Reduced digit spans and ear dominance using dichotic digits in bimodal cochlear-implant users.

Bimodal stimulation, a cochlear implant (CI) in one ear and a hearing aid (HA) in the other, provides highly asymmetrical inputs. To understand how asymmetry affects perception and memory, forward and backward digit spans were measured in nine bimodal listeners. Spans were unchanged from monotic to diotic presentation; there was an average two-digit decrease for dichotic presentation with some extreme cases of decreases to zero spans. Interaurally asymmetrical decreases were not predicted based on the device or better-functioning ear. Therefore, bimodal listeners can demonstrate a strong ear dominance, diminishing memory recall dichotically even when perception was intact monaurally.

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.

Computed-Tomography Estimates of Interaural Mismatch in Insertion Depth and Scalar Location in Bilateral Cochlear-Implant Users.

HYPOTHESIS: Bilateral cochlear-implant (BI-CI) users will have a range of interaural insertion-depth mismatch because of different array placement or characteristics. Mismatch will be larger for electrodes located near the apex or outside scala tympani, or for arrays that are a mix of precurved and straight types. BACKGROUND: Brainstem superior olivary-complex neurons are exquisitely sensitive to interaural-difference cues for sound localization. Because these neurons rely on interaurally place-of-stimulation-matched inputs, interaural insertion-depth or scalar-location differences for BI-CI users could cause interaural place-of-stimulation mismatch that impairs binaural abilities. METHODS: Insertion depths and scalar locations were calculated from temporal-bone computed-tomography scans for 107 BI-CI users (27 Advanced Bionics, 62 Cochlear, 18 MED-EL). RESULTS: Median interaural insertion-depth mismatch was 23.4 degrees or 1.3 mm. Mismatch in the estimated clinically relevant range expected to impair binaural processing (>75 degrees or 3 mm) occurred for 13 to 19% of electrode pairs overall, and for at least three electrode pairs for 23 to 37% of subjects. There was a significant three-way interaction between insertion depth, scalar location, and array type. Interaural insertion-depth mismatch was largest for apical electrodes, for electrode pairs in two different scala, and for arrays that were both-precurved. CONCLUSION: Average BI-CI interaural insertion-depth mismatch was small; however, large interaural insertion-depth mismatch-with the potential to degrade spatial hearing-occurred frequently enough to warrant attention. For new BICI users, improved surgical techniques to avoid interaural insertion-depth and scalar mismatch are recommended. For existing BI-CI users with interaural insertion-depth mismatch, interaural alignment of clinical frequency tables might reduce negative spatial-hearing consequences.

Single-Sided Deafness Cochlear Implant Sound-Localization Behavior With Multiple Concurrent Sources.

OBJECTIVES: For listeners with one deaf ear and the other ear with normal/near-normal hearing (single-sided deafness [SSD]) or moderate hearing loss (asymmetric hearing loss), cochlear implants (CIs) can improve speech understanding in noise and sound-source localization. Previous SSD-CI localization studies have used a single source with artificial sounds such as clicks or random noise. While this approach provides insights regarding the auditory cues that facilitate localization, it does not capture the complex nature of localization behavior in real-world environments. This study examined SSD-CI sound localization in a complex scenario where a target sound was added to or removed from a mixture of other environmental sounds, while tracking head movements to assess behavioral strategy. DESIGN: Eleven CI users with normal hearing or moderate hearing loss in the contralateral ear completed a sound-localization task in monaural (CI-OFF) and bilateral (CI-ON) configurations. Ten of the listeners were also tested before CI activation to examine longitudinal effects. Two-second environmental sound samples, looped to create 4- or 10-sec trials, were presented in a spherical array of 26 loudspeakers encompassing ±144° azimuth and ±30° elevation at a 1-m radius. The target sound was presented alone (localize task) or concurrently with one or three additional sources presented to different loudspeakers, with the target cued by being added to (Add) or removed from (Rem) the mixture after 6 sec. A head-mounted tracker recorded movements in six dimensions (three for location, three for orientation). Mixed-model regression was used to examine target sound-identification accuracy, localization accuracy, and head movement. Angular and translational head movements were analyzed both before and after the target was switched on or off. RESULTS: Listeners showed improved localization accuracy in the CI-ON configuration, but there was no interaction with test condition and no effect of the CI on sound-identification performance. Although high-frequency hearing loss in the unimplanted ear reduced localization accuracy and sound-identification performance, the magnitude of the CI localization benefit was independent of hearing loss. The CI reduced the magnitude of gross head movements used during the task in the azimuthal rotation and translational dimensions, both while the target sound was present (in all conditions) and during the anticipatory period before the target was switched on (in the Add condition). There was no change in pre- versus post-activation CI-OFF performance. CONCLUSIONS: These results extend previous findings, demonstrating a CI localization benefit in a complex listening scenario that includes environmental and behavioral elements encountered in everyday listening conditions. The CI also reduced the magnitude of gross head movements used to perform the task. This was the case even before the target sound was added to the mixture. This suggests that a CI can reduce the need for physical movement both in anticipation of an upcoming sound event and while actively localizing the target sound. Overall, these results show that for SSD listeners, a CI can improve localization in a complex sound environment and reduce the amount of physical movement used.

Interaural Place-of-Stimulation Mismatch Estimates Using CT Scans and Binaural Perception, But Not Pitch, Are Consistent in Cochlear-Implant Users.

Bilateral cochlear implants (BI-CIs) or a CI for single-sided deafness (SSD-CI; one normally functioning acoustic ear) can partially restore spatial-hearing abilities, including sound localization and speech understanding in noise. For these populations, however, interaural place-of-stimulation mismatch can occur and thus diminish binaural sensitivity that relies on interaurally frequency-matched neurons. This study examined whether plasticity-reorganization of central neural pathways over time-can compensate for peripheral interaural place mismatch. We hypothesized differential plasticity across two systems: none for binaural processing but adaptation for pitch perception toward frequencies delivered by the specific electrodes. Interaural place mismatch was evaluated in 19 BI-CI and 23 SSD-CI human subjects (both sexes) using binaural processing (interaural-time-difference discrimination with simultaneous bilateral stimulation), pitch perception (pitch ranking for single electrodes or acoustic tones with sequential bilateral stimulation), and physical electrode-location estimates from computed-tomography (CT) scans. On average, CT scans revealed relatively little BI-CI interaural place mismatch (26° insertion-angle mismatch) but a relatively large SSD-CI mismatch, particularly at low frequencies (166° for an electrode tuned to 300 Hz, decreasing to 14° at 7000 Hz). For BI-CI subjects, the three metrics were in agreement because there was little mismatch. For SSD-CI subjects, binaural and CT measurements were in agreement, suggesting little binaural-system plasticity induced by mismatch. The pitch measurements disagreed with binaural and CT measurements, suggesting place-pitch plasticity or a procedural bias. These results suggest that reducing interaural place mismatch and potentially improving binaural processing by reprogramming the CI frequency allocation would be better done using CT-scan than pitch information.SIGNIFICANCE STATEMENT Electrode-array placement for cochlear implants (bionic prostheses that partially restore hearing) does not explicitly align neural representations of frequency information. The resulting interaural place-of-stimulation mismatch can diminish spatial-hearing abilities. In this study, adults with two cochlear implants showed reasonable interaural alignment, whereas those with one cochlear implant but normal hearing in the other ear often showed mismatch. In cases of mismatch, binaural sensitivity was best when the same cochlear locations were stimulated in both ears, suggesting that binaural brainstem pathways do not experience plasticity to compensate for mismatch. In contrast, interaurally pitch-matched electrodes deviated from cochlear-location estimates and did not optimize binaural sensitivity. Clinical correction of interaural place mismatch using binaural or computed-tomography (but not pitch) information may improve spatial-hearing benefits.

Dichotic listening performance and effort as a function of spectral resolution and interaural symmetry.

One potential benefit of bilateral cochlear implants is reduced listening effort in speech-on-speech masking situations. However, the symmetry of the input across ears, possibly related to spectral resolution, could impact binaural benefits. Fifteen young adults with normal hearing performed digit recall with target and interfering digits presented to separate ears and attention directed to the target ear. Recall accuracy and pupil size over time (used as an index of listening effort) were measured for unprocessed, 16-channel vocoded, and 4-channel vocoded digits. Recall accuracy was significantly lower for dichotic (with interfering digits) than for monotic listening. Dichotic recall accuracy was highest when the target was less degraded and the interferer was more degraded. With matched target and interferer spectral resolution, pupil dilation was lower with more degradation. Pupil dilation grew more shallowly over time when the interferer had more degradation. Overall, interferer spectral resolution more strongly affected listening effort than target spectral resolution. These results suggest that interfering speech both lowers performance and increases listening effort, and that the relative spectral resolution of target and interferer affect the listening experience. Ignoring a clearer interferer is more effortful.

Aging Effects on Cortical Responses to Tones and Speech in Adult Cochlear-Implant Users.

Age-related declines in auditory temporal processing contribute to speech understanding difficulties of older adults. These temporal processing deficits have been established primarily among acoustic-hearing listeners, but the peripheral and central contributions are difficult to separate. This study recorded cortical auditory evoked potentials from younger to middle-aged (< 65 years) and older (≥ 65 years) cochlear-implant (CI) listeners to assess age-related changes in temporal processing, where cochlear processing is bypassed in this population. Aging effects were compared to age-matched normal-hearing (NH) listeners. Advancing age was associated with prolonged P2 latencies in both CI and NH listeners in response to a 1000-Hz tone or a syllable /da/, and with prolonged N1 latencies in CI listeners in response to the syllable. Advancing age was associated with larger N1 amplitudes in NH listeners. These age-related changes in latency and amplitude were independent of stimulus presentation rate. Further, CI listeners exhibited prolonged N1 and P2 latencies and smaller P2 amplitudes than NH listeners. Thus, aging appears to degrade some aspects of auditory temporal processing when peripheral-cochlear contributions are largely removed, suggesting that changes beyond the cochlea may contribute to age-related temporal processing deficits.

A Comparison of Place-Pitch-Based Interaural Electrode Matching Methods for Bilateral Cochlear-Implant Users.

Interaural place-of-stimulation mismatch for bilateral cochlear-implant (BI-CI) listeners is often evaluated using pitch-comparison tasks that can be susceptible to procedural biases. Bias effects were compared for three sequential interaural pitch-comparison tasks in six BI-CI listeners using single-electrode direct stimulation. The reference (right ear) was a single basal, middle, or apical electrode. The comparison electrode (left ear) was chosen from one of three ranges: basal half, full array, or apical half. In Experiment 1 (discrimination), interaural pairs were chosen randomly (method of constant stimuli). In Experiment 2 (ranking), an efficient adaptive procedure rank ordered 3 reference and 6 or 11 comparison electrodes. In Experiment 3 (matching), listeners adjusted the comparison electrode to pitch match the reference. Each experiment was evaluated for testing-range bias (point of subjective equality [PSE] vs. comparison-range midpoint) and reference-electrode slope bias (PSE vs. reference electrode). Discrimination showed large biases for both metrics; matching showed a smaller but significant reference-electrode bias; ranking showed no significant biases in either dimension. Ranking and matching were also evaluated for starting-point bias (PSE vs. adaptive-track starting point), but neither showed significant effects. A response-distribution truncation model explained a nonsignificant bias for ranking but it could not fully explain the observed biases for discrimination or matching. It is concluded that (a) BI-CI interaural pitch comparisons are inconsistent across test methods; (b) biases must be evaluated in more than one dimension before accepting the results as valid; and (c) of the three methods tested, ranking was least susceptible to biases and therefore emerged as the optimal approach.

Binaural Optimization of Cochlear Implants: Discarding Frequency Content Without Sacrificing Head-Shadow Benefit.

OBJECTIVES: Single-sided deafness cochlear-implant (SSD-CI) listeners and bilateral cochlear-implant (BI-CI) listeners gain near-normal levels of head-shadow benefit but limited binaural benefits. One possible reason for these limited binaural benefits is that cochlear places of stimulation tend to be mismatched between the ears. SSD-CI and BI-CI patients might benefit from a binaural fitting that reallocates frequencies to reduce interaural place mismatch. However, this approach could reduce monaural speech recognition and head-shadow benefit by excluding low- or high-frequency information from one ear. This study examined how much frequency information can be excluded from a CI signal in the poorer-hearing ear without reducing head-shadow benefits and how these outcomes are influenced by interaural asymmetry in monaural speech recognition. DESIGN: Speech-recognition thresholds for sentences in speech-shaped noise were measured for 6 adult SSD-CI listeners, 12 BI-CI listeners, and 9 normal-hearing listeners presented with vocoder simulations. Stimuli were presented using nonindividualized in-the-ear or behind-the-ear head-related impulse-response simulations with speech presented from a 70° azimuth (poorer-hearing side) and noise from 70° (better-hearing side), thereby yielding a better signal-to-noise ratio (SNR) at the poorer-hearing ear. Head-shadow benefit was computed as the improvement in bilateral speech-recognition thresholds gained from enabling the CI in the poorer-hearing, better-SNR ear. High- or low-pass filtering was systematically applied to the head-related impulse-response-filtered stimuli presented to the poorer-hearing ear. For the SSD-CI listeners and SSD-vocoder simulations, only high-pass filtering was applied, because the CI frequency allocation would never need to be adjusted downward to frequency-match the ears. For the BI-CI listeners and BI-vocoder simulations, both low and high pass filtering were applied. The normal-hearing listeners were tested with two levels of performance to examine the effect of interaural asymmetry in monaural speech recognition (vocoder synthesis-filter slopes: 5 or 20 dB/octave). RESULTS: Mean head-shadow benefit was smaller for the SSD-CI listeners (~7 dB) than for the BI-CI listeners (~14 dB). For SSD-CI listeners, frequencies <1236 Hz could be excluded; for BI-CI listeners, frequencies <886 or >3814 Hz could be excluded from the poorer-hearing ear without reducing head-shadow benefit. Bilateral performance showed greater immunity to filtering than monaural performance, with gradual changes in performance as a function of filter cutoff. Real and vocoder-simulated CI users with larger interaural asymmetry in monaural performance had less head-shadow benefit. CONCLUSIONS: The "exclusion frequency" ranges that could be removed without diminishing head-shadow benefit are interpreted in terms of low importance in the speech intelligibility index and a small head-shadow magnitude at low frequencies. Although groups and individuals with greater performance asymmetry gained less head-shadow benefit, the magnitudes of these factors did not predict the exclusion frequency range. Overall, these data suggest that for many SSD-CI and BI-CI listeners, the frequency allocation for the poorer-ear CI can be shifted substantially without sacrificing head-shadow benefit, at least for energetic maskers. Considering the two ears together as a single system may allow greater flexibility in discarding redundant frequency content from a CI in one ear when considering bilateral programming solutions aimed at reducing interaural frequency mismatch.

Acoustic Hearing Can Interfere With Single-Sided Deafness Cochlear-Implant Speech Perception.

OBJECTIVES: Cochlear implants (CIs) restore some spatial advantages for speech understanding in noise to individuals with single-sided deafness (SSD). In addition to a head-shadow advantage when the CI ear has a better signal-to-noise ratio, a CI can also provide a binaural advantage in certain situations, facilitating the perceptual separation of spatially separated concurrent voices. While some bilateral-CI listeners show a similar binaural advantage, bilateral-CI listeners with relatively large asymmetries in monaural speech understanding can instead experience contralateral speech interference. Based on the interference previously observed for asymmetric bilateral-CI listeners, this study tested the hypothesis that in a multiple-talker situation, the acoustic ear would interfere with rather than improve CI speech understanding for SSD-CI listeners. DESIGN: Experiment 1 measured CI-ear speech understanding in the presence of competing speech or noise for 13 SSD-CI listeners. Target speech from the closed-set coordinate response-measure corpus was presented to the CI ear along with one same-gender competing talker or stationary noise at target-to-masker ratios between -8 and 20 dB. The acoustic ear was presented with silence (monaural condition) or with a copy of the competing speech or noise (bilateral condition). Experiment 2 tested a subset of 6 listeners in the reverse configuration for which SSD-CI listeners have previously shown a binaural benefit (target and competing speech presented to the acoustic ear; silence or competing speech presented to the CI ear). Experiment 3 examined the possible influence of a methodological difference between experiments 1 and 2: whether the competing talker spoke keywords that were inside or outside the response set. For each experiment, the data were analyzed using repeated-measures logistic regression. For experiment 1, a correlation analysis compared the difference between bilateral and monaural speech-understanding scores to several listener-specific factors: speech understanding in the CI ear, preimplantation duration of deafness, duration of CI experience, ear of deafness (left/right), acoustic-ear audiometric thresholds, and listener age. RESULTS: In experiment 1, presenting a copy of the competing speech to the acoustic ear reduced CI speech-understanding scores for target-to-masker ratios ≥4 dB. This interference effect was limited to competing-speech conditions and was not observed for a noise masker. There was dramatic intersubject variability in the magnitude of the interference (range: 1 to 43 rationalized arcsine units), which was found to be significantly correlated with listener age. The interference effect contrasted sharply with the reverse configuration (experiment 2), whereby presenting a copy of the competing speech to the contralateral CI ear significantly improved performance relative to monaural acoustic-ear performance. Keyword condition (experiment 3) did not influence the observed pattern of interference. CONCLUSIONS: Most SSD-CI listeners experienced interference when they attended to the CI ear and competing speech was added to the acoustic ear, although there was a large amount of intersubject variability in the magnitude of the effect, with older listeners particularly susceptible to interference. While further research is needed to investigate these effects under free-field listening conditions, these results suggest that for certain spatial configurations in a multiple-talker situation, contralateral speech interference could reduce the benefit that an SSD-CI otherwise provides.

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.

Interaural Pitch-Discrimination Range Effects for Bilateral and Single-Sided-Deafness Cochlear-Implant Users.

By allowing bilateral access to sound, bilateral cochlear implants (BI-CIs) or unilateral CIs for individuals with single-sided deafness (SSD; i.e., normal or near-normal hearing in one ear) can improve sound localization and speech understanding in noise. Spatial hearing in the horizontal plane is primarily conveyed by interaural time and level differences computed from neurons in the superior olivary complex that receive frequency-matched inputs. Because BI-CIs and SSD-CIs do not necessarily convey frequency-matched information, it is critical to understand how to align the inputs to CI users. Previous studies show that interaural pitch discrimination for SSD-CI listeners is highly susceptible to contextual biases, questioning its utility for establishing interaural frequency alignment. Here, we replicate this finding for SSD-CI listeners and show that these biases also extend to BI-CI listeners. To assess the testing-range bias, three ranges of comparison electrodes (BI-CI) or pure-tone frequencies (SSD-CI) were tested: full range, apical/lower half, or basal/upper half. To assess the reference bias, the reference electrode was either held fixed throughout a testing block or randomly chosen from three electrodes (basal end, middle, or apical end of the array). Results showed no effect of reference electrode randomization, but a large testing range bias; changing the center of the testing-range shifted the pitch match by an average 63 % (BI-CI) or 43 % (SSD-CI) of the change magnitude. This bias diminished pitch-match accuracy, with a change in reference electrode shifting the pitch match only an average 34 % (BI-CI) or 40 % (SSD-CI) of the expected amount. Because these effects extended to the relatively more symmetric BI-CI listeners, the results suggest that the bias cannot be attributed to interaural asymmetry. Unless the range effect can be minimized or accounted for, a pitch-discrimination task will produce interaural place-of-stimulation estimates that are highly influenced by the conditions tested, rather than reflecting a true interaural place-pitch comparison.

Interaural Time-Difference Discrimination as a Measure of Place of Stimulation for Cochlear-Implant Users With Single-Sided Deafness.

Current clinical practice in programming a cochlear implant (CI) for individuals with single-sided deafness (SSD) is to maximize the transmission of speech information via the implant, with the implicit assumption that this will also result in improved spatial-hearing abilities. However, binaural sensitivity is reduced by interaural place-of-stimulation mismatch, a likely occurrence with a standard CI frequency-to-electrode allocation table (FAT). As a step toward reducing interaural mismatch, this study investigated whether a test of interaural-time-difference (ITD) discrimination could be used to estimate the acoustic frequency yielding the best place match for a given CI electrode. ITD-discrimination performance was measured by presenting 300-ms bursts of 100-pulses-per-second electrical pulse trains to a single CI electrode and band-limited pulse trains with variable carrier frequencies to the acoustic ear. Listeners discriminated between two reference intervals (four bursts each with constant ITD) and a moving target interval (four bursts with variable ITD). For 17 out of the 26 electrodes tested across eight listeners, the function describing the relationship between ITD-discrimination performance and carrier frequency had a discernable peak where listeners achieved 70% to 100% performance. On average, this peak occurred 1.15 octaves above the CI manufacturer's default FAT. ITD discrimination shows promise as a method of estimating the cochlear place of stimulation for a given electrode, thereby providing information to optimize the FAT for SSD-CI listeners.

Across-channel interaural-level-difference processing demonstrates frequency dependence.

Accurate localization of complex sounds involves combining interaural information across frequencies to produce a single location percept. Interaural level differences (ILDs) are highly frequency dependent and it is unclear how the auditory system combines differing ILDs across frequency. Therefore, ILD just noticeable differences (JNDs) and intracranial lateralization were measured in young normal-hearing listeners using single- and multi-band stimuli. The bands were 300-ms, 10-Hz narrowband noises; the multi-band condition had three bands; they started and ended synchronously; they were located around three different frequency regions (750, 2000, or 4000 Hz); they had five different frequency separations that ranged from unresolved to resolved; the bands were dichotic with the same non-zero ILD (targets) or were diotic with zero ILD (interferers). Results showed single-band ILD JNDs were marginally frequency dependent. If unresolved diotic interferers were added, ILD JNDs increased greatly because of interaural decorrelation. If well-resolved diotic interferers were added, ILD JNDs were frequency dependent and the worst performance occurred when targets were near 1000 or 4000 Hz. This frequency dependence might be partially explained by ILD vs azimuth non-monotonicities for free-field sound sources in this frequency region. These results suggest that binaural processing models need revision for the processing of complex sounds.

Contralateral Interference Caused by Binaurally Presented Competing Speech in Adult Bilateral Cochlear-Implant Users.

OBJECTIVES: Bilateral cochlear implants (BI-CIs) are intended to improve sound localization and speech understanding in the presence of interfering sounds. For normal-hearing listeners, improved speech understanding in the presence of interfering sounds can be achieved with monaural head shadow and binaural unmasking. While some BI-CI listeners experience binaural unmasking under certain testing conditions, others appear to not. This study tested a group of BI-CI users with hearing histories that have been linked to poor binaural processing-early onset of deafness or long duration of deafness in just one ear. We predicted that these listeners would experience the opposite of binaural unmasking (i.e., contralateral interference) when trying to understand speech in the presence of a competing talker. DESIGN: Nine adult BI-CI users who were deafened early in life or had an asymmetric hearing history (e.g., a much longer duration of deafness in one ear) participated in this study. The coordinate response measure corpus was used to assess speech understanding for a male target talker in quiet or in the presence of one male competing talker. Experiment 1 measured binaural unmasking in a paradigm that provided no head-shadow component. The target was always presented monaurally, while the interferer was presented either monaurally or diotically. Experiment 2 measured spatial release from masking in a paradigm that included both a head-shadow component and possible binaural-unmasking component. Nonindividualized head-related transfer functions were used to simulate talker locations in the front or 90° to the left or right. RESULTS: In experiment 1, all nine listeners experienced contralateral interference (9 dB on average). Four listeners demonstrated roughly symmetric contralateral interference; five listeners experienced asymmetrical contralateral interference. In experiment 2, the listeners experienced only 1 dB of spatial release from masking on average; this small amount was possibly a result of the contralateral interference observed in experiment 1. The results were best explained by individual differences in speech understanding in quiet, which significantly correlated with the duration of deafness in the ipsilateral ear. Specifically, instances of asymmetrical contralateral interference could correspond to asymmetrical hearing histories. CONCLUSIONS: Bilateral cochlear implantation should provide a hearing benefit to the recipient. For the BI-CI listeners specifically recruited for this study, there seems to be a conflict with processing the auditory information across the two ears, which produced the opposite of the desired hearing benefit. This suggests that there may be a subset of potential BI-CI users for whom contralateral interference offsets much of the potential head-shadow benefit. If so, earlier implantation in the second implanted ear might have produced larger binaural benefits, which is important information for clinicians advising patients considering bilateral implantation.

Across-frequency processing of interaural time and level differences in perceived lateralization.

Interaural time and level differences (ITDs and ILDs) contribute to the localization of sound sources; however, reverberation or use of cochlear implants diminishes the role of ITDs. Intracranial lateralization was investigated in normal-hearing listeners using correlated or uncorrelated narrowband noises, where ITDs and/or ILDs from a typical headrelated transfer function were applied. Results showed that ITDs and ILDs contributed to lateralization for correlated noises. ILDs contributed to lateralization for uncorrelated noises. Frequency-dependent ITD and ILD weighting occurred. These data help understand the across-channel processing of ITDs and ILDs, particularly when ITDs may not be available to the listener.

Auditory and otologic profile of Alström syndrome: Comprehensive single center data on 38 patients.

Alström syndrome (AS) is a rare autosomal recessive ciliopathy caused by mutations in the ALMS1 gene. Hallmark characteristics include childhood onset of severe retinal degeneration, sensorineural hearing loss, obesity, insulin-resistant diabetes, and cardiomyopathy. Here we comprehensively characterize the auditory and otologic manifestations in a prospective case series of 38 individuals, aged 1.7-37.9 years, with genetically confirmed AS. Hearing loss was preceded by retinal dystrophy in all cases, and had an average age of detection of 7.45 years (range 1.5-15). Audiometric assessments showed mean pure tone averages (0.5, 1, 2, 4 kHz) of 48.6 and 47.5 dB HL in the right and left ears, respectively. Hearing was within normal limits for only 8/74 ears (11%). For the 66 ears with hearing loss, the degree was mild (12%), moderate (54%), or severe (8%). Type of hearing loss was predominantly sensorineural (77%), while three ears had mixed loss, no ears had conductive loss, and type of hearing loss was indeterminate for the remaining 12 ears. Serial audiograms available for 33 patients showed hearing loss progression of approximately 10-15 dB/decade. Our data show that hearing loss associated with AS begins in childhood and is a predominantly symmetric, sensory hearing loss that may progress to a severe degree. Absent otoacoustic emissions, intact speech discrimination, and disproportionately normal auditory brainstem responses suggest an outer hair cell site of lesion. These findings indicate that individuals with AS would benefit from sound amplification and if necessary, cochlear implantation.

Lateralization of Interaural Level Differences with Multiple Electrode Stimulation in Bilateral Cochlear-Implant Listeners.

OBJECTIVE: There is currently no accepted method of mapping bilateral cochlear-implant (BiCI) users to maximize binaural performance, but the current approach of mapping one ear at a time could produce spatial perceptions that are not consistent with a sound's physical location in space. The goal of this study was to investigate the perceived intracranial lateralization of bilaterally synchronized electrical stimulation with a range of interaural level differences (ILDs) and to determine a method to produce relatively more centered auditory images when provided multielectrode stimulation. DESIGN: Using direct stimulation, lateralization curves were measured in nine BiCI listeners using 1000-pulses per second (pps), 500-msec constant-amplitude pulse trains with ILDs that ranged from -20 to +20 clinical current units (CUs). The stimuli were presented bilaterally at 70 to 80% of the dynamic range on single or multiple electrode pairs. For the multielectrode pairs, the ILD was applied consistently across all the pairs. The lateralization response range and the bias magnitude at 0 CU ILD (i.e., the number of CUs needed to produce a centered auditory image) were computed. Then the levels that elicit a centered auditory image with single-electrode stimulation were used with multielectrode stimulation to determine if this produced fewer significant biases at 0 CU ILD. Lastly, a multichannel ILD processing model was used to predict lateralization for the multielectrode stimulation from the single-electrode stimulation. RESULTS: BiCI listeners often perceived both single- and multielectrode stimulation at 0-CU ILD as not intracranially centered. For single-electrode stimulation, 44% of the lateralization curves had relatively large (≥5 CU) bias magnitudes. For the multielectrode stimulation, 25% of the lateralization curves had large bias magnitudes. After centering the single-electrode pairs, the percentage of multielectrode combinations that produced large biases significantly decreased to only 4% (p < 0.001, McNemar's test). The lateralization with multielectrode stimulation was well predicted by a model that used unweighted or weighted average single-electrode lateralization percepts across electrode pairs (87 or 90%, respectively). CONCLUSION: Current BiCI mapping procedures can produce an inconsistent association between a physical ILD and the perceived location across electrodes for both single- and multielectrode stimulation. Explicit centering of single-electrode pairs using the perceived centered intracranial location almost entirely corrects this problem and such an approach is supported by our understanding and model of across-frequency ILD processing. Such adjustments might be achieved by clinicians using single-electrode binaural comparisons. Binaural abilities, like sound localization and understanding speech in noise, may be improved if these across-electrode perceptual inconsistencies are removed.

Effects of brain-derived neurotrophic factor (BDNF) on the cochlear nucleus in cats deafened as neonates.

Many previous studies have shown significant neurotrophic effects of intracochlear delivery of BDNF in preventing degeneration of cochlear spiral ganglion (SG) neurons after deafness in rodents and our laboratory has shown similar results in developing cats deafened prior to hearing onset. This study examined the morphology of the cochlear nucleus (CN) in a group of neonatally deafened cats from a previous study in which infusion of BDNF elicited a significant improvement in survival of the SG neurons. Five cats were deafened by systemic injections of neomycin sulfate (60 mg/kg, SQ, SID) starting one day after birth, and continuing for 16-18 days until auditory brainstem response (ABR) testing demonstrated profound bilateral hearing loss. The animals were implanted unilaterally at about 1 month of age using custom-designed electrodes with a drug-delivery cannula connected to an osmotic pump. BDNF (94 µg/ml; 0.25 µl/hr) was delivered for 10 weeks. The animals were euthanized and studied at 14-23 weeks of age. Consistent with the neurotrophic effects of BDNF on SG survival, the total CN volume in these animals was significantly larger on the BDNF-treated side than on the contralateral side. However, total CN volume, both ipsi- and contralateral to the implants in these deafened juvenile animals, was markedly smaller than the CN in normal adult animals, reflecting the severe effects of deafness on the central auditory system during development. Data from the individual major CN subdivisions (DCN, Dorsal Cochlear Nucleus; PVCN, Posteroventral Cochlear Nucleus; AVCN, Anteroventral Cochlear Nucleus) also were analyzed. A significant difference was observed between the BDNF-treated and control sides only in the AVCN. Measurements of the cross-sectional areas of spherical cells showed that cells were significantly larger in the AVCN ipsilateral to the implant than on the contralateral side. Further, the numerical density of spherical cells was significantly lower in the AVCN ipsilateral to the implant than on the contralateral side, consistent with the larger AVCN volume observed with BDNF treatment. Together, findings indicate significant neurotrophic effects of intracochlear BDNF infusion on the developing CN.

Effects of brain-derived neurotrophic factor (BDNF) and electrical stimulation on survival and function of cochlear spiral ganglion neurons in deafened, developing cats.

Both neurotrophic support and neural activity are required for normal postnatal development and survival of cochlear spiral ganglion (SG) neurons. Previous studies in neonatally deafened cats demonstrated that electrical stimulation (ES) from a cochlear implant can promote improved SG survival but does not completely prevent progressive neural degeneration. Neurotrophic agents combined with an implant may further improve neural survival. Short-term studies in rodents have shown that brain-derived neurotrophic factor (BDNF) promotes SG survival after deafness and may be additive to trophic effects of stimulation. Our recent study in neonatally deafened cats provided the first evidence of BDNF neurotrophic effects in the developing auditory system over a prolonged duration Leake et al. (J Comp Neurol 519:1526-1545, 2011). Ten weeks of intracochlear BDNF infusion starting at 4 weeks of age elicited significant improvement in SG survival and larger soma size compared to contralateral. In the present study, the same deafening and BDNF infusion procedures were combined with several months of ES from an implant. After combined BDNF + ES, a highly significant increase in SG numerical density (>50 % improvement re: contralateral) was observed, which was significantly greater than the neurotrophic effect seen with ES-only over comparable durations. Combined BDNF + ES also resulted in a higher density of myelinated radial nerve fibers within the osseous spiral lamina. However, substantial ectopic and disorganized sprouting of these fibers into the scala tympani also occurred, which may be deleterious to implant function. EABR thresholds improved (re: initial thresholds at time of implantation) on the chronically stimulated channels of the implant. Terminal electrophysiological studies recording in the inferior colliculus (IC) revealed that the basic cochleotopic organization was intact in the midbrain in all studied groups. In deafened controls or after ES-only, lower IC thresholds were correlated with more selective activation widths as expected, but no such correlation was seen after BDNF + ES due to much greater variability in both measures.