Effect of experimentally introduced interaural frequency mismatch on sentence recognition in bilateral cochlear-implant listeners.

Bilateral cochlear-implant users experience interaural frequency mismatch because of asymmetries in array insertion and frequency-to-electrode assignment. To explore the acute perceptual consequences of such mismatch, sentence recognition in quiet was measured in nine bilateral cochlear-implant listeners as frequency allocations in the poorer ear were shifted by ±1.5, ±3, and ±4.5 mm using experimental programs. Shifts in frequency allocation >3 mm reduced bilateral sentence scores below those for the better ear alone, suggesting that the poorer ear interfered with better-ear perception. This was not a result of fewer active channels; deactivating electrodes without frequency shifting had minimal effect.

Effect of experimentally introduced interaural frequency mismatch on sentence recognition in bilateral cochlear-implant listeners.

Bilateral cochlear-implant users experience interaural frequency mismatch because of asymmetries in array insertion and frequency-to-electrode assignment. To explore the acute perceptual consequences of such mismatch, sentence recognition in quiet was measured in nine bilateral cochlear-implant listeners as frequency allocations in the poorer ear were shifted by ±1.5, ±3 and ±4.5 mm using experimental programs. Shifts in frequency allocation >3 mm were found to reduce bilateral sentence scores below those for the better ear alone, suggesting that the poorer ear interfered with better-ear perception. This was not a result of fewer active channels; deactivating electrodes without frequency shifting had minimal effect.

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.

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.

Effect of Stimulation Rate on Speech Understanding in Older Cochlear-Implant Users.

OBJECTIVES: Cochlear implants (CIs) are considered a safe and effective intervention for more severe degrees of hearing loss in adults of all ages. Although older CI users ≥65 years of age can obtain large benefits in speech understanding from a CI, there is a growing body of literature suggesting that older CI users may not perform as well as younger CI users. One reason for this potential age-related limitation could be that default CI stimulation settings are not optimal for older CI users. The goal of this study was to determine whether improvements in speech understanding were possible when CI users were programmed with nondefault stimulation rates and to determine whether lower-than-default stimulation rates improved older CI users' speech understanding. DESIGN: Sentence recognition was measured acutely using different stimulation rates in 37 CI users ranging in age from 22 to 87 years. Maps were created using rates of 500, 720, 900, and 1200 pulses per second (pps) for each subject. An additional map using a rate higher than 1200 pps was also created for individuals who used a higher rate in their clinical processors. Thus, the clinical rate of each subject was also tested, including non-default rates above 1200 pps for Cochlear users and higher rates consistent with the manufacturer defaults for subjects implanted with Advanced Bionics and Med-El devices. Speech understanding performance was evaluated at each stimulation rate using AzBio and Perceptually Robust English Sentence Test Open-set (PRESTO) sentence materials tested in quiet and in noise. RESULTS: For Cochlear-brand users, speech understanding performance using non-default rates was slightly poorer when compared with the default rate (900 pps). However, this effect was offset somewhat by age, in which older subjects were able to maintain comparable performance using a 500-pps map compared with the default rate map when listening to the more difficult PRESTO sentence material. Advanced Bionics and Med-El users showed modest improvements in their overall performance using 720 pps compared with the default rate (>1200 pps). On the individual-subject level, 10 subjects (11 ears) showed a significant effect of stimulation rate, with 8 of those ears performing best with a lower-than-default rate. CONCLUSIONS: Results suggest that default stimulation rates are likely sufficient for many CI users, but some CI users at any age can benefit from a lower-than-default rate. Future work that provides experience with novel rates in everyday life has the potential to identify more individuals whose performance could be improved with changes to stimulation rate.

Memory Span for Spoken Digits in Adults With Cochlear Implants or Typical Hearing: Effects of Age and Identification Ability.

Purpose: Short-term and working memory vary across individuals and life span. Studies of how cochlear implant (CI) users remember spoken words often do not fully disentangle perceptual influences from memory assessment because stimulus identification is rarely checked; instead, correct perception is assumed by using simple or practiced stimuli. Here, we examine whether the accuracy and speed of auditory digit identification are associated with short-term and working memory in adults with CIs as a function of age. Do perceptual errors and longer processing times depress memory span scores for such listeners? An intensive digit span assessment using manual responses is compared with traditional digit span, which uses only a few speech-based trials. We also assess how vocoder-based degradation impacts identification and digit span in older versus younger listeners with normal hearing (NH). Method: We measured forward and backward recall for spoken digits in 25 adults with CIs (M = 57 years; range = 20-82 years), 10 older adults with NH (> 55 years; M = 64 years), and 11 younger adults with NH (< 22 years). Listeners identified recorded digits one at a time by clicking numerals on a computer screen and then were cued to recall the list. In a comparison condition, listeners only identified each item. Listeners with NH also completed the tasks under four conditions of signal degradation implemented using a channel vocoder: four or eight channels crossed with 0- or 6-mm simulated tonotopic shift, administered in randomized blocks. A mixed design was used, with the effects of degradation examined within subject, across different listener groups, and as a function of age. Results: CI users were near perfect at identifying auditory digits (> 99% correct, on average), both in isolation and while experiencing a memory load. Perceptual accuracy, thus, did not affect digit span in these listeners. Identification times, however, were significantly slower under memory load. Age was only weakly related to span in CI listeners. Instead, individual differences in span in the CI group were better predicted by simple no-load identification times. Among listeners with NH, spans for vocoded digits that were not frequency shifted did not differ from spans for unprocessed stimuli. Frequency-shifted vocoded digits, however, yielded lower scores that could mostly, but not entirely, be accounted for by crediting for perceptual errors. Frequency-shifted vocoded digits were particularly challenging for older adult listeners with NH to identify. Traditional digit span correlated more strongly with our primary span measure for listeners with CIs than for listeners with NH. Conclusions: Digit span can be studied independently of perception in many adult CI users. For both CI listeners and listeners with NH, stimulus degradation does not appear to significantly affect memory span, unless an effect on simple identification is also present. Auditory degradation that may slow, but which does not ultimately prevent identification, appears to have negligible impact on short-term and working memory spans.

Picture naming and verbal fluency in children with cochlear implants.

PURPOSE: In the present study, the authors examined lexical naming in children with cochlear implants (CIs). The goal was to determine whether children with CIs have deficits in lexical access and organization as revealed through reaction time in picture-naming and verbal fluency (VF) experiments. METHOD: Children with CIs (n = 20, ages 7-10) were compared with 20 children with normal hearing (NH) matched for age and nonverbal IQ. Lexical abilities were examined using two naming tasks: a timed picture-naming task and a phonological and semantic VF naming task. Picture naming taps into lexical access capabilities and the VF task elucidates lexical organization. RESULTS: No group differences were found between children with CIs and children with NH on the timed picture-naming task. Children with CIs generated significantly fewer words than the children with NH on the VF tasks. Larger group differences were found for the phonological VF task compared with the semantic VF task. CONCLUSIONS: Limited early linguistic and auditory experiences may affect lexical representations and organization (lexical-semantic connections) in school-age children with hearing loss who use CIs. Further analyses and studies should continue to examine these underlying linguistic deficits. The present results suggest a need to emphasize not only increasing the size of children's vocabularies during therapy, but also expanding and increasing the semantic and phonological richness of their lexical representations.

Influence of voice similarity on talker discrimination in children with normal hearing and children with cochlear implants.

The perception of voice similarity was examined in 5-year-old children with normal hearing sensitivity and in pediatric cochlear implant users, 5-12 years of age. Recorded sentences were manipulated to form a continuum of similar-sounding voices. An adaptive procedure was then used to determine how acoustically different, in terms of average fundamental and formant frequencies, 2 sentences needed to be for a child to categorize the sentences as spoken by 2 different talkers. The average spectral characteristics of 2 utterances (including their fundamental frequencies) needed to differ by at least 11%-16% (2-2.5 semitones) for normal-hearing children to perceive the voices as belonging to different talkers. Introducing differences in the linguistic content of the 2 sentences to be compared did not change performance. Although several children with cochlear implants performed similarly to normal-hearing children, most found the task very difficult. Pediatric cochlear implant users who scored above the group mean of 64% of words correct on a monosyllabic open-set word identification task categorized the voices more like children with normal hearing sensitivity.

Nonword repetition by children with cochlear implants: accuracy ratings from normal-hearing listeners.

Seventy-six children with cochlear implants completed a nonword repetition task. The children were presented with 20 nonword auditory patterns over a loud-speaker and were asked to repeat them aloud to the experimenter. The children's responses were recorded on digital audiotape and then played back to normal-hearing adult listeners to obtain accuracy ratings on a 7-point scale. The children's nonword repetition performance, as measured by these perceptual accuracy ratings, could be predicted in large part by their performance on independently collected measures of speech perception, verbal rehearsal speed, and speech production. The strongest contributing variable was speaking rate, which is widely argued to reflect verbal rehearsal speed in phonological working memory. Children who had become deaf at older ages received higher perceptual ratings. Children whose early linguistic experience and educational environments emphasized oral communication methods received higher perceptual ratings than children enrolled in total communication programs. The present findings suggest that individual differences in performance on nonword repetition are strongly related to variability observed in the component processes involved in language imitation tasks, including measures of speech perception, speech production, and especially verbal rehearsal speed in phonological working memory. In addition, onset of deafness at a later age and an educational environment emphasizing oral communication may be beneficial to the children's ability to develop the robust phonological processing skills necessary to accurately repeat novel, nonword sound patterns.

Nonword imitation by children with cochlear implants: consonant analyses.

OBJECTIVES: To complete detailed linguistic analyses of archived recordings of pediatric cochlear implant users' imitations of nonwords; to gain insight into the children's developing phonological systems and the wide range of variability in nonword responses. DESIGN: Nonword repetition: repetition of 20 auditory-only English-sounding nonwords. SETTING: Central Institute for the Deaf "Education of the Deaf Child" research program, St Louis, Mo. PARTICIPANTS: Eighty-eight 8- to 10-year-old experienced pediatric cochlear implant users. MAIN OUTCOME MEASURES: Several different consonant accuracy scores based on the linguistic structure (voicing, place, and manner of articulation) of the consonants being imitated; analysis of the errors produced for all consonants imitated incorrectly. RESULTS: Seventy-six children provided a response to at least 75% of the nonword stimuli. In these children's responses, 33% of the target consonants were imitated correctly, 25% of the target consonants were deleted, and substitutions were provided for 42% of the target consonants. The children tended to correctly reproduce target consonants with coronal place (which involve a mid-vocal tract constriction) more often than other consonants. Poorer performers tended to produce more deletions than the better performers, but their production errors tended to follow the same patterns as the better performers. CONCLUSIONS: Poorer performance on labial consonants suggests that scores were affected by the lack of visual cues such as lip closure. Oral communication users tended to perform better than total communication users, indicating that oral communication methods are beneficial to the development of pediatric cochlear implant users' phonological processing skills.

Imitation of nonwords by hearing-impaired children with cochlear implants: segmental analyses.

The phonological processing skills of 24 pre-lingually deaf 8- and 9-year-old experienced cochlear implant users were measured using a nonword repetition task. The children heard recordings of 20 nonwords and were asked to repeat each pattern as accurately as possible. Detailed segmental analyses of the consonants in the children's imitation responses were carried out. Overall, 39% of the consonants were imitated correctly. Coronals were produced correctly more often than labials or dorsals. There was no difference in the proportion of correctly reproduced stops, fricatives, nasals, and liquids, or voiced and voiceless consonants. Although nonword repetition performance was not correlated with the children's demographic characteristics, the nonword repetition scores were strongly correlated with other measures of the component processes required for the immediate reproduction of a novel sound pattern: spoken word recognition, language comprehension, working memory, and speech production.

Measures of working memory span and verbal rehearsal speed in deaf children after cochlear implantation.

Large individual differences in spoken word recognition performance have been found in deaf children after cochlear implantation. Recently, Pisoni and Geers (2000) reported that simple forward digit span measures of verbal working memory were significantly correlated with spoken word recognition scores even after potentially confounding variables were statistically controlled for. The present study replicates and extends these initial findings to the full set of 176 participants in the CID cochlear implant study. The pooled data indicate that despite statistical "partialling-out" of differences in chronological age, communication mode, duration of deafness, duration of device use, age at onset of deafness, number of active electrodes, and speech feature discrimination, significant correlations still remain between digit span and several measures of spoken word recognition. Strong correlations were also observed between speaking rate and both forward and backward digit span, a result that is similar to previously reported findings in normal-hearing adults and children. The results suggest that perhaps as much as 20% of the currently unexplained variance in spoken word recognition scores may be independently accounted for by individual differences in cognitive factors related to the speed and efficiency with which phonological and lexical representations of spoken words are maintained in and retrieved from working memory. A smaller percentage, perhaps about 7% of the currently unexplained variance in spoken word recognition scores, may be accounted for in terms of working memory capacity. We discuss how these relationships may arise and their contribution to subsequent speech and language development in prelingually deaf children who use cochlear implants.

Talker discrimination by prelingually deaf children with cochlear implants: preliminary results.

Forty-four school-age children who had used a multichannel cochlear implant (CI) for at least 4 years were tested to assess their ability to discriminate differences between recorded pairs of female voices uttering sentences. Children were asked to respond "same voice" or "different voice" on each trial. Two conditions were examined. In one condition, the linguistic content of the sentence was always held constant and only the talker's voice varied from trial to trial. In another condition, the linguistic content of the utterance also varied so that to correctly respond "same voice," the child needed to recognize that two different sentences were spoken by the same talker. Data from normal-hearing children were used to establish that these tasks were well within the capabilities of children without hearing impairment. For the children with CIs, in the "fixed sentence condition" the mean proportion correct was 68%, which, although significantly different from the 50% score expected by chance, suggests that the children with CIs found this discrimination task rather difficult. In the "varied sentence condition," however, the mean proportion correct was only 57%, indicating that the children were essentially unable to recognize an unfamiliar talker's voice when the linguistic content of the paired sentences differed. Correlations with other speech and language outcome measures are also reported.

Imitation of nonwords by deaf children after cochlear implantation: preliminary findings.

Fourteen prelingually deafened pediatric users of the Nucleus-22 cochlear implant were asked to imitate auditorily presented nonwords. The children's utterances were recorded, digitized, and broadly transcribed. The target patterns and the children's imitations were then played back to normal-hearing adult listeners in order to obtain perceptual judgments of repetition accuracy. The results revealed wide variability in the children's ability to repeat the novel sound sequences. Individual differences in the component processes of encoding, memory, and speech production were strongly reflected in the nonword repetition scores. Duration of deafness before implantation also appeared to be a factor associated with imitation performance. Linguistic analyses of the initial consonants in the nonwords revealed that coronal stops were imitated best, followed by the coronal fricative /s/, and then the labial and velar stops. Labial fricatives were poorly imitated. The theoretical significance of the nonword repetition task as it has been used in past studies of working memory and vocabulary development in normal-hearing children is discussed.

Working Memory Spans as Predictors of Spoken Word Recognition and Receptive Vocabulary in Children with Cochlear Implants.

The present study investigated whether individual differences in working memory could account for a significant proportion of the variance in the open-set word recognition and receptive vocabulary skills of prelingually deafened, pediatric cochlear implant recipients, after the contribution of known predictors was taken into account. The contributions of four measures of working memory were examined separately for children using oral communication (OC) (n = 32) and Total Communication (TC) (n = 29). Wechsler Intelligence Scale for Children-Third Edition (WISC) digit-spans, requiring immediate recall of auditory-only lists in both forwards and backwards directions were, collected. Two versions of a novel "memory span game" were also administered: One required memory for sequences of colored lights; the other assessed memory for colored lights presented in conjunction with auditory color-names. A contribution from working memory was observed only for the span tasks that incorporated an auditory processing component. These results suggest a relationship between working memory and the examined outcome measures that is specific to the auditory modality, partially linked to communication mode, and not related to individual differences in a general-purpose component of working memory.

Individual Differences in Effectiveness of Cochlear Implants in Children Who Are Prelingually Deaf: New Process Measures of Performance.

The efficacy of cochlear implants in children who are deaf has been firmly established in the literature. However, the effectiveness of cochlear implants varies widely and is influenced by demographic and experiential factors. Several key findings suggest new directions for research on central auditory factors that underlie the effectiveness of cochlear implants. First, enormous individual differences have been observed in both adults and children on a wide range of audiological outcome measures. Some patients show large increases in speech perception scores after implantation, whereas others display only modest gains on standardized tests. Second, age of implantation and length of deafness affect all outcome measures. Children implanted at younger ages do better than children implanted at older ages, and children who have been deaf for shorter periods do better than children who have been deaf for longer periods. Third, communication mode affects outcome measures. Children from "oral-only" environments do much better on standardized tests that assess phonological processing skills than children who use Total Communication. Fourth, at the present time there are no preimplant predictors of outcome performance in young children. The underlying perceptual, cognitive, and linguistic abilities and skills emerge after implantation and improve over time. Finally, there are no significant differences in audiological outcome measures among current implant devices or processing strategies. This finding suggests that the major source of variance in outcome measures lies in the neural and cognitive information processing operations that the user applies to the signal provided by the implant. Taken together, this overall pattern of results suggests that higher-level central processes such as perception, attention, learning, and memory may play important roles in explaining the large individual differences observed among users of cochlear implants. Investigations of the content and flow of information in the central nervous system and interactions between sensory input and stored knowledge may provide important new insights into the basis of individual differences. Knowledge about the underlying basis of individual differences may also help in developing new intervention strategies to improve the effectiveness of cochlear implants in children who show relatively poor development of oral/aural language skills.