Cochlear-Implant Simulated Signal Degradation Exacerbates Listening Effort in Older Listeners.

OBJECTIVES: Individuals with cochlear implants (CIs) often report that listening requires high levels of effort. Listening effort can increase with decreasing spectral resolution, which occurs when listening with a CI, and can also increase with age. What is not clear is whether these factors interact; older CI listeners potentially experience even higher listening effort with greater signal degradation than younger CI listeners. This study used pupillometry as a physiological index of listening effort to examine whether age, spectral resolution, and their interaction affect listening effort in a simulation of CI listening. DESIGN: Fifteen younger normal-hearing listeners (ages 18 to 31 years) and 15 older normal-hearing listeners (ages 65 to 75 years) participated in this experiment; they had normal hearing thresholds from 0.25 to 4 kHz. Participants repeated sentences presented in quiet that were either unprocessed or vocoded, simulating CI listening. Stimuli frequency spectra were limited to below 4 kHz (to control for effects of age-related high-frequency hearing loss), and spectral resolution was decreased by decreasing the number of vocoder channels, with 32-, 16-, and 8-channel conditions. Behavioral speech recognition scores and pupil dilation were recorded during this task. In addition, cognitive measures of working memory and processing speed were obtained to examine if individual differences in these measures predicted changes in pupil dilation. RESULTS: For trials where the sentence was recalled correctly, there was a significant interaction between age and spectral resolution, with significantly greater pupil dilation in the older normal-hearing listeners for the 8- and 32-channel vocoded conditions. Cognitive measures did not predict pupil dilation. CONCLUSIONS: There was a significant interaction between age and spectral resolution, such that older listeners appear to exert relatively higher listening effort than younger listeners when the signal is highly degraded, with the largest effects observed in the eight-channel condition. The clinical implication is that older listeners may be at higher risk for increased listening effort with a CI.

Binaural fusion: Complexities in definition and measurement.

Despite the growing interest in studying binaural fusion, there is little consensus over its definition or how it is best measured. This review seeks to describe the complexities of binaural fusion, highlight measurement challenges, provide guidelines for rigorous perceptual measurements, and provide a working definition that encompasses this information. First, it is argued that binaural fusion may be multidimensional and might occur in one domain but not others, such as fusion in the spatial but not the spectral domain or vice versa. Second, binaural fusion may occur on a continuous scale rather than on a binary one. Third, binaural fusion responses are highly idiosyncratic, which could be a result of methodology, such as the specific experimental instructions, suggesting a need to explicitly report the instructions given. Fourth, it is possible that direct ("Did you hear one sound or two?") and indirect ("Where did the sound come from?" or "What was the pitch of the sound?") measurements of fusion will produce different results. In conclusion, explicit consideration of these attributes and reporting of methodology are needed for rigorous interpretation and comparison across studies and listener populations.

How to vocode: Using channel vocoders for cochlear-implant research.

The channel vocoder has become a useful tool to understand the impact of specific forms of auditory degradation-particularly the spectral and temporal degradation that reflect cochlear-implant processing. Vocoders have many parameters that allow researchers to answer questions about cochlear-implant processing in ways that overcome some logistical complications of controlling for factors in individual cochlear implant users. However, there is such a large variety in the implementation of vocoders that the term "vocoder" is not specific enough to describe the signal processing used in these experiments. Misunderstanding vocoder parameters can result in experimental confounds or unexpected stimulus distortions. This paper highlights the signal processing parameters that should be specified when describing vocoder construction. The paper also provides guidance on how to determine vocoder parameters within perception experiments, given the experimenter's goals and research questions, to avoid common signal processing mistakes. Throughout, we will assume that experimenters are interested in vocoders with the specific goal of better understanding cochlear implants.

Temporal speech cue perception in listeners with cochlear implants depends on the time between those cues and previous sound energy.

Cochlear implants (CIs) provide precise temporal information that listeners use to understand speech. Other acoustic cues are not conveyed as precisely, making unambiguous temporal speech cues vital to a listener's ability to understand speech. Several speech sounds are differentiated by small differences in the timing of acoustic features. Previous studies have shown differences in the perception of these differences, depending on whether the speech sound was heard in a single word or embedded in a sentence. This study expands on previous research by exploring forward masking as a possible contributor to the mechanisms driving the effects observed when temporal cues were embedded in sentences. Listeners using CIs performed a phoneme categorization task on words from four continua that each varied mainly on a single temporal dimension. The differentiating phonemes were located at the beginning of the word in two continua and at the end of the word in two others. Silent intervals of 0, 25, 50, 75, and 100 ms between the preceding sentence and the target words were tested. Results showed an increasing effect on performance as the inter-stimulus interval duration decreased for the two word-initial phonemic contrasts, lending support to forward masking as an influence on speech understanding.

Spectral degradation and carrier sentences increase age-related temporal processing deficits in a cue-specific manner.

Advancing age is associated with decreased sensitivity to temporal cues in word segments, particularly when target words follow non-informative carrier sentences or are spectrally degraded (e.g., vocoded to simulate cochlear-implant stimulation). This study investigated whether age, carrier sentences, and spectral degradation interacted to cause undue difficulty in processing speech temporal cues. Younger and older adults with normal hearing performed phonemic categorization tasks on two continua: a Buy/Pie contrast with voice onset time changes for the word-initial stop and a Dish/Ditch contrast with silent interval changes preceding the word-final fricative. Target words were presented in isolation or after non-informative carrier sentences, and were unprocessed or degraded via sinewave vocoding (2, 4, and 8 channels). Older listeners exhibited reduced sensitivity to both temporal cues compared to younger listeners. For the Buy/Pie contrast, age, carrier sentence, and spectral degradation interacted such that the largest age effects were seen for unprocessed words in the carrier sentence condition. This pattern differed from the Dish/Ditch contrast, where reducing spectral resolution exaggerated age effects, but introducing carrier sentences largely left the patterns unchanged. These results suggest that certain temporal cues are particularly susceptible to aging when placed in sentences, likely contributing to the difficulties of older cochlear-implant users in everyday environments.

Performance on stochastic figure-ground perception varies with individual differences in speech-in-noise recognition and working memory capacity.

Speech recognition in noisy environments can be challenging and requires listeners to accurately segregate a target speaker from irrelevant background noise. Stochastic figure-ground (SFG) tasks in which temporally coherent inharmonic pure-tones must be identified from a background have been used to probe the non-linguistic auditory stream segregation processes important for speech-in-noise processing. However, little is known about the relationship between performance on SFG tasks and speech-in-noise tasks nor the individual differences that may modulate such relationships. In this study, 37 younger normal-hearing adults performed an SFG task with target figure chords consisting of four, six, eight, or ten temporally coherent tones amongst a background of randomly varying tones. Stimuli were designed to be spectrally and temporally flat. An increased number of temporally coherent tones resulted in higher accuracy and faster reaction times (RTs). For ten target tones, faster RTs were associated with better scores on the Quick Speech-in-Noise task. Individual differences in working memory capacity and self-reported musicianship further modulated these relationships. Overall, results demonstrate that the SFG task could serve as an assessment of auditory stream segregation accuracy and RT that is sensitive to individual differences in cognitive and auditory abilities, even among younger normal-hearing adults.

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.

The effect of target and interferer frequency on across-frequency binaural interference of interaural-level-difference sensitivity.

Across-frequency binaural interference occurs when the sensitivity to changes in interaural differences in a target sound is decreased by a spectrally remote diotic interfering sound. For interaural time differences (ITDs), low-frequency (e.g., 0.5 kHz) interferers cause more interference on high-frequency (e.g., 4 kHz) targets than vice versa. For interaural level differences (ILDs), however, it is unclear if a frequency dependence exists. Therefore, ILD discrimination thresholds and across-frequency binaural interference were measured for target and interferer frequencies between 0.5 and 8 kHz (for tones) or 12 kHz (for narrowband noises). For tones, 8-kHz targets experienced the least interference and 8-kHz interferers produced the most interference, suggesting that higher-frequency ILDs are a more heavily weighted localization cue than lower-frequency ILDs. For narrowband noises, the frequency-dependent interference patterns increased in complexity in comparison to tones. Low-frequency ITD dominance (from randomly varying onset ITDs) and grouping cues (e.g., envelope modulations) might explain some of the complexity in the interference patterns for the noises. These data contribute to a better understanding of across-frequency ILD processing, which remains poorly understood.

Stimulus context affects the phonemic categorization of temporally based word contrasts in adult cochlear-implant users.

Cochlear-implant (CI) users rely heavily on temporal envelope cues for speech understanding. This study examined whether their sensitivity to temporal cues in word segments is affected when the words are preceded by non-informative carrier sentences. Thirteen adult CI users performed phonemic categorization tasks that present primarily temporally based word contrasts: Buy-Pie contrast with word-initial stop of varying voice-onset time (VOT), and Dish-Ditch contrast with varying silent intervals preceding the word-final fricative. These words were presented in isolation or were preceded by carrier stimuli including a sentence, a sentence-envelope-modulated noise, or an unmodulated speech-shaped noise. While participants were able to categorize both word contrasts, stimulus context effects were observed primarily for the Buy-Pie contrast, such that participants reported more "Buy" responses for words with longer VOTs in conditions with carrier stimuli than in isolation. The two non-speech carrier stimuli yielded similar or even greater context effects than sentences. The context effects disappeared when target words were delayed from the carrier stimuli for ≥75 ms. These results suggest that stimulus contexts affect auditory temporal processing in CI users but the context effects appear to be cue-specific. The context effects may be governed by general auditory processes, not those specific to speech processing.

Effects of better-ear glimpsing, binaural unmasking, and spectral resolution on spatial release from masking in cochlear-implant users.

Bilateral cochlear-implant (BICI) listeners obtain less spatial release from masking (SRM; speech-recognition improvement for spatially separated vs co-located conditions) than normal-hearing (NH) listeners, especially for symmetrically placed maskers that produce similar long-term target-to-masker ratios at the two ears. Two experiments examined possible causes of this deficit, including limited better-ear glimpsing (using speech information from the more advantageous ear in each time-frequency unit), limited binaural unmasking (using interaural differences to improve signal-in-noise detection), or limited spectral resolution. Listeners had NH (presented with unprocessed or vocoded stimuli) or BICIs. Experiment 1 compared natural symmetric maskers, idealized monaural better-ear masker (IMBM) stimuli that automatically performed better-ear glimpsing, and hybrid stimuli that added worse-ear information, potentially restoring binaural cues. BICI and NH-vocoded SRM was comparable to NH-unprocessed SRM for idealized stimuli but was 14%-22% lower for symmetric stimuli, suggesting limited better-ear glimpsing ability. Hybrid stimuli improved SRM for NH-unprocessed listeners but degraded SRM for BICI and NH-vocoded listeners, suggesting they experienced across-ear interference instead of binaural unmasking. In experiment 2, increasing the number of vocoder channels did not change NH-vocoded SRM. BICI SRM deficits likely reflect a combination of across-ear interference, limited better-ear glimpsing, and poorer binaural unmasking that stems from cochlear-implant-processing limitations other than reduced spectral resolution.

Impacts of signal processing factors on perceptual restoration in cochlear-implant users.

Cochlear-implant (CI) users have previously demonstrated perceptual restoration, or successful repair of noise-interrupted speech, using the interrupted sentences paradigm [Bhargava, Gaudrain, and Baskent (2014). "Top-down restoration of speech in cochlear-implant users," Hear. Res. 309, 113-123]. The perceptual restoration effect was defined experimentally as higher speech understanding scores with noise-burst interrupted sentences compared to silent-gap interrupted sentences. For the perceptual restoration illusion to occur, it is often necessary for the masking or interrupting noise bursts to have a higher intensity than the adjacent speech signal to be perceived as a plausible masker. Thus, signal processing factors like noise reduction algorithms and automatic gain control could have a negative impact on speech repair in this population. Surprisingly, evidence that participants with cochlear implants experienced the perceptual restoration illusion was not observed across the two planned experiments. A separate experiment, which aimed to provide a close replication of previous work on perceptual restoration in CI users, also found no consistent evidence of perceptual restoration, contrasting the original study's previously reported findings. Typical speech repair of interrupted sentences was not observed in the present work's sample of CI users, and signal-processing factors did not appear to affect speech repair.

Effects of aging and hearing loss on perceptual and electrophysiological measures of pulse-rate discrimination.

Auditory temporal processing declines with age, leading to potential deleterious effects on communication. In young normal-hearing listeners, perceptual rate discrimination is rate limited around 300 Hz. It is not known whether this rate limitation is similar in older listeners with hearing loss. The purpose of this study was to investigate age- and hearing-loss-related rate limitations on perceptual rate discrimination, and age- and hearing-loss-related effects on neural representation of these stimuli. Younger normal-hearing, older normal-hearing, and older hearing-impaired listeners performed a pulse-rate discrimination task at rates of 100, 200, 300, and 400 Hz. Neural phase locking was assessed using the auditory steady-state response. Finally, a battery of non-auditory cognitive tests was administered. Younger listeners had better rate discrimination, higher phase locking, and higher cognitive scores compared to both groups of older listeners. Aging, but not hearing loss, diminished neural-rate encoding and perceptual performance; however, there was no relationship between the perceptual and neural measures. Higher cognitive scores were correlated with improved perceptual performance, but not with neural phase locking. This study shows that aging, rather than hearing loss, may be a stronger contributor to poorer temporal processing, and cognitive factors such as processing speed and inhibitory control may be related to these declines.

Children's syntactic parsing and sentence comprehension with a degraded auditory signal.

During sentence comprehension, young children anticipate syntactic structures using early-arriving words and have difficulties revising incorrect predictions using late-arriving words. However, nearly all work to date has focused on syntactic parsing in idealized speech environments, and little is known about how children's strategies for predicting and revising meanings are affected by signal degradation. This study compares comprehension of active and passive sentences in natural and vocoded speech. In a word-interpretation task, 5-year-olds inferred the meanings of novel words in sentences that (1) encouraged agent-first predictions (e.g., The blicket is eating the seal implies The blicket is the agent), (2) required revising predictions (e.g., The blicket is eaten by the seal implies The blicket is the theme), or (3) weakened predictions by placing familiar nouns in sentence-initial position (e.g., The seal is eating/eaten by the blicket). When novel words promoted agent-first predictions, children misinterpreted passives as actives, and errors increased with vocoded compared to natural speech. However, when familiar words were sentence-initial that weakened agent-first predictions, children accurately interpreted passives, with no signal-degradation effects. This demonstrates that signal quality interacts with interpretive processes during sentence comprehension, and the impacts of speech degradation are greatest when late-arriving information conflicts with predictions.

Access to semantic cues does not lead to perceptual restoration of interrupted speech in cochlear-implant users.

Cochlear-implant (CI) users experience less success in understanding speech in noisy, real-world listening environments than normal-hearing (NH) listeners. Perceptual restoration is one method NH listeners use to repair noise-interrupted speech. Whereas previous work has reported that CI users can use perceptual restoration in certain cases, they failed to do so under listening conditions in which NH listeners can successfully restore. Providing increased opportunities to use top-down linguistic knowledge is one possible method to increase perceptual restoration use in CI users. This work tested perceptual restoration abilities in 18 CI users and varied whether a semantic cue (presented visually) was available prior to the target sentence (presented auditorily). Results showed that whereas access to a semantic cue generally improved performance with interrupted speech, CI users failed to perceptually restore speech regardless of the semantic cue availability. The lack of restoration in this population directly contradicts previous work in this field and raises questions of whether restoration is possible in CI users. One reason for speech-in-noise understanding difficulty in CI users could be that they are unable to use tools like restoration to process noise-interrupted speech effectively.

Benefits of triple acoustic beamforming during speech-on-speech masking and sound localization for bilateral cochlear-implant users.

Bilateral cochlear-implant (CI) users struggle to understand speech in noisy environments despite receiving some spatial-hearing benefits. One potential solution is to provide acoustic beamforming. A headphone-based experiment was conducted to compare speech understanding under natural CI listening conditions and for two non-adaptive beamformers, one single beam and one binaural, called "triple beam," which provides an improved signal-to-noise ratio (beamforming benefit) and usable spatial cues by reintroducing interaural level differences. Speech reception thresholds (SRTs) for speech-on-speech masking were measured with target speech presented in front and two maskers in co-located or narrow/wide separations. Numerosity judgments and sound-localization performance also were measured. Natural spatial cues, single-beam, and triple-beam conditions were compared. For CI listeners, there was a negligible change in SRTs when comparing co-located to separated maskers for natural listening conditions. In contrast, there were 4.9- and 16.9-dB improvements in SRTs for the beamformer and 3.5- and 12.3-dB improvements for triple beam (narrow and wide separations). Similar results were found for normal-hearing listeners presented with vocoded stimuli. Single beam improved speech-on-speech masking performance but yielded poor sound localization. Triple beam improved speech-on-speech masking performance, albeit less than the single beam, and sound localization. Thus, triple beam was the most versatile across multiple spatial-hearing domains.

Accuracy and cue use in word segmentation for cochlear-implant listeners and normal-hearing listeners presented vocoded speech.

Cochlear-implant (CI) listeners experience signal degradation, which leads to poorer speech perception than normal-hearing (NH) listeners. In the present study, difficulty with word segmentation, the process of perceptually parsing the speech stream into separate words, is considered as a possible contributor to this decrease in performance. CI listeners were compared to a group of NH listeners (presented with unprocessed speech and eight-channel noise-vocoded speech) in their ability to segment phrases with word segmentation ambiguities (e.g., "an iceman" vs "a nice man"). The results showed that CI listeners and NH listeners were worse at segmenting words when hearing processed speech than NH listeners were when presented with unprocessed speech. When viewed at a broad level, all of the groups used cues to word segmentation in similar ways. Detailed analyses, however, indicated that the two processed speech groups weighted top-down knowledge cues to word boundaries more and weighted acoustic cues to word boundaries less relative to NH listeners presented with unprocessed speech.

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.

Spectral-Temporal Trade-Off in Vocoded Sentence Recognition: Effects of Age, Hearing Thresholds, and Working Memory.

OBJECTIVES: Cochlear implant (CI) signal processing degrades the spectral components of speech. This requires CI users to rely primarily on temporal cues, specifically, amplitude modulations within the temporal envelope, to recognize speech. Auditory temporal processing ability for envelope modulations worsens with advancing age, which may put older CI users at a disadvantage compared with younger users. To evaluate how potential age-related limitations for processing temporal envelope modulations impact spectrally degraded sentence recognition, noise-vocoded sentences were presented to younger and older normal-hearing listeners in quiet. Envelope modulation rates were varied from 10 to 500 Hz by adjusting the low-pass filter cutoff frequency (LPF). The goal of this study was to evaluate if age impacts recognition of noise-vocoded speech and if this age-related limitation existed for a specific range of envelope modulation rates. DESIGN: Noise-vocoded sentence recognition in quiet was measured as a function of number of spectral channels (4, 6, 8, and 12 channels) and LPF (10, 20, 50, 75, 150, 375, and 500 Hz) in 15 younger normal-hearing listeners and 15 older near-normal-hearing listeners. Hearing thresholds and working memory were assessed to determine the extent to which these factors were related to recognition of noise-vocoded sentences. RESULTS: Younger listeners achieved significantly higher sentence recognition scores than older listeners overall. Performance improved in both groups as the number of spectral channels and LPF increased. As the number of spectral channels increased, the differences in sentence recognition scores between groups decreased. A spectral-temporal trade-off was observed in both groups in which performance in the 8- and 12-channel conditions plateaued with lower-frequency amplitude modulations compared with the 4- and 6-channel conditions. There was no interaction between age group and LPF, suggesting that both groups obtained similar improvements in performance with increasing LPF. The lack of an interaction between age and LPF may be due to the nature of the task of recognizing sentences in quiet. Audiometric thresholds were the only significant predictor of vocoded sentence recognition. Although performance on the working memory task declined with advancing age, working memory scores did not predict sentence recognition. CONCLUSIONS: Younger listeners outperformed older listeners for recognizing noise-vocoded sentences in quiet. The negative impact of age was reduced when ample spectral information was available. Age-related limitations for recognizing vocoded sentences were not affected by the temporal envelope modulation rate of the signal, but instead, appear to be related to a generalized task limitation or to reduced audibility of the signal.

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.