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Differences in current spread and/or spread of excitation may differently affect integration of acoustic and electric hearing within (electric-acoustic stimulation, or EAS) or across ears (bimodal). In this study, vowel recognition was measured in normal-hearing subjects listening to simulations of cochlear implant (CI), EAS, and bimodal listening. Residual acoustic hearing was limited between 0.1 and 0.6 kHz. The carrier bandwidth in the CI simulations was varied across carriers: broad-band noise, narrow-band noise, and sine waves. Results showed that reducing the bandwidth (and the inherent noise fluctuations) in the CI simulations significantly affected CI-only and bimodal performance, but not EAS performance.

For bimodal cochlear implant users, acoustic and electric hearing has been shown to contribute differently to speech and music perception. However, differences in test paradigms and stimuli in speech and music testing can make it difficult to assess the relative contributions of each device. To address these concerns, the Sung Speech Corpus (SSC) was created. The SSC contains 50 monosyllable words sung over an octave range and can be used to test both speech and music perception using the same stimuli. Here SSC data are presented with normal hearing listeners and any advantage of musicianship is examined.

In normal hearing (NH), the perception of the gender of a speaker is strongly affected by two anatomically related vocal characteristics: the fundamental frequency (F0), related to vocal pitch, and the vocal tract length (VTL), related to the height of the speaker. Previous studies on gender categorization in cochlear implant (CI) users found that performance was variable, with few CI users performing at the level of NH listeners. Data collected with recorded speech produced by multiple talkers suggests that CI users might rely more on F0 and less on VTL than NH listeners. However, because VTL cannot be accurately estimated from recordings, it is difficult to know how VTL contributes to gender categorization. In the present study, speech was synthesized to systematically vary F0, VTL, or both. Gender categorization was measured in CI users, as well as in NH participants listening to unprocessed (only synthesized) and vocoded (and synthesized) speech. Perceptual weights for F0 and VTL were derived from the performance data. With unprocessed speech, NH listeners used both cues (normalized perceptual weight: F0 = 3.76, VTL = 5.56). With vocoded speech, NH listeners still made use of both cues but less efficiently (normalized perceptual weight: F0 = 1.68, VTL = 0.63). CI users relied almost exclusively on F0 while VTL perception was profoundly impaired (normalized perceptual weight: F0 = 6.88, VTL = 0.59). As a result, CI users' gender categorization was abnormal compared to NH listeners. Future CI signal processing should aim to improve the transmission of both F0 cues and VTL cues, as a normal gender categorization may benefit speech understanding in competing talker situations.

Objective: Most studies have evaluated cochlear implant (CI) performance using "clear" speech materials, which are highly intelligible and well articulated. CI users may encounter much greater variability in speech patterns in the "real world," including synthetic speech. In this study, the authors measured sentence recognition with multiple talkers and speaking rates, and with naturally produced and synthetic speech in listeners with normal hearing (NH) and CIs., Design: NH and CI subjects were asked to recognize naturally produced or synthetic sentences, presented at a slow, normal, or fast speaking rate. Natural speech was produced by one male and one female talker; synthetic speech was generated to simulate a male and female talker. For natural speech, the speaking rate was time-scaled while preserving voice pitch and formant frequency information. For synthetic speech, the speaking rate was adjusted within the speech synthesis engine. NH subjects were tested while listening to unprocessed speech or to an eight-channel acoustic CI simulation. CI subjects were tested while listening with their clinical processors and the recommended microphone sensitivity and volume settings., Results: The NH group performed significantly better than did the CI-simulation group, and the CI-simulation group performed significantly better than did the CI group. For all subject groups, sentence recognition was significantly better with natural speech than with synthetic speech. The performance deficit with synthetic speech was relatively small for NH subjects listening to unprocessed speech. However, the performance deficit with synthetic speech was much greater for CI subjects and for CI-simulation subjects. There was significant effect of talker gender, with slightly better performance with the female talker for CI subjects and slightly better performance with the male talker for the CI simulations. For all subject groups, sentence recognition was significantly poorer only at the fast rate. CI performance was very poor (approximately 10% correct) at the fast rate., Conclusions: CI listeners are susceptible to variability in speech patterns caused by speaking rate and production style (natural versus synthetic). CI performance with clear speech materials may overestimate performance in real-world listening conditions. The poorer CI performance may be because of other factors besides reduced spectro-temporal resolution, such the quality of electric stimulation, duration of deafness, or cortical processing. Optimizing the input or training may improve CI users' tolerance for variability in speech patterns.

Objective: The full benefit of bilateral cochlear implants may depend on the unilateral performance with each device, the speech materials, processing ability of the user, and/or the listening environment. In this study, bilateral and unilateral speech performances were evaluated in terms of recognition of phonemes and sentences presented in quiet or in noise., Design: Speech recognition was measured for unilateral left, unilateral right, and bilateral listening conditions; speech and noise were presented at 0° azimuth. The 'binaural benefit' was defined as the difference between bilateral performance and unilateral performance with the better ear., Study Sample: Nine adults with bilateral cochlear implants participated., Results: On average, results showed a greater binaural benefit in noise than in quiet for all speech tests. More importantly, the binaural benefit was greater when unilateral performance was similar across ears. As the difference in unilateral performance between ears increased, the binaural advantage decreased; this functional relationship was observed across the different speech materials and noise levels even though there was substantial intra- and inter-subject variability., Conclusions: The results indicate that subjects who show symmetry in speech recognition performance between implanted ears in general show a large binaural benefit.

Objectives: (1) To investigate whether voice gender discrimination (VGD) could be a useful indicator of the spectral and temporal processing abilities of individual cochlear implant (CI) users; (2) To examine the relationship between VGD and speech recognition with CI when comparable acoustic cues are used for both perception processes., Design: VGD was measured using two talker sets with different inter-gender fundamental frequencies (F(0)), as well as different acoustic CI simulations. Vowel and consonant recognition in quiet and noise were also measured and compared with VGD performance., Study Sample: Eleven postlingually deaf CI users., Results: The results showed that (1) mean VGD performance differed for different stimulus sets, (2) VGD and speech recognition performance varied among individual CI users, and (3) individual VGD performance was significantly correlated with speech recognition performance under certain conditions., Conclusions: VGD measured with selected stimulus sets might be useful for assessing not only pitch-related perception, but also spectral and temporal processing by individual CI users. In addition to improvements in spectral resolution and modulation detection, the improvement in higher modulation frequency discrimination might be particularly important for CI users in noisy environments.

Currently there are few standardized speech testing materials for Mandarin-speaking cochlear implant (CI) listeners. In this study, Mandarin speech perception (MSP) sentence test materials were developed and validated in normal-hearing subjects listening to acoustic simulations of CI processing. Percent distribution of vowels, consonants, and tones within each MSP sentence list was similar to that observed across commonly used Chinese characters. There was no significant difference in sentence recognition across sentence lists. Given the phonetic balancing within lists and the validation with spectrally degraded speech, the present MSP test materials may be useful for assessing speech performance of Mandarin-speaking CI listeners., (© 2011 Acoustical Society of America)

Cochlear implant (CI) users' speech understanding may be influenced by different speaking styles. In this study, speech recognition was measured in Mandarin-speaking CI and normal-hearing (NH) subjects for sentences produced according to four styles: slow, normal, fast, and whispered. CI subjects were tested using their clinical processors; NH subjects were tested while listening to a four-channel CI simulation. Performance gradually worsened with increasing speaking rate and was much poorer with whispered speech. CI performance was generally similar to NH performance with the four-channel simulation. Results suggest that some speaking styles, especially whispering, may negatively affect Mandarin-speaking CI users' speech understanding., (© 2011 Acoustical Society of America)

Because of the poor spectral resolution in cochlear implants (CIs), fundamental frequency (F0) cues are not well preserved. Chinese-speaking CI users may have great difficulty understanding speech produced by competing talkers, due to conflicting tones. In this study, normal-hearing listeners' concurrent Chinese syllable recognition was measured with unprocessed speech and CI simulations. Concurrent syllables were constructed by summing two vowels from a male talker (with identical mean F0's) or one vowel from each of a male and a female talker (with a relatively large F0 separation). CI signal processing was simulated using four- and eight-channel noise-band vocoders; the degraded spectral resolution may limit listeners' ability to utilize talker and/or tone differences. The results showed that concurrent speech recognition was significantly poorer with the CI simulations than with unprocessed speech. There were significant interactions between the talker and speech-processing conditions, e.g., better tone and syllable recognitions with the male-female condition for unprocessed speech, and with the male-male condition for eight-channel speech. With the CI simulations, competing tones interfered with concurrent-tone and syllable recognitions, but not vowel recognition. Given limited pitch cues, subjects were unable to use F0 differences between talkers or tones for concurrent Chinese syllable recognition.

Chinese sentence recognition strongly relates to the reception of tonal information. For cochlear implant (CI) users with residual acoustic hearing, tonal information may be enhanced by restoring low-frequency acoustic cues in the nonimplanted ear. The present study investigated the contribution of low-frequency acoustic information to Chinese speech recognition in Mandarin-speaking normal-hearing subjects listening to acoustic simulations of bilaterally combined electric and acoustic hearing. Subjects listened to a 6-channel CI simulation in one ear and low-pass filtered speech in the other ear. Chinese tone, phoneme, and sentence recognition were measured in steady-state, speech-shaped noise, as a function of the cutoff frequency for low-pass filtered speech. Results showed that low-frequency acoustic information below 500 Hz contributed most strongly to tone recognition, while low-frequency acoustic information above 500 Hz contributed most strongly to phoneme recognition. For Chinese sentences, speech reception thresholds (SRTs) improved with increasing amounts of low-frequency acoustic information, and significantly improved when low-frequency acoustic information above 500 Hz was preserved. SRTs were not significantly affected by the degree of spectral overlap between the CI simulation and low-pass filtered speech. These results suggest that, for CI patients with residual acoustic hearing, preserving low-frequency acoustic information can improve Chinese speech recognition in noise.

Because of the limited spectra-temporal resolution associated with cochlear implants, implant patients often have greater difficulty with multitalker speech recognition. The present study investigated whether multitalker speech recognition can be improved by applying speaker normalization techniques to cochlear implant speech processing. Multitalker Chinese vowel recognition was tested with normal-hearing Chinese-speaking subjects listening to a 4-channel cochlear implant simulation, with and without speaker normalization. For each subject, speaker normalization was referenced to the speaker that produced the best recognition performance under conditions without speaker normalization. To match the remaining speakers to this "optimal" output pattern, the overall frequency range of the analysis filter bank was adjusted for each speaker according to the ratio of the mean third formant frequency values between the specific speaker and the reference speaker. Results showed that speaker normalization provided a small but significant improvement in subjects' overall recognition performance. After speaker normalization, subjects' patterns of recognition performance across speakers changed, demonstrating the potential for speaker-dependent effects with the proposed normalization technique.

After implantation, postlingually deafened cochlear implant (CI) patients must adapt to both spectrally reduced and spectrally shifted speech, due to the limited number of electrodes and the limited length of the electrode array. This adaptation generally occurs during the first three to six months of implant use and may continue for many years. To see whether moderate speech training can accelerate this learning process, 16 naïve, normal-hearing listeners were trained with spectrally shifted speech via an eight-channel acoustic simulation of CI speech processing. Baseline vowel and consonant recognition was measured for both spectrally shifted and unshifted speech. Short daily training sessions were conducted over five consecutive days, using four different protocols. For the test-only protocol, no improvement was seen over the five-day period. Similarly, sentence training provided little benefit for vowel recognition. However, after five days of targeted phoneme training, subjects' recognition of spectrally shifted vowels significantly improved in most subjects. This improvement did not generalize to the spectrally unshifted vowel and consonant tokens, suggesting that subjects adapted to the specific spectral shift, rather than to the eight-channel processing in general. Interestingly, significant improvement was also observed for the recognition of spectrally shifted consonants. The largest improvement was observed with targeted vowel contrast training, which did not include any explicit consonant training. These results suggest that targeted phoneme training can accelerate adaptation to spectrally shifted speech. Given these results with normal-hearing listeners, auditory rehabilitation tools that provide targeted phoneme training may be effective in improving the speech recognition performance of adult CI users.

Tone recognition is important for speech understanding in tonal languages such as Mandarin Chinese. Cochlear implant patients are able to perceive some tonal information by using temporal cues such as periodicity-related amplitude fluctuations and similarities between the fundamental frequency (F0) contour and the amplitude envelope. The present study investigates whether modifying the amplitude envelope to better resemble the F0 contour can further improve tone recognition in multichannel cochlear implants. Chinese tone and vowel recognition were measured for six native Chinese normal-hearing subjects listening to a simulation of a four-channel cochlear implant speech processor with and without amplitude envelope enhancement. Two algorithms were proposed to modify the amplitude envelope to more closely resemble the F0 contour. In the first algorithm, the amplitude envelope as well as the modulation depth of periodicity fluctuations was adjusted for each spectral channel. In the second algorithm, the overall amplitude envelope was adjusted before multichannel speech processing, thus reducing any local distortions to the speech spectral envelope. The results showed that both algorithms significantly improved Chinese tone recognition. By adjusting the overall amplitude envelope to match the F0 contour before multichannel processing, vowel recognition was better preserved and less speech-processing computation was required. The results suggest that modifying the amplitude envelope to more closely resemble the F0 contour may be a useful approach toward improving Chinese-speaking cochlear implant patients' tone recognition.

Objective: To evaluate the effects of speech processing strategy on Chinese tone recognition by Nucleus-24 cochlear implant users., Methods: Recognition of Chinese tones was measured with the spectral peak (SPEAK), advanced combination encoders (ACE), and continuous interleaved sampling (CIS) strategies in nine Nucleus-24 cochlear implant users. Recognition of Chinese sentences was also measured with the ACE strategy (1200 pulse-per-sec on each electrode). As the stimulation rate in the SPEAK processor is fixed at 250 pulses per sec per electrode (ppspe), only one stimulation rate was tested with the SPEAK processor. Three stimulation rates (900, 1200, and 1800 ppspe) were tested in the ACE processing conditions. Four stimulation rates (1200, 1800, 2400, and 3600 ppspe) were tested in the CIS processing conditions. Because the number of stimulating electrodes determines the stimulation rate in CIS processors, the number of electrodes was reduced from 12 to 8 to 6 to 4 to achieve stimulation rates of 1200, 1800, 2400, to 3600 pps., Results: Results showed that mean Chinese tone recognition scores with the ACE and CIS strategies were significantly higher than those with the SPEAK strategy. For the ACE strategy, there was no significant difference in performance among the various stimulation rates (for the ACE processing conditions, the stimulation rate was varied while keeping the number of stimulating electrodes constant). For the CIS strategy, there was no significant difference among the various stimulation rates and electrode configurations (for the CIS processing conditions, the stimulation rate was varied by changing the number of stimulating electrodes). There was also a significant correlation between Chinese tone recognition and the recognition of everyday sentences., Conclusions: The latest-generation speech processing strategies were able to provide cochlear implant users with considerable amounts of tonal information. However, Chinese tone recognition was significantly affected by the choice in speech processing strategy. Relatively higher stimulation rates (>900 ppspe) may improve Chinese tone recognition. Furthermore, it may be necessary to develop speech processing strategies to specifically improve tone recognition, which is critical for understanding tonal languages.

The present study examined the effects of short-term perceptual training on normal-hearing listeners' ability to adapt to spectrally altered speech patterns. Using noise-band vocoder processing, acoustic information was spectrally distorted by shifting speech information from one frequency region to another. Six subjects were tested with spectrally shifted sentences after five days of practice with upwardly shifted training sentences. Training with upwardly shifted sentences significantly improved recognition of upwardly shifted speech; recognition of downwardly shifted speech was nearly unchanged. Three subjects were later trained with downwardly shifted speech. Results showed that the mean improvement was comparable to that observed with the upwardly shifted training. In this retrain and retest condition, performance was largely unchanged for upwardly shifted sentence recognition, suggesting that these listeners had retained some of the improved speech perception resulting from the previous training. The results suggest that listeners are able to partially adapt to a spectral shift in acoustic speech patterns over the short-term, given sufficient training. However, the improvement was localized to where the spectral shift was trained, as no change in performance was observed for spectrally altered speech outside of the trained regions.

Current multichannel cochlear implant devices provide high levels of speech performance in quiet. However, performance deteriorates rapidly with increasing levels of background noise. The goal of this study was to investigate whether the noise susceptibility of cochlear implant users is primarily due to the loss of fine spectral information. Recognition of vowels and consonants was measured as a function of signal-to-noise ratio in four normal-hearing listeners in conditions simulating cochlear implants with both CIS and SPEAK-like strategies. Six conditions were evaluated: 3-, 4-, 8-, and 16-band processors (CIS-like), a 6/20 band processor (SPEAK-like), and unprocessed speech. Recognition scores for vowels and consonants decreased as the S/N level worsened in all conditions, as expected. Phoneme recognition threshold (PRT) was defined as the S/N at which the recognition score fell to 50% of its level in quiet. The unprocessed speech had the best PRT, which worsened as the number of bands decreased. Recognition of vowels and consonants was further measured in three Nucleus-22 cochlear implant users using either their normal SPEAK speech processor or a custom processor with a four-channel CIS strategy. The best cochlear implant user showed similar performance with the CIS strategy in quiet and in noise to that of normal-hearing listeners when listening to correspondingly spectrally degraded speech. These findings suggest that the noise susceptibility of cochlear implant users is at least partly due to the loss of spectral resolution. Efforts to improve the effective number of spectral information channels should improve implant performance in noise.