Effect of noise and reverberation on speech intelligibility for cochlear implant recipients in realistic sound environments.

Previous studies have suggested a strong effect of reverberation on speech intelligibility (SI) in cochlear implant (CI) recipients. In many of them, different reverberation conditions were obtained by altering the acoustic absorption of a single room, thereby omitting the effect of the room volume. In addition, studies that have investigated the combined effects of reverberation and noise on SI have overlooked the effect of reverberation on the modulation of the noise. In the present study, SI was measured unilaterally in 12 CI recipients in quiet and in noise using a three-dimensional loudspeaker array. Target speech was convolved with room impulse responses (RIRs) recorded at three talker-to-listener distances in five physical rooms with distinct reverberation times. Noise consisted of four two-talker dialogues convolved with RIRs measured at fixed positions around the listener. Results in quiet suggest that a significant drop in SI occurs mainly at long talker-to-listener distances, and small reverberant rooms affect SI the most. In noise, the most detrimental type of noise is anechoic as it is the most modulated. Overall, the results suggest that at fixed signal-to-noise ratios the effects of noise and reverberation are smallest at short distances in large rooms or in small rooms with some reverberation.

Sequential stream segregation with bilateral cochlear implants.

Sequential stream segregation on the basis of binaural 'ear-of-entry', modulation rate and electrode place-of-stimulation cues was investigated in bilateral cochlear implant (CI) listeners using a rhythm anisochrony detection task. Sequences of alternating 'A' and 'B' bursts were presented via direct electrical stimulation and comprised either an isochronous timing structure or an anisochronous structure that was generated by delaying just the 'B' bursts. 'B' delay thresholds that enabled rhythm anisochrony detection were determined. Higher thresholds were assumed to indicate a greater likelihood of stream segregation, resulting specifically from stream integration breakdown. Results averaged across subjects showed that thresholds were significantly higher when monaural 'A' and 'B' bursts were presented contralaterally rather than ipsilaterally, and that diotic presentation of 'A', with a monaural 'B', yielded intermediate thresholds. When presented monaurally and ipsilaterally, higher thresholds were also found when successive bursts had mismatched rather than matched modulation rates. In agreement with previous studies, average delay thresholds also increased as electrode separation between bursts increased when presented ipsilaterally. No interactions were found between ear-of-entry, modulation rate and place-of-stimulation. However, combining moderate electrode difference cues with either diotic-'A' ear-of-entry cues or modulation-rate mismatch cues did yield greater threshold increases than observed with any of those cues alone. The results from the present study indicate that sequential stream segregation can be elicited in bilateral CI users by differences in the signal across ears (binaural cues), in modulation rate (monaural cues) and in place-of-stimulation (monaural cues), and that those differences can be combined to further increase segregation.

Speech Detection in Noise for Young Bilaterally Implanted Children: Is There Evidence of Binaural Benefit Over the Shadowed Ear Alone?

OBJECTIVES: To measure binaural benefit over the shadowed ear alone for young bilateral cochlear implant (CI) users. It was hypothesized that children who received bilateral CIs at a young age (<4 years), and had significant bilateral experience, would demonstrate lower detection thresholds for speech sounds in background noise in the bilateral CI over the unilateral CI condition when the added CI was ipsilateral to the noise source. DESIGN: Children receiving bilateral CIs at the Eye and Ear Hospital Clinic in Melbourne were invited to participate in a wider research project evaluating outcomes; those participating in the wider project who were bilaterally implanted by 4 years and were approximately 2 years postoperative were included in the present study. For 20 participants, detection signal to noise ratios (SNRs) were measured for speech presented from in front and noise from 90° in at least 3 of 4 device/noise conditions, namely left CI/noise right and right CI/noise left, plus bilateral CIs/noise right and bilateral CIs/noise left. RESULTS: As some participants could only complete testing in 3 conditions within the 1 test block, the unilateral versus bilateral comparison was performed for 1 CI (i.e., 1 noise direction) for 15 participants and for both CIs (i.e., noise left and noise right) for 5 participants. Group analysis indicated no significant difference in detection SNR between the unilateral and bilateral CI conditions when adding the left CI or right CI (for the overall group) or when adding the first or second CI (for the 15 participants with sequential bilateral CIs). Separate analyses indicated no significant difference in detection SNR between the unilateral and bilateral CI conditions for the majority of individuals; this occurred irrespective of whether the analysis indicated that the CI added in the bilateral condition was poorer-performing, better-performing, or not significantly different compared with the other CI. Four individuals demonstrated a significant improvement in the bilateral condition when the CI added in the bilateral condition was a better-performing (n = 1), poorer-performing (n = 2), or not significantly different CI (n = 1). There was no relationship between the detection SNR difference between each CI and the detection SNR difference between the unilateral and bilateral conditions. CONCLUSIONS: The hypothesis of a lower detection SNR in the bilateral condition was not supported by the group results or by the results for the majority of individuals. For the 4 participants who did demonstrate benefit over the shadowed ear alone, that benefit cannot be separated from the potential benefit gained as a result of the CI added in the bilateral condition being the better-performing CI for 1 of the 4. Variation in outcomes could not be related to demographic factors for this group, which was relatively homogeneous for age at bilateral CI and experience; an older, more experienced group may demonstrate greater binaural benefit in these conditions. These results can be used during counseling for families regarding postoperative expectations for young children, especially in the first 2 years.

Influence of contralateral acoustic hearing on adult bimodal outcomes after cochlear implantation.

OBJECTIVE: To examine post-implantation benefit and time taken to acclimate to the cochlear implant for adult candidates with more hearing in the contralateral non-implanted ear than has been previously considered within local candidacy guidelines. DESIGN: Prospective, within-subject experimental design. STUDY SAMPLE: Forty postlingual hearing-impaired adult subjects with a contralateral ear word score in quiet ranging from 27% to 100% (median 67%). RESULTS: Post-implantation improvement of 2.4 dB and 4.0 dB was observed on a sentence in coincident babble test at presentation levels of 65 and 55 dB SPL respectively, and a 2.1 dB benefit in spatial release from masking (SRM) advantage observed when the noise location favoured the implanted side. Significant post-operative group mean change of between 2.1 and 3.0 was observed on the sub-scales of the speech, spatial, and qualities (SSQ) questionnaire. Degree of post-implantation speech reception threshold (SRT) benefit on the coincident babble test and on perception of soft speech and sounds in the environment was greater for subjects with less contralateral hearing. The degree of contralateral acoustic hearing did not affect time taken to acclimate to the device. CONCLUSIONS: The findings from this study support cochlear implantation for candidates with substantial acoustic hearing in the contralateral ear, and provide guidance regarding post-implantation expectations.

Factors Predicting Postoperative Unilateral and Bilateral Speech Recognition in Adult Cochlear Implant Recipients with Acoustic Hearing.

OBJECTIVES: The first objective was to examine factors that could be predictive of postoperative unilateral (cochlear implant alone) speech recognition ability in a group of subjects with greater degrees of preoperative acoustic hearing than has been previously examined. Second, the study aimed to identify factors predictive of speech recognition in the best-aided, bilateral listening condition. DESIGN: Participants were 65 postlinguistically hearing-impaired adults with preoperative phoneme in quiet scores of greater than or equal to 46% in one or both ears. Preoperative demographic and audiometric factors were assessed as predictors of 12-month postoperative unilateral and bilateral monosyllabic word scores in quiet and of bilateral speech reception threshold (SRT) in babble. RESULTS: The predictive regression model accounted for 34.1% of the variance in unilateral word recognition scores in quiet. Factors that predicted better scores included: a shorter duration of severe to profound hearing loss in the implanted ear; and poorer pure-tone-averaged thresholds in the contralateral ear. Predictive regression models of postimplantation bilateral function accounted for 36.0% of the variance for word scores in quiet, and 30.9% of the variance for SRT in noise. A shorter duration of severe to profound hearing loss in the implanted ear, a lower age at the time of implantation, and better contralateral hearing thresholds were associated with higher bilateral word recognition in quiet and SRT in noise. CONCLUSIONS: In this group of cochlear implant recipients with preoperative acoustic hearing, a shorter duration of severe to profound hearing loss in the implanted ear was shown to be predictive of better unilateral and bilateral outcomes. However, further research is warranted to better understand the impact of that factor in a larger number of subjects with long-term hearing impairment of greater than 30 years. Better contralateral hearing was associated with poorer unilateral word scores with the implanted ear alone, but better absolute bilateral speech recognition. As a result, it is clear that different models would need to be developed to predict unilateral and bilateral postimplantation scores.

An evaluation of the performance of two binaural beamformers in complex and dynamic multitalker environments.

OBJECTIVE: Binaural beamformers are super-directional hearing aids created by combining microphone outputs from each side of the head. While they offer substantial improvements in SNR over conventional directional hearing aids, the benefits (and possible limitations) of these devices in realistic, complex listening situations have not yet been fully explored. In this study we evaluated the performance of two experimental binaural beamformers. DESIGN: Testing was carried out using a horizontal loudspeaker array. Background noise was created using recorded conversations. Performance measures included speech intelligibility, localization in noise, acceptable noise level, subjective ratings, and a novel dynamic speech intelligibility measure. STUDY SAMPLE: Participants were 27 listeners with bilateral hearing loss, fitted with BTE prototypes that could be switched between conventional directional or binaural beamformer microphone modes. RESULTS: Relative to the conventional directional microphones, both binaural beamformer modes were generally superior for tasks involving fixed frontal targets, but not always for situations involving dynamic target locations. CONCLUSIONS: Binaural beamformers show promise for enhancing listening in complex situations when the location of the source of interest is predictable.

Audio-visual speech intelligibility benefits with bilateral cochlear implants when talker location varies.

One of the key benefits of using cochlear implants (CIs) in both ears rather than just one is improved localization. It is likely that in complex listening scenes, improved localization allows bilateral CI users to orient toward talkers to improve signal-to-noise ratios and gain access to visual cues, but to date, that conjecture has not been tested. To obtain an objective measure of that benefit, seven bilateral CI users were assessed for both auditory-only and audio-visual speech intelligibility in noise using a novel dynamic spatial audio-visual test paradigm. For each trial conducted in spatially distributed noise, first, an auditory-only cueing phrase that was spoken by one of four talkers was selected and presented from one of four locations. Shortly afterward, a target sentence was presented that was either audio-visual or, in another test configuration, audio-only and was spoken by the same talker and from the same location as the cueing phrase. During the target presentation, visual distractors were added at other spatial locations. Results showed that in terms of speech reception thresholds (SRTs), the average improvement for bilateral listening over the better performing ear alone was 9 dB for the audio-visual mode, and 3 dB for audition-alone. Comparison of bilateral performance for audio-visual and audition-alone showed that inclusion of visual cues led to an average SRT improvement of 5 dB. For unilateral device use, no such benefit arose, presumably due to the greatly reduced ability to localize the target talker to acquire visual information. The bilateral CI speech intelligibility advantage over the better ear in the present study is much larger than that previously reported for static talker locations and indicates greater everyday speech benefits and improved cost-benefit than estimated to date.

Clinical outcomes for adult cochlear implant recipients experiencing loss of usable acoustic hearing in the implanted ear.

OBJECTIVES: The first aim of the study was to quantify the change in clinical performance after cochlear implantation for adults who had pre-operative levels of acoustic hearing in each ear of greater than or equal to 46% phoneme score on an open-set monosyllabic word test, and who subsequently experienced loss of useable acoustic hearing in the implanted ear. Pre- and postoperative spatial hearing abilities were assessed, because a clinical consideration for candidates with bilateral acoustic hearing is the potential for post-operative reduction in spatial hearing ability. Second, it was of interest to examine whether preoperative localization ability, as an indicator of access to interaural timing and level cues preoperatively, might be correlated with post-operative change in spatial hearing abilities. DESIGN: Clinical performance measures in the binaural condition were obtained preoperatively and at 12 months postoperatively in 19 postlinguistically hearing-impaired adult subjects. Preoperative localization ability was investigated as a potential correlate with post-operative change in spatial hearing abilities. RESULTS: Significant postoperative group mean improvement in speech perception was observed on measures of open-set monosyllabic word perception in quiet and on an adaptive sentence test presented in coincident 4-talker babble. Observed benefit was greater for a lower presentation level of 55 dB SPL as compared with a conversational speech level of 65 dB SPL. Self-reported ratings of benefit also improved for all questionnaires administered. Objective assessment of localization ability revealed poorer localization postoperatively, although subjective ratings of post-operative change in localization ability in real-world environments were more variable. Postoperative spatial release from masking was not different to that measured preoperatively for the configuration where the side of the head with the hearing aid was advantaged, but improved postoperatively for the configuration that advantaged the implanted side. Preoperative binaural localization ability was not correlated with postoperative spatial hearing abilities. CONCLUSIONS: The findings from this study support cochlear implantation for candidates with pre-operative levels of binaural acoustic hearing within the range examined within the present study. This includes subjects with preoperative open-set monosyllabic word scores ranging from 11 to 62% in the implanted ear, and from 16 to 75% on the contralateral side. Post-operative improvement would be expected for those subjects on a range of clinical measures, even when acoustic hearing was lost in the implanted ear after implantation.

Training of cochlear implant users to improve pitch perception in the presence of competing place cues.

OBJECTIVES: Perception of musical pitch in cochlear implant (CI) systems is relatively poor compared with normal hearing and can be adversely affected by changes in spectral timbre coded by stimulation place. In this study, we evaluated whether the perception of musical pitch could be improved through specific training designed to teach listeners to attend to fundamental frequency (F0) exclusively for judgment of pitch and to spectral envelope exclusively for discrimination of spectral timbre. DESIGN: A computer-based training program to improve musical pitch perception was developed that required listeners to match acoustic patterns of pitch and spectral timbre to visual patterns. Ten adult CI recipients participated: five used the training program and five acted as controls. Before training, F0 and center frequency discrimination for band-limited complex harmonic stimuli presented in the sound field were measured in all subjects using their standard clinical device(s). The F0 tests were conducted with and without spectral variations. The trainees subsequently used the training program at home for a period of 4 months, during which they were asked to train for approximately 30 min per day. The training schedule comprised two successive phases, each lasting 2 months. In the first phase, training employed a single cue (i.e., F0 for pitch or center frequency for spectral timbre) in the absence of other cue variations. In the second phase, training incorporated more complex sounds in which multiple cues were varied. Discrimination thresholds were remeasured in all subjects after each phase and again with trainees 3 months after training had ceased. RESULTS: Trainees obtained significant improvements in F0 and center frequency discrimination as compared with control subjects for tests conducted at 2 months. The improvements in F0 discrimination were observed both in the absence and presence of small variations in place. However, the effect of training diminished for large variations in place or for higher F0s. Neither group showed further improvement in tests after additional training in the second phase. Tests conducted with trainees after training had ceased showed that F0 discrimination improvements were retained. CONCLUSIONS: The results showed that performance on pitch and timbre discrimination can be improved by training with single cues (F0 and center frequency) in the absence of other cue variations. Although results indicated that training with single cues can improve F0 discrimination within more complex sounds in which multiple cues vary, little improvement was seen when large variations in place were present, which suggests a diminishing effect of the training with increased influence of place-pitch. These data imply that although such training techniques may help listeners to follow melody in music, changes in instrument are likely to affect perception of melody. Results of subsequent training with complex sounds in which multiple cues varied were less conclusive and showed no further improvement. Follow-up evaluations with trainees conducted well after training had ceased verified the robustness of the training effect on F0 but not center frequency discrimination. Further studies are needed, however, to determine whether and to what degree subject motivation may be an important factor in these outcomes.

Factors influencing electrical place pitch perception in bimodal listeners.

Factors that might affect perceptual pitch match between acoustic and electric stimulation were examined in 25 bimodal listeners using magnitude estimation. Pre-operative acoustic thresholds in both ears, and duration of severe-profound loss, were first examined as correlates with degree of match between the measured pitch and that predicted by the spiral ganglion frequency-position model. The degree of match was examined with respect to (1) the ratio between the measured and predicted pitch percept on the most apical electrode and (2) the ratio between the slope of the measured and predicted pitch function. Second, effect of listening experience was examined to assess whether adaptation occurred over time to match the frequency assignment to electrodes. Pre-experience pitch estimates on the apical electrode were within the predicted range in only 28% of subjects, and the slope of the electrical pitch function was lower than predicted in all except one subject. Subjects with poorer hearing tended to have a lower pitch and a shallower electrical pitch function than predicted by the model. Pre-operative hearing thresholds in the contralateral ear and hearing loss duration were not correlated with the degree of pitch match, and there was no significant group effect of listening experience.

Pitch and loudness matching of unmodulated and modulated stimuli in cochlear implantees.

The pitch elicited by unmodulated and amplitude modulated electrical pulse trains was examined with six adult cochlear implantees. In addition, for three of those subjects who had some hearing in their contralateral ear, the pitch of unmodulated electrical pulse trains was compared to that of complex harmonic acoustic tones. In the first experiment, pulse rate discrimination and the effects of place and level differences on pitch were examined for unmodulated pulse trains. General results were consistent with previous studies showing that variations in pulse rate, while holding loudness fixed, elicit changes in pitch at low rates, but become progressively harder to discriminate as rates approach approximately 300 pulses-per-second. Variations in place or level of stimulation generally produced changes in pitch consistent with tonotopic place and spread of excitation. In the second experiment, pitch and loudness of unmodulated pulse trains were compared with those of amplitude modulated stimuli as a function of modulation depth, rate, and shape, and presentation level. The pitch elicited by an amplitude modulated pulse train was generally higher than that of an unmodulated pulse train with a pulse rate equal to the modulation rate, and generally decreased toward that of the unmodulated pulse train as modulation depth or rate increased, or as presentation level decreased. Sharper/narrower modulation produced lower pitch. In the final experiment, the pitch heights of acoustic complex harmonic tones and unmodulated pulse trains were compared. When electrical pulse rate was equal to the fundamental frequency of the acoustic tone, similar pitch heights were elicited. The results from these experiments indicate that F0 rate pitch derived from the temporal envelope in existing clinical cochlear implant strategies may often be higher than that of acoustic harmonic tones at the same F0 in normal hearing, and that pitch growth with increasing F0 may be shallower. The relationship between F0 and rate pitch is expected to be more similar to acoustic stimulation for low F0 rates when using new pitch coding strategies that code F0 information via deep (narrow) amplitude modulation of the stimulus envelope. Although that similarity reduces as F0 approaches the upper limit of rate-pitch discrimination, that limit is reached sooner for the shallow (or broad) modulators used in existing clinical strategies.

Enhancement of temporal cues to pitch in cochlear implants: effects on pitch ranking.

The abilities to hear changes in pitch for sung vowels and understand speech using an experimental sound coding strategy (eTone) that enhanced coding of temporal fundamental frequency (F0) information were tested in six cochlear implant users, and compared with performance using their clinical (ACE) strategy. In addition, rate- and modulation rate-pitch difference limens (DLs) were measured using synthetic stimuli with F0s below 300 Hz to determine psychophysical abilities of each subject and to provide experience in attending to rate cues for the judgment of pitch. Sung-vowel pitch ranking tests for stimuli separated by three semitones presented across an F0 range of one octave (139-277 Hz) showed a significant benefit for the experimental strategy compared to ACE. Average d-prime (d') values for eTone (d' = 1.05) were approximately three time larger than for ACE (d' = 0.35). Similar scores for both strategies in the speech recognition tests showed that coding of segmental speech information by the experimental strategy was not degraded. Average F0 DLs were consistent with results from previous studies and for all subjects were less than or equal to approximately three semitones for F0s of 125 and 200 Hz.

Binaural interactions develop in the auditory brainstem of children who are deaf: effects of place and level of bilateral electrical stimulation.

Bilateral cochlear implants (CIs) might promote development of binaural hearing required to localize sound sources and hear speech in noise for children who are deaf. These hearing skills improve in children implanted bilaterally but remain poorer than normal. We thus questioned whether the deaf and immature human auditory system is able to integrate input delivered from bilateral CIs. Using electrophysiological measures of brainstem activity that include the Binaural Difference (BD), a measure of binaural processing, we showed that a period of unilateral deprivation before bilateral CI use prolonged response latencies but that amplitudes were not significantly affected. Tonotopic organization was retained to some extent as evidenced by an elimination of the BD with large mismatches in place of stimulation between the two CIs. Smaller place mismatches did not affect BD latency or amplitude, indicating that the tonotopic organization of the auditory brainstem is underdeveloped and/or not well used by CI stimulation. Finally, BD amplitudes decreased when the intensity of bilateral stimulation became weighted to one side and this corresponded to a perceptual shift of sound away from midline toward the side of increased intensity. In summary, bilateral CI stimulation is processed by the developing human auditory brainstem leading to perceptual changes in sound location and potentially improving hearing for children who are deaf.

Contrasting benefits from contralateral implants and hearing aids in cochlear implant users.

In recent years a substantial number of studies have reported results from cochlear implant users who increasingly are being fitted with a contralateral hearing aid or second implant. Often outcomes are discussed in relation to the benefits available to listeners with normal hearing in both ears. The objective of this paper is to consider the available cues that are degraded in different ways when a cochlear implant is combined with a contralateral hearing aid or second implant, and to review the literature in that context. It is found that the data largely confirm the expectations that arise from those considerations, and that outcomes differ substantially for the two types of listeners, with a greater emphasis on better ear selection and comparison of information at the two ears for bilateral implant users, and conversely, on the complementary use of information from the two ears in bimodal listeners.

Development of a temporal fundamental frequency coding strategy for cochlear implants.

A sound-coding strategy for users of cochlear implants, named enhanced-envelope-encoded tone (eTone), was developed to improve coding of fundamental frequency (F0) in the temporal envelopes of the electrical stimulus signals. It is based on the advanced combinational encoder (ACE) strategy and includes additional processing that explicitly applies F0 modulation to channel envelope signals that contain harmonics of prominent complex tones. Channels that contain only inharmonic signals retain envelopes normally produced by ACE. The strategy incorporates an F0 estimator to determine the frequency of modulation and a harmonic probability estimator to control the amount of modulation enhancement applied to each channel. The F0 estimator was designed to provide an accurate estimate of F0 with minimal processing lag and robustness to the effects of competing noise. Error rates for the F0 estimator and accuracy of the harmonic probability estimator were compared with previous approaches and outcomes demonstrated that the strategy operates effectively across a range of signals and conditions that are relevant to cochlear implant users.

Statistical bias in the assessment of binaural benefit relative to the better ear.

The comparison of measured binaural performance with the better of two monaural measures (one from each ear) may lead to underestimated binaural benefit due to statistical sampling bias that favors the monaural condition. The mathematical basis of such bias is reviewed and applied to speech reception thresholds measured in 32 bilateral cochlear implant (CI) users for coincident and spatially separated speech and noise. It is shown that the bias increases with test-retest variation and is maximal for uncorrelated samples of identical underlying performance in each ear. When measured differences between ears were assumed to reflect actual underlying performance differences, the bias averaged across the CI users was about 0.2 dB for coincident target and noise, and 0.1 dB for spatially separated conditions. An upper-bound estimate of the bias, based on the assumption that both ears have the same underlying performance and observed differences were due to test-retest variation, was about 0.7 dB regardless of noise location. To the extent that the test-retest variation in these data is comparable to other studies, the results indicate that binaural benefits in bilateral cochlear implant users are not substantially underestimated (on for average) when binaural performance is compared with the better ear in each listening configuration.

Lateralization of interimplant timing and level differences in children who use bilateral cochlear implants.

OBJECTIVES: Interaural level differences (ILD) and interaural timing differences (ITD) are important cues for locating sounds in space. Adult bilateral cochlear implant (CI) users use ILDs more effectively than ITDs. Few studies investigated the ability of children who use bilateral CIs to make use of these binaural cues. Our working hypothesis was that children using bilateral CIs are able to perceive changes in ITDs and ILDs similar to their normal-hearing (NH) peers. DESIGN: Participants were two groups of children; 19 bilateral implant users (CI) and nine NH children. The children in the CI group had received a second CI after 4.9 +/- 2.8 yrs of unilateral use. Children performed a four alternative forced-choice lateralization task in which they were asked to describe stimuli as coming from the left side, right side, middle of the head, or from both sides simultaneously. Stimuli were 500 msec trains of electrical pulses delivered to apical electrode no. 18 (CI group) or clicks (NH group) presented 11 times per second with either ITDs (0, 400, 1000, or 2000 microsec delay between sides) or level differences (0, 10, or 20 Current Units (CI group) or 0, 10, or 20 dB (NH group) difference between sides). ITDs were presented using current levels that were balanced using left and right electrically evoked brain stem responses. Stimulus levels evoking response amplitudes that were most similar were used. RESULTS: Responses from children in the CI group changed significantly with changes in ILD of bilateral stimuli, but not with changes in ITD. Responses from children in the CI group were significantly different from those in the NH group in three ways. Children in the CI group perceived bilaterally presented electrical pulses: (1) to come from the second implanted side more often than the first, (2) to rarely come from the middle, and (3) to come from both sides of the head simultaneously. Perceived changes in lateralization with ILD changes were correlated with differences in amplitudes of electrically evoked brain stem responses by the left versus right CI. CONCLUSIONS: The results of this study illustrate that children who use bilateral CIs can lateralize stimuli on the basis of level cues, but have difficulty interpreting interimplant timing differences. Perceived lateralization of bilaterally presented stimuli to the second implanted side in many of the stimulus conditions may relate to the use of different device generations between sides. Further differences from normal lateralization responses could be due to abnormal binaural processing, possibly resulting from a period of unilateral hearing before the provision of a second implant or due to insufficiently matched interimplant stimuli. It may be possible to use objective measures such as electrically evoked auditory brain stem responses wave eV amplitudes to provide balanced levels of bilateral stimulation in children who have had no binaural hearing experience.

Effect of age at onset of deafness on binaural sensitivity in electric hearing in humans.

Sensitivity to binaural cues was studied in 11 bilateral cochlear implant users, all of whom received both of their cochlear implants as adults, but who varied in the age at onset of deafness, from pre-lingual to childhood-onset to adult-onset. Sensitivity to interaural timing difference (ITD) and interaural level difference (ILD) cues was measured at basal, middle, and apical pitch-matched places of stimulation along the cochlear arrays, using a stimulation rate of 100 Hz. Results show that there is a trend for people whose onset of deafness occurred during adult life or late childhood to retain at least some sensitivity to ITDs, whereas people with onset of deafness earlier in life were insensitive to ITDs. In contrast, ILD cue sensitivity was present in all subjects. There were no effects of place of stimulation on binaural sensitivity, suggesting that there is no indication of a dependence of ITD sensitivity on apical vs basal electrode location.

Interaural time-delay sensitivity in bilateral cochlear implant users: effects of pulse rate, modulation rate, and place of stimulation.

Electrical interaural time delay (ITD) discrimination was measured using 300-ms bursts applied to binaural pitch matched electrodes at basal, mid, and apical locations in each ear. Six bilateral implant users, who had previously shown good ITD sensitivity at a pulse rate of 100 pulses per second (pps), were assessed. Thresholds were measured as a function of pulse rate between 100 and 1,000 Hz, as well as modulation rate over that same range for high-rate pulse trains at 6,000 pps. Results were similar for all three places of stimulation and showed decreasing ITD sensitivity as either pulse rate or modulation rate increased, although the extent of that effect varied across subjects. The results support a model comprising a common ITD mechanism for high- and low-frequency places of stimulation, which, for electrical stimulation, is rate-limited in the same way across electrodes because peripheral temporal responses are largely place invariant. Overall, ITD sensitivity was somewhat better with unmodulated pulse trains than with high-rate pulse trains modulated at matched rates, although comparisons at individual rates showed that difference to be significant only at 300 Hz. Electrodes presenting with the lowest thresholds at 600 Hz were further assessed using bursts with a ramped onset of 10 ms. The slower rise time resulted in decreased performance in four of the listeners, but not in the two best performers, indicating that those two could use ongoing cues at 600 Hz. Performance at each place was also measured using single-pulse stimuli. Comparison of those data with the unmodulated 300-ms burst thresholds showed that on average, the addition of ongoing cues beyond the onset enhanced overall ITD sensitivity at 100 and 300 Hz, but not at 600 Hz. At 1,000 Hz, the added ongoing cues actually decreased performance. That result is attributed to the introduction of ambiguous cues within the physiologically relevant range and increased dichotic firing.