The effect of interaural fluctuation rate on correlation change discrimination

Review of Binaural Processing With Asymmetrical Hearing Outcomes in Patients With Bilateral Cochlear Implants

Bilateral cochlear implants (BiCIs) result in several benefits, including improvements in speech understanding in noise and sound source localization. However, the benefit bilateral implants provide among recipients varies considerably across individuals. Here we consider one of the reasons for this variability: difference in hearing function between the two ears, that is, interaural asymmetry. Thus far, investigations of interaural asymmetry have been highly specialized within various research areas. The goal of this review is to integrate these studies in one place, motivating future research in the area of interaural asymmetry. We first consider bottom-up processing, where binaural cues are represented using excitation-inhibition of signals from the left ear and right ear, varying with the location of the sound in space, and represented by the lateral superior olive in the auditory brainstem. We then consider top-down processing via predictive coding, which assumes that perception stems from expectations based on context and prior sensory experience, represented by cascading series of cortical circuits. An internal, perceptual model is maintained and updated in light of incoming sensory input. Together, we hope that this amalgamation of physiological, behavioral, and modeling studies will help bridge gaps in the field of binaural hearing and promote a clearer understanding of the implications of interaural asymmetry for future research on optimal patient interventions.

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.

Large group differences in binaural sensitivity are represented in preattentive responses from auditory cortex

Correlated sounds presented to two ears are perceived as compact and centrally lateralized, whereas decorrelation between ears leads to intracranial image widening. Though most listeners have fine resolution for perceptual changes in interaural correlation (IAC), some investigators have reported large variability in IAC thresholds, and some normal-hearing listeners even exhibit seemingly debilitating IAC thresholds. It is unknown whether or not this variability across individuals and outlier manifestations are a product of task difficulty, poor training, or a neural deficit in the binaural auditory system. The purpose of this study was first to identify listeners with normal and abnormal IAC resolution, second to evaluate the neural responses elicited by IAC changes, and third to use a well-established model of binaural processing to determine a potential explanation for observed individual variability. Nineteen subjects were enrolled in the study, eight of whom were identified as poor performers in the IAC-threshold task. Global scalp responses (N1 and P2 amplitudes of an auditory change complex) in the individuals with poor IAC behavioral thresholds were significantly smaller than for listeners with better IAC resolution. Source-localized evoked responses confirmed this group effect in multiple subdivisions of the auditory cortex, including Heschl's gyrus, planum temporale, and the temporal sulcus. In combination with binaural modeling results, this study provides objective electrophysiological evidence of a binaural processing deficit linked to internal noise, that corresponds to very poor IAC thresholds in listeners that otherwise have normal audiometric profiles and lack spatial hearing complaints.NEW & NOTEWORTHY Group differences in the perception of interaural correlation (IAC) were observed in human adults with normal audiometric sensitivity. These differences were reflected in cortical-evoked activity measured via electroencephalography (EEG). For some participants, weak representation of the binaural cue at the cortical level in preattentive N1-P2 cortical responses may be indicative of a potential processing deficit. Such a deficit may be related to a poorly understood condition known as hidden hearing loss.

Impaired interaural correlation processing in people with schizophrenia

Detection of transient changes in interaural correlation is based on the temporal precision of the central representations of acoustic signals. Whether schizophrenia impairs the temporal precision in the interaural correlation process is not clear. In both participants with schizophrenia and matched healthy-control participants, this study examined the detection of a break in interaural correlation (BIC, a change in interaural correlation from 1 to 0 and back to 1), including the longest interaural delay at which a BIC was just audible, representing the temporal extent of the primitive auditory memory (PAM). Moreover, BIC-induced electroencephalograms (EEGs) and the relationships between the early binaural psychoacoustic processing and higher cognitive functions, which were assessed by the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS), were examined. The results showed that compared to healthy controls, participants with schizophrenia exhibited poorer BIC detection, PAM and RBANS score. Both the BIC-detection accuracy and the PAM extent were correlated with the RBANS score. Moreover, participants with schizophrenia showed weaker BIC-induced N1-P2 amplitude which was correlated with both theta-band power and inter-trial phase coherence. These results suggested that schizophrenia impairs the temporal precision of the central representations of acoustic signals, affecting both interaural correlation processing and higher-order cognitions.

Dichotic listening performance with cochlear-implant simulations of ear asymmetry is consistent with difficulty ignoring clearer speech

There are an increasing number of bilateral and single-sided-deafness cochlear-implant (CI) users who hope to achieve improved spatial-hearing abilities through access to sound in both ears. It is, however, unclear how speech is processed when inputs are functionally asymmetrical, which may have an impact on spatial-hearing abilities. Therefore, functionally asymmetrical hearing was controlled and parametrically manipulated using a channel vocoder as a CI simulation. In Experiment 1, normal-hearing (NH) listeners performed a dichotic listening task (i.e., selective attention to one ear, ignoring the other) using asymmetrical signal degradation. Spectral resolution varied independently in each ear (4, 8, 16 channels, and unprocessed control). Performance decreased with decreasing resolution in the target ear and increasing resolution in the interferer ear. In Experiment 2, these results were replicated using a divided attention task (attend to both ears, report one after sentence completion) in both NH and bilateral CI listeners, although overall performance was lower than in Experiment 1. In Experiment 3, frequency-to-place mismatch simulated shallow CI insertion depths (0, 3, 6-mm shifts, and unprocessed control). Performance mostly decreased with increasing shift in the target ear and decreasing shift in the interferer ear; however, performance nonmonotonicities occurred. The worst performance occurred when the shift matched across ears, suggesting that pitch similarity increases difficulty. The results show that it is more difficult to attend an ear that is relatively degraded or distorted, which may set spatial-hearing limitations for CI users when trying to attend to a target in complex auditory scenes.

Counting or discriminating the number of voices to assess binaural fusion with single-sided vocoders

For single-sided deafness cochlear-implant (SSD-CI) listeners, different peripheral representations for electric versus acoustic stimulation, combined with interaural frequency mismatch, might limit the ability to perceive bilaterally presented speech as a single voice. The assessment of binaural fusion often relies on subjective report, which requires listeners to have some understanding of the perceptual phenomenon of object formation. Two experiments explored whether binaural fusion could instead be assessed using judgments of the number of voices in a mixture. In an SSD-CI simulation, normal-hearing listeners were presented with one or two "diotic" voices (i.e., unprocessed in one ear and noise-vocoded in the other) in a mixture with additional monaural voices. In experiment 1, listeners reported how many voices they heard. Listeners generally counted the diotic speech as two separate voices, regardless of interaural frequency mismatch. In experiment 2, listeners identified which of two mixtures contained diotic speech. Listeners performed significantly better with interaurally frequency-matched than with frequency-mismatched stimuli. These contrasting results suggest that listeners experienced partial fusion: not enough to count the diotic speech as one voice, but enough to detect its presence. The diotic-speech detection task (experiment 2) might provide a tool to evaluate fusion and optimize frequency mapping for SSD-CI patients.

The effect of envelope modulations on binaural processing

An experiment was performed with 10 young normal-hearing listeners that attempted to determine if envelope modulations affected binaural processing in bandlimited pulse trains. Listeners detected an interaurally out-of-phase carrier pulse train in the presence of different amplitude modulations. The peaks of the pulses were constant (called "flat" or F), followed envelope modulations from an interaurally correlated 50-Hz bandwidth noise (called CM), or followed modulations from an interaurally uncorrelated noise (called UM). The pulse rate was varied from 50 to 500 pulses per second (pps) and the center frequency (CF) was 4 or 8 kHz. It was hypothesized that CM would cause no change or an increase in performance compared to F; UM would cause a decrease because of the blurring of the binaural detection cue. There was a small but significant decrease from F to CM (inconsistent with the hypothesis) and a further decrease from CM to UM (consistent with the hypothesis). Critically, there was a significant envelope by rate interaction caused by a decrease from F to CM for the 200-300 pps rates. The data can be explained by a subject-based factor, where some listeners experienced interaural envelope decorrelation when the sound was encoded by the auditory system that reduced performance when the modulations were present. Since the decrease in performance between F and CM conditions was small, it seems that most young normal-hearing listeners have very similar encoding of modulated stimuli across the ears. This type of task, when further optimized, may be able to assess if hearing-impaired populations experience interaural decorrelation from encoding modulated stimuli and therefore could help better understand the limited spatial hearing in populations like cochlear-implant users.

The Effect of Simulated Interaural Frequency Mismatch on Speech Understanding and Spatial Release From Masking

OBJECTIVE

The binaural-hearing system interaurally compares inputs, which underlies the ability to localize sound sources and to better understand speech in complex acoustic environments. Cochlear implants (CIs) are provided in both ears to increase binaural-hearing benefits; however, bilateral CI users continue to struggle with understanding speech in the presence of interfering sounds and do not achieve the same level of spatial release from masking (SRM) as normal-hearing listeners. One reason for diminished SRM in CI users could be that the electrode arrays are inserted at different depths in each ear, which would cause an interaural frequency mismatch. Because interaural frequency mismatch diminishes the salience of interaural differences for relatively simple stimuli, it may also diminish binaural benefits for spectral-temporally complex stimuli like speech. This study evaluated the effect of simulated frequency-to-place mismatch on speech understanding and SRM.

DESIGN

Eleven normal-hearing listeners were tested on a speech understanding task. There was a female target talker who spoke five-word sentences from a closed set of words. There were two interfering male talkers who spoke unrelated sentences. Nonindividualized head-related transfer functions were used to simulate a virtual auditory space. The target was presented from the front (0°), and the interfering speech was either presented from the front (colocated) or from 90° to the right (spatially separated). Stimuli were then processed by an eight-channel vocoder with tonal carriers to simulate aspects of listening through a CI. Frequency-to-place mismatch ("shift") was introduced by increasing the center frequency of the synthesis filters compared with the corresponding analysis filters. Speech understanding was measured for different shifts (0, 3, 4.5, and 6 mm) and target-to-masker ratios (TMRs: +10 to -10 dB). SRM was calculated as the difference in the percentage of correct words for the colocated and separated conditions. Two types of shifts were tested: (1) bilateral shifts that had the same frequency-to-place mismatch in both ears, but no interaural frequency mismatch, and (2) unilateral shifts that produced an interaural frequency mismatch.

RESULTS

For the bilateral shift conditions, speech understanding decreased with increasing shift and with decreasing TMR, for both colocated and separate conditions. There was, however, no interaction between shift and spatial configuration; in other words, SRM was not affected by shift. For the unilateral shift conditions, speech understanding decreased with increasing interaural mismatch and with decreasing TMR for both the colocated and spatially separated conditions. Critically, there was a significant interaction between the amount of shift and spatial configuration; in other words, SRM decreased for increasing interaural mismatch.

CONCLUSIONS

A frequency-to-place mismatch in one or both ears resulted in decreased speech understanding. SRM, however, was only affected in conditions with unilateral shifts and interaural frequency mismatch. Therefore, matching frequency information between the ears provides listeners with larger binaural-hearing benefits, for example, improved speech understanding in the presence of interfering talkers. A clinical procedure to reduce interaural frequency mismatch when programming bilateral CIs may improve benefits in speech segregation that are due to binaural-hearing abilities.

Relating interaural difference sensitivities for several parameters measured in normal-hearing and hearing-impaired listeners

Just-noticeable differences (JNDs) in interaural time delay (ITD), interaural level difference (ILD), and interaural cross-correlation (ICC) were measured with low- and high-frequency noise bands over multiple sessions for 10 normal-hearing (NH) and 11 hearing-impaired (HI) listeners. Individual subject thresholds tended to improve with training then stabilize. Measured JNDs varied over these experienced listeners, for both subject groups and all tasks. Group JNDs were seldom predictable from hearing level. Individual listeners' JNDs were highly correlated across frequency for each task and group, except for ICC in the HI listeners. Further, ITD JNDs almost always significantly correlated with ILD JNDs within a group. Finally, although the ICC JNDs always significantly correlated with the ITD or ILD JNDs for the NH listeners, they often did not for the HI listeners. These findings suggest that little information about binaural sensitivity is added for NH listeners with multiple ITD, ILD, and ICC measures. For HI listeners, however, while ITD and ILD measures are well correlated, information is added with ICC measures. In general, the results suggest that less information is added with JND measures for NH listeners (15 significant correlations) than for HI listeners (six significant correlations).

Auditory Model-Based Sound Direction Estimation With Bilateral Cochlear Implants

Users of bilateral cochlear implants (CIs) show above-chance performance in localizing the source of a sound in the azimuthal (horizontal) plane; although localization errors are far worse than for normal-hearing listeners, they are considerably better than for CI listeners with only one implant. In most previous studies, subjects had access to interaural level differences and to interaural time differences conveyed in the temporal envelope. Here, we present a binaural model that predicts the azimuthal direction of sound arrival from a two-channel input signal as it is received at the left and right CI processor. The model includes a replication of a clinical speech-coding strategy, a model of the electrode-nerve interface and binaural brainstem neurons, and three different prediction stages that are trained to map the neural response rate to an azimuthal angle. The model is trained and tested with various noise and speech stimuli created by means of virtual acoustics. Localization error patterns of the model match experimental data and are explicable largely in terms of the nonmonotonic relationship between interaural level difference and azimuthal angle.

Untrained listeners experience difficulty detecting interaural correlation changes in narrowband noises

Interaural correlation change detection was measured in untrained normal-hearing listeners. Narrowband (10-Hz) noises were varied by center frequency (CF; 500 or 4000 Hz) and diotic level roving (absent or present). For the 500-Hz CF, 96% of listeners could achieve threshold (79.4% correct at the easiest testing level) if roving was absent, but only 36% of listeners could if level roving was present. No one could achieve threshold at the 4000-Hz CF, unlike trained listeners in the literature. The results raise questions about how individual differences affect learning and generalization of monaural and binaural cues related to interaural correlation detection.

Interaural envelope correlation change discrimination in bilateral cochlear implantees: effects of mismatch, centering, and onset of deafness

Bilateral cochlear implant (CI) listeners can perform binaural tasks, but they are typically worse than normal-hearing (NH) listeners. To understand why this difference occurs and the mechanisms involved in processing dynamic binaural differences, interaural envelope correlation change discrimination sensitivity was measured in real and simulated CI users. In experiment 1, 11 CI (eight late deafened, three early deafened) and eight NH listeners were tested in an envelope correlation change discrimination task. Just noticeable differences (JNDs) were best for a matched place-of-stimulation and increased for an increasing mismatch. In experiment 2, attempts at intracranially centering stimuli did not produce lower JNDs. In experiment 3, the percentage of correct identifications of antiphasic carrier pulse trains modulated by correlated envelopes was measured as a function of mismatch and pulse rate. Sensitivity decreased for increasing mismatch and increasing pulse rate. The experiments led to two conclusions. First, envelope correlation change discrimination necessitates place-of-stimulation matched inputs. However, it is unclear if previous experience with acoustic hearing is necessary for envelope correlation change discrimination. Second, NH listeners presented with CI simulations demonstrated better performance than real CI listeners. If the simulations are realistic representations of electrical stimuli, real CI listeners appear to have difficulty processing interaural information in modulated signals.

Sensitivity to interaural envelope correlation changes in bilateral cochlear-implant users

Provision of bilateral cochlear implants (CIs) to people who are deaf is partially justified by improved abilities to understand speech in noise when comparing bilateral vs unilateral listening conditions. However, bilateral CI listeners generally show only monaural head shadow with little improvement in speech understanding due to binaural unmasking. Sensitivity to change in interaural envelope correlation, which is related to binaural speech unmasking, was investigated. Bilateral CI users were tested with bilaterally synchronized processors at single, pitch-matched electrode pairs. First, binaural masking level differences (BMLDs) were measured using 1000 pulse-per-second (pps) carriers, yielding BMLDs of 11.1 ± 6.5 and 8.5 ± 4.2 dB for 10- and 50-Hz bandwidth masking noises, respectively. Second, envelope correlation change just-noticeable differences (JNDs) were measured. Stimuli presented at 1000 pps yielded lower JNDs than those presented at 100 pps. Furthermore, perfectly correlated reference stimuli produced lower JNDs than uncorrelated references, and uncorrelated references generally produced immeasurable JNDs. About 25% of JNDs measured in the CI listeners were in the range of JNDs observed in normal-hearing listeners presented CI simulations. In conclusion, CI listeners can perceive changes in interaural envelope correlation, but the poor performance may be a major limiting factor in binaural unmasking tested to date in realistic listening environments.

Comparing sound localization deficits in bilateral cochlear-implant users and vocoder simulations with normal-hearing listeners

Bilateral cochlear-implant (BiCI) users are less accurate at localizing free-field (FF) sound sources than normal-hearing (NH) listeners. This performance gap is not well understood but is likely due to a combination of compromises in acoustic signal representation by the two independent speech processors and neural degradation of auditory pathways associated with a patient's hearing loss. To exclusively investigate the effect of CI speech encoding on horizontal-plane sound localization, the present study measured sound localization performance in NH subjects listening to vocoder processed and nonvocoded virtual acoustic space (VAS) stimuli. Various aspects of BiCI stimulation such as independently functioning devices, variable across-ear channel selection, and pulsatile stimulation were simulated using uncorrelated noise (Nu), correlated noise (N0), or Gaussian-enveloped tone (GET) carriers during vocoder processing. Additionally, FF sound localization in BiCI users was measured in the same testing environment for comparison. Distinct response patterns across azimuthal locations were evident for both listener groups and were analyzed using a multilevel regression analysis. Simulated implant speech encoding, regardless of carrier, was detrimental to NH localization and the GET vocoder best simulated BiCI FF performance in NH listeners. Overall, the detrimental effect of vocoder processing on NH performance suggests that sound localization deficits may persist even for BiCI patients who have minimal neural degradation associated with their hearing loss and indicates that CI speech encoding plays a significant role in the sound localization deficits experienced by BiCI users.

Binaural hearing with electrical stimulation

Bilateral cochlear implantation is becoming a standard of care in many clinics. While much benefit has been shown through bilateral implantation, patients who have bilateral cochlear implants (CIs) still do not perform as well as normal hearing listeners in sound localization and understanding speech in noisy environments. This difference in performance can arise from a number of different factors, including the areas of hardware and engineering, surgical precision and pathology of the auditory system in deaf persons. While surgical precision and individual pathology are factors that are beyond careful control, improvements can be made in the areas of clinical practice and the engineering of binaural speech processors. These improvements should be grounded in a good understanding of the sensitivities of bilateral CI patients to the acoustic binaural cues that are important to normal hearing listeners for sound localization and speech in noise understanding. To this end, we review the current state-of-the-art in the understanding of the sensitivities of bilateral CI patients to binaural cues in electric hearing, and highlight the important issues and challenges as they relate to clinical practice and the development of new binaural processing strategies. This article is part of a Special Issue entitled <Lasker Award>.

Internalized elevation perception of simple stimuli in cochlear-implant and normal-hearing listeners

In normal-hearing (NH) listeners, elevation perception is produced by the spectral cues imposed by the pinna, head, and torso. Elevation perception in cochlear-implant (CI) listeners appears to be non-existent; this may be a result of poorly encoded spectral cues. In this study, an analog of elevation perception was investigated by having 15 CI and 8 NH listeners report the intracranial location of spectrally simple signals (single-electrode or bandlimited acoustic stimuli, respectively) in both horizontal and vertical dimensions. Thirteen CI listeners and all of the NH listeners showed an association between place of stimulation (i.e., stimulus frequency) and perceived elevation, generally responding with higher elevations for more basal stimulation. This association persisted in the presence of a randomized temporal pitch, suggesting that listeners were not associating pitch with elevation. These data provide evidence that CI listeners might perceive changes in elevation if they were presented stimuli with sufficiently salient elevation cues.