharpness (concomitantly absolute bandwidth in Hz) did not affect performance. Rather, sensitivity was most easily explained by recruitment of additional auditory channels.

Effects of place of stimulation on the interaural time difference sensitivity in bilateral electrical intracochlear stimulations: Neurophysiological study in a rat model.

We examined the sensitivity of the neurons in the inferior colliculus (IC) in male and female rats to the interaural time differences (ITDs) conveyed in electrical pulse trains. Using bipolar pairs of electrodes that selectively activate the auditory nerve fibers at different intracochlear locations, we assessed whether the responses to electrical stimulation with ITDs in different frequency regions were processed differently. Most well-isolated single units responded to the electrical stimulation in only one of the apical or basal cochlear regions, and they were classified as either apical or basal units. Regardless of the cochlear stimulating location, more than 70% of both apical and basal units were sensitive to ITDs of electrical stimulation. However, the pulse rate dependence of neural ITD sensitivity differed significantly depending on the location of the stimulation. Moreover, ITD discrimination thresholds and the relative incidence of ITD tuning type markedly differed between units activated by apical and basal stimulations. With apical stimulation, IC neurons had a higher incidence of peak-type ITD function, which mostly exhibited the steepest position of the tuning curve within the rat's physiological ITD range of ±160μs and, accordingly, had better ITD discrimination thresholds than those with basal stimulation. These results support the idea that ITD processing in the IC might be determined by functionally segregated frequency-specific pathways from the cochlea to the auditory midbrain.

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.

Are there binaural consequences of speech envelope enhancement?

Fluctuations in the amplitude envelope play a critical role in the spatial perception of sounds. For example, listeners place increased perceptual weight on binaural cues occurring at onsets, and the steepness of onset slopes can influence binaural sensitivity. For speech stimuli, we recently showed that the temporal weighting of binaural cues is non-uniform and depends in part on the detailed behavior or the envelope. Here, we examined the potential binaural consequences of two monaural speech enhancement strategies that operate by altering the speech envelope: consonant enhancement (CE) and envelope expansion (EE). While CE and EE improve speech intelligibility under certain conditions, their effect on binaural perception has not been considered. We measured sensitivity to interaural time differences (ITDs) carried by single-word stimuli that were either unprocessed or processed with CE/EE. Thresholds were measured in quiet, in the presence of multi-talker babble, or for a vocoded condition designed to limit the availability of fine-structure ITDs. For ten listeners with normal hearing, while sensitivity depended on the specific word token, there were no systematic effects of speech enhancement on ITD sensitivity. Ongoing work is considering supra-threshold binaural tasks and listeners with hearing loss who have reduced spatial sensitivity. [Work supported by NIDCD.]

A rapid measurement for the effect of interaural mismatch on interaural time difference sensitivity

Cochlear implant (CI) users often have difficulties utilizing binaural cues such as interaural time difference (ITD) because of a mismatch in terms of the cochlear stimulation location in each ear. Methods for measuring this mismatch often rely on psychophysical approaches that can require hours of testing. The goal of this study was to evaluate a rapid method for measuring the effects of mismatch on ITD sensitivity. Seven normal hearing listeners were tested with a vocoder that simulated current spread. Participants were tested with a traditional adaptive ITD protocol and a rapid descending ITD protocol with fixed ITDs starting at 2000 μs and decreasing in two dB steps to approximately 50 μs. In both protocols, four stimuli were presented. The first and last stimuli always had an ITD of 0 μs. One of the two center stimuli had a non-zero ITD. Participants had to identify which had the non-zero ITD. The carriers were altered to simulate varying mismatch across ears. The preliminary results indicated a similar effect of mismatch for both protocols, with considerably shorter testing time for the descending ITD protocol. This suggests that the rapid descending ITD protocol is a valid method for assessing the effects of interaural mismatch.