Supra-normal skills in processing of visuo-auditory prosodic information by cochlear-implanted deaf patients.

Cochlear implanted (CI) adults with acquired deafness are known to depend on multisensory integration skills (MSI) for speech comprehension through the fusion of speech reading skills and their deficient auditory perception. But, little is known on how CI patients perceive prosodic information relating to speech content. Our study aimed to identify how CI patients use MSI between visual and auditory information to process paralinguistic prosodic information of multimodal speech and the visual strategies employed. A psychophysics assessment was developed, in which CI patients and hearing controls (NH) had to distinguish between a question and a statement. The controls were separated into two age groups (young and aged-matched) to dissociate any effect of aging. In addition, the oculomotor strategies used when facing a speaker in this prosodic decision task were recorded using an eye-tracking device and compared to controls. This study confirmed that prosodic processing is multisensory but it revealed that CI patients showed significant supra-normal audiovisual integration for prosodic information compared to hearing controls irrespective of age. This study clearly showed that CI patients had a visuo-auditory gain more than 3 times larger than that observed in hearing controls. Furthermore, CI participants performed better in the visuo-auditory situation through a specific oculomotor exploration of the face as they significantly fixate the mouth region more than young NH participants who fixate the eyes, whereas the aged-matched controls presented an intermediate exploration pattern equally reported between the eyes and mouth. To conclude, our study demonstrated that CI patients have supra-normal skills MSI when integrating visual and auditory linguistic prosodic information, and a specific adaptive strategy developed as it participates directly in speech content comprehension.

An Integrative Review of Current Practice Models and/or Process of Family-Centered Early Intervention for Children Who Are Deaf or Hard of Hearing.

Over the past few decades, there has been an increasing shift toward emphasizing the importance of the child's family taking an active role in the habilitation process through family-centered early intervention (FCEI) programs. Accordingly, the Health Professions Council of South Africa recommends that early intervention services following confirmation of hearing loss must be family-centered within a community-based model of service delivery that is culturally congruent. The aim of this study was to explore and document current evidence reflecting trends in FCEI for children who are deaf or hard of hearing (DHH) by identifying and describing current practice models and/or processes of FCEI for these children. This study describes our first steps in formulating a framework for FCEI for children who are DHH in South Africa. An integrative literature review was conducted. Sage, Science Direct, PubMed, and Google Scholar databases were searched for studies published in English between January 2009 and January 2019 reporting on FCEI programs for children who are DHH. Studies that focused on the following were excluded from the study: speech and language outcomes of children, youth, and adults who are DHH; education for children who are DHH; universal newborn hearing screening; professionals' roles in early hearing detection and intervention; diagnosis of hearing loss; and sign language. Kappa statistics were performed to determine agreement between reviewers. Twenty-two studies were included in the review. Cohen's kappa revealed a substantial agreement (κ = 0.8) between reviewers for data extraction and synthesis in terms of the articles that met the criteria for inclusion in the review. Findings were discussed under 5 themes: caregiver involvement; caregiver coaching/information sharing; caregiver satisfaction; challenges with FCEI; and telehealth. Generally, there is sufficient evidence for FCEI, with caregivers indicating the need for full involvement in their children's care. Methods of caregiver involvement involving caregiver coaching/information sharing need to be culturally and linguistically appropriate, with sensitivities around time and manner. This increases caregiver satisfaction with intervention programs and improves outcomes for children who are DHH. Challenges identified by the studies raise implications for early hearing detection and intervention programs, as well as Departments of Health and Social Welfare. These included logistical challenges, professional-related challenges, and caregiver-related challenges. Various aspects of FCEI have been reported in the review. Findings of these studies have significant implications for the formulation of quality FCEI programs to ensure contextually relevant and contextually responsive care of children who are DHH.

Sensitivity to binaural temporal-envelope beats with single-sided deafness and a cochlear implant as a measure of tonotopic match (L).

For cochlear-implant users with near-normal contralateral hearing, a mismatch between the frequency-to-place mapping in the two ears could produce a suboptimal performance. This study assesses tonotopic matches via binaural interactions. Dynamic interaural time-difference sensitivity was measured using bandpass-filtered pulse trains at different rates in the acoustic and implanted ear, creating binaural envelope beats. Sensitivity to beats should peak when the same tonotopic region is stimulated in both ears. All nine participants detected dynamic interaural timing differences and demonstrated some frequency selectivity. This method provides a guide to frequency-to-place mapping without compensation for inherent latency differences between the acoustic and implanted ears.

Dynamic current steering with phantom electrode in cochlear implants.

Phantom electrode (PE) stimulation can extend the lower limit of pitch perception with cochlear implants (CIs) by using simultaneous out-of-phase stimulation of the most apical primary electrode and the adjacent basal compensating electrode. The total electrical field may push the excitation pattern beyond the most apical electrode to elicit a lower pitch, depending on the ratio of current between the compensating and primary electrodes (i.e., the compensation coefficient σ). This study tested the hypothesis that dynamic current steering of PE stimuli can be implemented by varying σ over time to encode spectral details in low frequencies. To determine the range of σ for current steering and the corresponding current levels, Experiment 1 tested CI users' loudness balance and pitch ranking of static PE stimuli with σ from 0 to 0.6 in steps of 0.2. It was found that the equal-loudness most comfortable level significantly increased with σ and can be modeled by a piecewise linear function of σ. Consistent with the previous findings, higher σ elicited either lower or similar pitches without salient pitch reversals than lower σ. Based on the results of Experiment 1, Experiment 2 created flat, rising, and falling pitch contours of 300-1000 ms using dynamic PE stimuli with time-varying σ from 0 to 0.6 and equal-loudness current levels. In a pitch contour identification (PCI) task, CI users scored 80% and above on average. Increasing the stimulus duration from 300 to 1000 ms slightly but did not significantly improve the PCI scores. Across subjects, the 1000-ms PCI scores in Experiment 2 were significantly correlated with the cumulative pitch-ranking sensitivity in Experiment 1. It is thus feasible to use dynamic current steering with PE to encode low-frequency pitch cues for CI users.

Forward masking patterns by low and high-rate stimulation in cochlear implant users: Differences in masking effectiveness and spread of neural excitation.

The goal of the present study was to compare forward masking patterns by stimulation of low and high rates in cochlear implant users. Postlingually deafened Cochlear Nucleus® device users participated in the study. In experiment 1, two maskers of different rates (250 and 1000 pulses per second) were set at levels that produced equal masking for a probe presented at the same electrode as the maskers. This aligned the two masking functions at the on-site probe location. Then their forward masking patterns for the far probes were compared. Results showed that slope of the masked probe-threshold decay as a function of probe-masker separation was steeper for the high-rate than the low-rate masker. A linear model indicated that this difference in spread of neural excitation (SOE) was accounted for by two factors that were not correlated with each other. One factor was that the low-rate masker required a considerably higher current level to be equally effective in masking as the high-rate masker. The second factor was the effect of stimulation rate on loudness, i.e., integration of multiple pulses. This was consistent with our hypothesis that if an increase in stimulation rate does not result in an increased total neural response, then it is unlikely that the change in rate would change spatial distribution of the neural activity. Interestingly, the difference in masking effectiveness of the maskers predicted subjects' speech recognition. Poorer performers were those who showed more comparable masking effects by maskers of different rates. The difference in the masking effectiveness may indirectly measure the auditory neurons' excitability, which predicts speech recognition. In experiment 2, SOE of the high-rate and low-rate maskers were compared at a level that is clinically relevant, i.e., equal loudness. At equal loudness, high-rate stimulation not only produced an overall greater amount of forward masking, but also a shallower decay of masking with probe-masker separation (wider SOE), compared to low rate. The difference in SOE was the opposite to the findings from experiment 1. Whether the maskers were calibrated for equal masking or loudness, the absolute current level was always higher for the low-rate masker, which suggests that the SOE patterns cannot be explained by current spread alone. The fact that high-rate stimulation produced greater masking and wider SOE at equal loudness may explain why using high stimulation rates has not produced consistent benefits for speech recognition, and why lowering stimulation rate from the manufacturer's default sometimes results in improved speech recognition for subjects.

Effect on vestibular function of cochlear implantation by partial deafness treatment-electro acoustic stimulation (PDT-EAS).

PURPOSE: Although the cochlear implantation procedure does not interfere with vestibular structures directly, both the vestibulum and the cochlea share the same inner ear fluid space, and this fluid may be responsible for transferring possibly damaging forces from one to the other. The purpose of the study is to assess postoperative vestibular function after partial deafness treatment-electro-acoustic stimulation (PDT-EAS) cochlear implantation. METHODS: Fifty-five patients were included in the study (30 females, 25 males, age 11-80, mean 41.8 ± 19.35). cVEMP and oVEMP were performed preoperatively and 1-3 months after cochlear implantation. Caloric and vHIT tests were conducted preoperatively and 4-6 months after cochlear implantation. RESULTS: Our study shows that, based on a wide range of electrodes, use of PDT-EAS is protective in terms of preserving vestibular function. It gives a rate of saccular damage of 15.79%, utricular damage of 19.04%, and a horizontal semicircular canal response reduction of 15.79%. CONCLUSIONS: PDT-EAS is protective in terms of preserving vestibular function. Nevertheless, it should be emphasized that the risk of vestibular damage cannot be totally eliminated even when hearing preservation techniques are adopted.

Perceptual Differences Between Low-Frequency Analog and Pulsatile Stimulation as Shown by Single- and Multidimensional Scaling.

Cochlear-implant users who have experienced both analog and pulsatile sound coding strategies often have strong preferences for the sound quality of one over the other. This suggests that analog and pulsatile stimulation may provide different information or sound quality to an implant listener. It has been well documented that many implant listeners both prefer and perform better with multichannel analog than multichannel pulsatile strategies, although the reasons for these differences remain unknown. Here, we examine the perceptual differences between analog and pulsatile stimulation on a single electrode. A multidimensional scaling task, analyzed across two dimensions, suggested that pulsatile stimulation was perceived to be considerably different from analog stimulation. Two associated tasks using single-dimensional scaling showed that analog stimulation was perceived to be less Clean on average than pulsatile stimulation and that the perceptual differences were not related to pitch. In a follow-up experiment, it was determined that the perceptual differences between analog and pulsatile stimulation were not dependent on the interpulse gap present in pulsatile stimulation. Although the results suggest that there is a large perceptual difference between analog and pulsatile stimulation, further work is needed to determine the nature of these differences.

Current Focusing to Reduce Channel Interaction for Distant Electrodes in Cochlear Implant Programs.

Speech understanding abilities are highly variable among cochlear implant (CI) listeners. Poor electrode-neuron interfaces (ENIs) caused by sparse neural survival or distant electrode placement may lead to increased channel interaction and reduced speech perception. Currently, it is not possible to directly measure neural survival in CI listeners; therefore, obtaining information about electrode position is an alternative approach to assessing ENIs. This information can be estimated with computerized tomography (CT) imaging; however, postoperative CT imaging is not often available. A reliable method to assess channel interaction, such as the psychophysical tuning curve (PTC), offers an alternative way to identify poor ENIs. This study aimed to determine (a) the within-subject relationship between CT-estimated electrode distance and PTC bandwidths, and (b) whether using focused stimulation on channels with suspected poor ENI improves vowel identification and sentence recognition. In 13 CI listeners, CT estimates of electrode-to-modiolus distance and PTCs bandwidths were measured for all available electrodes. Two test programs were created, wherein a subset of electrodes used focused stimulation based on (a) broad PTC bandwidth (Tuning) and (b) far electrode-to-modiolus distance (Distance). Two control programs were also created: (a) Those channels not focused in the Distance program (Inverse-Control), and (b) an all-channel monopolar program (Monopolar-Control). Across subjects, scores on the Distance and Tuning programs were significantly higher than the Inverse-Control program, and similar to the Monopolar-Control program. Subjective ratings were similar for all programs. These findings suggest that focusing channels suspected to have a high degree of channel interaction result in quite different outcomes, acutely.

A directional remote-microphone for bimodal cochlear implant recipients.

To evaluate whether speech recognition in noise differs according to whether a wireless remote microphone is connected to just the cochlear implant (CI) or to both the CI and to the hearing aid (HA) in bimodal CI users. The second aim was to evaluate the additional benefit of the directional microphone mode compared with the omnidirectional microphone mode of the wireless microphone. This prospective study measured Speech Recognition Thresholds (SRT) in babble noise in a 'within-subjects repeated measures design' for different listening conditions. Eighteen postlingually deafened adult bimodal CI users. No difference in speech recognition in noise in the bimodal listening condition was found between the wireless microphone connected to the CI only and to both the CI and the HA. An improvement of 4.1 dB was found for switching from the omnidirectional microphone mode to the directional mode in the CI only condition. The use of a wireless microphone improved speech recognition in noise for bimodal CI users. The use of the directional microphone mode led to a substantial additional improvement of speech perception in noise for situations with one target signal.

Assessing auditory nerve condition by tone decay in deaf subjects with a cochlear implant.

The condition of the auditory nerve is a factor determining hearing performance of cochlear implant (CI) recipients. Abnormal loudness adaptation is associated with poor auditory nerve survival. We examined which stimulus conditions are suitable for tone decay measurements to differentiate between CI recipients with respect to their speech perception. Tone decay was defined here as occurring when the percept disappears before the stimulus stops. We measured the duration of the percept of a 60-s pulse train. Current levels ranged from below threshold up to maximum acceptable loudness, pulse rates from 250 to 5000 pulses/s, and duty cycles (percentages of time the burst of pulses is on) from 10% to 100%. Ten adult CI recipients were included: seven with good and three with poor speech perception. Largest differences among the subjects were found at 5000 pulses/s and 100% duty cycle. The well performing subjects had a continuous percept of the 60-s stimulus within 3 dB above threshold. Two poorly performing subjects showed abnormal loudness adaptation, that is, no continuous percept even at levels greater than 6 dB above threshold. We conclude that abnormal loudness adaptation can be detected via an electric tone decay test using a high pulse rate and 100% duty cycle.

When singing with cochlear implants, are two ears worse than one for perilingually/postlingually deaf individuals?

Many individuals with bilateral cochlear implants hear different pitches when listening with their left versus their right cochlear implant. This conflicting information could potentially increase the difficulty of singing with cochlear implants. To determine if bilateral cochlear implants are detrimental for singing abilities, ten perilingually/postlingually deaf bilateral adult cochlear implant users were asked to sing "Happy Birthday" when using their left, right, both, or neither cochlear implant. The results indicated that bilateral cochlear implant users have more difficulty singing the appropriate pitch contour when using both cochlear implants as opposed to the better ear alone.

Development of electrophysiological and behavioural measures of electrode discrimination in adult cochlear implant users.

The plasticity of the auditory system enables it to adjust to electrical stimulation from cochlear implants (CI). Whilst speech perception may develop for many years after implant activation, very little is known about the changes in auditory processing that underpin these improvements. Such an understanding could help guide interventions that improve hearing performance. In this longitudinal study, we examine how electrode discrimination ability changes over time in newly implanted adult CI users. Electrode discrimination was measured with a behavioural task as well as the spatial auditory change complex (ACC), which is a cortical response to a change in place of stimulation. We show that there was significant improvement in electrode discrimination ability over time, though in certain individuals the process of accommodation was slower and more limited. We found a strong relationship between objective and behavioural measures of electrode discrimination using pass-fail rules. In several cases, the development of the spatial ACC preceded accurate behavioural discrimination. These data provide evidence for plasticity of auditory processing in adult CI users. Behavioural electrode discrimination score but not spatial ACC amplitude was found to be a significant predictor of speech perception. We suggest that it would be beneficial to measure electrode discrimination in CI users and that interventions that exploit the plastic capacity of the auditory system to improve basic auditory processing, could be used to optimize performance in CI users.

Verbal learning and memory in prelingually deaf children with cochlear implants.

OBJECTIVE: Deaf children with cochlear implants (CIs) show poorer verbal working memory compared to normal-hearing (NH) peers, but little is known about their verbal learning and memory (VLM) processes involving multi-trial free recall. DESIGN: Children with CIs were compared to NH peers using the California Verbal Learning Test for Children (CVLT-C). STUDY SAMPLE: Participants were 21 deaf (before age 6 months) children (6-16 years old) implanted prior to age 3 years, and 21 age-IQ matched NH peers. RESULTS: Results revealed no differences between groups in number of words recalled. However, CI users showed a pattern of increasing use of serial clustering strategies across learning trials, whereas NH peers decreased their use of serial clustering strategies. In the CI sample (but not in the NH sample), verbal working memory test scores were related to resistance to the build-up of proactive interference, and sentence recognition was associated with performance on the first exposure to the word list and to the use of recency recall strategies. CONCLUSIONS: Children with CIs showed robust evidence of VLM comparable to NH peers. However, their VLM processing (especially recency and proactive interference) was related to speech perception outcomes and verbal WM in different ways from NH peers.

Categorisation of natural sounds at different stages of auditory recovery in cochlear implant adult deaf patients.

Previous studies have demonstrated that cochlear implant (CI) patients are more efficient at performing sound categorisation than sound identification. However, it remains unclear how this categorisation capacity develops with time during the rehabilitation period after implantation. To investigate the role of the post-implantation auditory experience in the broad sound categorisation in CI patients, we recruited CI patients with different durations of CI experience: Newly implanted CI patients (less than six months), Intermediate CI patients (6-14 months) and Experienced CI patients with a duration of implantation greater than 14 months. The patients completed a Free Sorting Task (FST), which allowed them to categorise 16 natural sounds based on their own criteria. We found an early deficit in categorisation, especially for vocal sounds; the categorisation started to improve after approximately six months post-implantation with a change of categorisation strategy which relied on different acoustic cues as a function of time after CI. The separation of the category of vocal sounds from other sounds significantly increased between the Newly implanted and Intermediate groups, i.e. as experience with the cochlear implant was acquired. The categorisation accuracy of vocal sounds was significantly correlated with the post-implantation period only in the group of newly implanted CI patients. This is the first study to show that the categorisation of vocal sounds with respect to non-vocal sounds improves during the rehabilitation period post-implantation. The first six-month post-implantation period appears to be crucial in this process. Our results demonstrate that patients in different rehabilitation periods use different acoustic cues, which increase their complexity with the CI experience.

Examination of Prosody and Timbre Perception in Adults With Cochlear Implants Comparing Different Fine Structure Coding Strategies.

PURPOSE: This study aimed to investigate whether adults with cochlear implants benefit from a change of fine structure (FS) coding strategies regarding the discrimination of prosodic speech cues, timbre cues, and the identification of natural instruments. The FS processing (FSP) coding strategy was compared to 2 settings of the FS4 strategy. METHOD: A longitudinal crossover, double-blinded study was conducted. This study consisted of 2 parts, with 14 participants in the first part and 12 participants in the second part. Each part lasted 3 months, in which participants were alternately fitted with either the established FSP strategy or 1 of the 2 newly developed FS4 settings. Participants had to complete an intonation identification test; a timbre discrimination test in which 1 of 2 isolated cues changed, either the spectral centroid or the spectral irregularity; and an instrument identification test. RESULTS: A significant effect was seen in the discrimination of spectral irregularity with 1 of the 2 FS4 settings. The improvement was seen in the FS4 setting in which the upper envelope channels had a low stimulation rate. This improvement was not seen with the FS4 setting that had a higher stimulation rate on the envelope channels. CONCLUSIONS: In general, the FSP strategy and the 2 settings of the FS4 strategy provided similar levels in the perception of prosody and timbre cues, as well as in the identification of instruments.

Why Do Hearing Aids Fail to Restore Normal Auditory Perception?

Hearing loss is a widespread condition that is linked to declines in quality of life and mental health. Hearing aids remain the treatment of choice, but, unfortunately, even state-of-the-art devices provide only limited benefit for the perception of speech in noisy environments. While traditionally viewed primarily as a loss of sensitivity, hearing loss is also known to cause complex distortions of sound-evoked neural activity that cannot be corrected by amplification alone. This Opinion article describes the effects of hearing loss on neural activity to illustrate the reasons why current hearing aids are insufficient and to motivate the use of new technologies to explore directions for improving the next generation of devices.

What can stimulus polarity and interphase gap tell us about auditory nerve function in cochlear-implant recipients?

Modeling studies suggest that differences in neural responses between polarities might reflect underlying neural health. Specifically, large differences in electrically evoked compound action potential (eCAP) amplitudes and amplitude-growth-function (AGF) slopes between polarities might reflect poorer peripheral neural health, whereas more similar eCAP responses between polarities might reflect better neural health. The interphase gap (IPG) has also been shown to relate to neural survival in animal studies. Specifically, healthy neurons exhibit larger eCAP amplitudes, lower thresholds, and steeper AGF slopes for increasing IPGs. In ears with poorer neural survival, these changes in neural responses are generally less apparent with increasing IPG. The primary goal of this study was to examine the combined effects of stimulus polarity and IPG within and across subjects to determine whether both measures represent similar underlying mechanisms related to neural health. With the exception of one measure in one group of subjects, results showed that polarity and IPG effects were generally not correlated in a systematic or predictable way. This suggests that these two effects might represent somewhat different aspects of neural health, such as differences in site of excitation versus integrative membrane characteristics, for example. Overall, the results from this study suggest that the underlying mechanisms that contribute to polarity and IPG effects in human CI recipients might be difficult to determine from animal models that do not exhibit the same anatomy, variance in etiology, electrode placement, and duration of deafness as humans.

Perceptual changes with monopolar and phantom electrode stimulation.

Phantom electrode (PE) stimulation is achieved by simultaneously stimulating out-of-phase from two adjacent intra-cochlear electrodes with different amplitudes. If the basal electrode stimulates with a smaller amplitude than the apical electrode of the pair, the resulting electrical field is pushed away from the basal electrode producing a lower pitch. There is great interest in using PE stimulation in a processing strategy as it can be used to provide stimulation to regions of the cochlea located more apically than the most apical contact on the electrode array. The result is that even lower pitch sensations can be provided without additional risk of a deeper insertion. However, it is unknown if there are perceptual differences between monopolar (MP) and PE stimulation other than a shift in place pitch. Furthermore, it is unknown if the effect and magnitude of changing from MP to PE stimulation is dependent on electrode location. This study investigates the perceptual differences (including pitch and other sound quality differences) at multiple electrode positions using MP and PE stimulation using both a multidimensional scaling procedure (MDS) and a traditional scaling procedure. 10 Advanced Bionics users reported the perceptual distances between 5 single electrode (typically 1, 3, 5, 7, and 9) stimuli in either MP or PE (σ = 0.5) mode. Subjects were asked to report how perceptually different each pair of stimuli were using any perceived differences except loudness. Subsequently, each stimulus was presented in isolation and subjects scaled how "high" or how "clean" each sounded. Results from the MDS task suggest that perceptual differences between MP and PE stimulation can be explained by a single dimension. The traditional scaling suggests that the single dimension is place pitch. PE stimulation elicits lower pitch perceptions in all cochlear regions. Analysis of Cone Beam Computer Tomography (CBCT) data suggests that PE stimulation may be more effective at the apical part of the cochlea. PE stimulation can be used for new sound coding strategies in order to extend the pitch range for cochlear implant (CI) users without perceptual side effects.

Optimal gain control step sizes for bimodal stimulation.

OBJECTIVE: Cochlear implants (CI) and hearing aids (HA) have a gain control that allows the bimodal user to change the loudness. Due to differences in dynamic range between CI and HA, an equal change of the gains of the two devices results in different changes in loudness. The objective was to relate and individualise the step sizes of the loudness controls to obtain a similar perceptual effect in the two ears. DESIGN: We used loudness models parametrised for individual users to find a relation between the controls of the CI and the HA such that each step resulted in an equal change in loudness. We conducted loudness balancing experiments to validate the results. STUDY SAMPLE: Eleven bimodal users of whom six were tested in a prior study. RESULTS: The difference between the optimal gain from the loudness balancing procedure and actual gain was 3.3 dB when the new relation was applied. In contrast, the difference was 8 dB if equal step sized were applied at both sides. CONCLUSION: We can relate the controls such that each step results in a similar loudness difference.

Pitch matching in bimodal cochlear implant patients: Effects of frequency, spectral envelope, and level.

This study systematically investigated the effects of frequency, level, and spectral envelope on pitch matching in twelve bimodal cochlear implant (CI) users. The participants were asked to vary the frequency and level of a pure or complex tone (adjustable sounds) presented in the non-implanted ear to match the pitch and loudness of different reference stimuli presented to the implanted ear. Three reference sounds were used: single electrode pulse trains, pure tones, and piano notes. The data showed a significant effect of the frequency and complexity of the reference sounds. No significant effect of the level of the reference sounds was found. The magnitude of effect of frequency was compressed in the implanted ear: on average a difference of seven semitones in the non-implanted ear induced the same pitch change as a difference of 19 to 24 semitones for a stimulus presented to the implanted ear. The spectral envelope of the adjustable sound presented to the non-implanted ear also had a significant effect. The matched frequencies were higher by an average of six semitones for the pure tone compared to a complex tone. Overall, the CI listeners might have matched the stimuli based on timbre characteristics such as brightness.