Andrew E. Vandali

Speech perception as a function of electrical stimulation rate: using the nucleus 24 cochlear implant system

Objective To investigate the effect of varying electrical stimulation rate on speech comprehension by cochlear implant users, while keeping the number of stimulated channels constant. Design Three average rates of electrical stimulation, 250, 807, and 1615 pulses per second per channel (pps/ch), were compared using a speech processing strategy that employed an electrode selection technique similar to that used in the Spectral Ma-ima Sound Processor strategy (McDermott, McKay, & Vandali, 1992; McDermott & Vandali, Reference Note 1; McKay, McDermott, Vandali, & Clark, 1991) and the Spectral Peak strategy (Skinner et al., 1994; Whitford et al., 1995). Speech perception tests with five users of the Nucleus 24 cochlear implant system were conducted over a 21-wk period. Subjects were given take-home experience with each rate condition. A repeated ABC evaluation protocol with alternating order was employed so as to account for learning effects and to minimize order effects. Perception of open-set monosyllabic words in quiet and open-set sentences at signal to noise ratios ranging from +20 to 0 dB, depending on the subject’s ability, were tested. A comparative performance questionnaire was also administered. Results No statistical differences in group performance between the 250 and 807 pps/ch rates were observed in any of the speech perception tests. However, significantly poorer group performance was observed for the 1615 pps/ch rate for some tests due predominantly to the results of one subject. Analysis of individual scores showed considerable variation across subjects. For some subjects, one or more of the three rate conditions evaluated provided benefits on some speech perception tasks. The results of the comparative performance questionnaire indicated a preference for the 250 and 807 pps/ch rates over the 1615 pps/ch rate for most listening situations. Conclusions For the speech processing strategy, implant system, and subjects evaluated in this study, the group results indicated that the use of electrical stimulation rates higher than 250 pps/ch (up to 1615 pps/ch) generally provided no significant improvement to speech comprehension. However, individual results indicated that perceptual benefits could be obtained by adjusting rate of stimulation optimally to suit each subject. Results from one subject, together with tinnitus problems arising from high-rate stimulation for another subject, indicated that high rates of stimulation may in fact be undesirable for some subjects.

A new portable sound processor for the University of Melbourne/ Nucleus Limited multielectrode cochlear implant

A new processor, called the spectral maxima sound processor (SMSP), has been developed for the University of Melbourne/Nucleus Limited multielectrode cochlear implant. The SMSP analyses sound signals by means of a bandpass filterbank having 16 channels which are allocated tonotopically to the implanted electrodes. Every 4 ms, typically, the six channels with the largest amplitudes are selected, and six corresponding electrodes are activated. In an ongoing study the performance of the SMSP is being compared with that of the Mini Speech Processor (MSP). Some results of speech perception tests from the first two SMSP users are presented, in which scores for the recognition of vowels, consonants, and words all showed significant increases over the corresponding MSP scores.

Pitch ranking ability of cochlear implant recipients: A comparison of sound-processing strategies

Pitch ranking of sung vowel stimuli, separated in fundamental frequency (F0) by half an octave, was measured with a group of eleven Nucleus 24 cochlear implant recipients using different sound coding strategies. In three consecutive studies, either two or three different sound coding strategies were compared to the Advanced Combinational Encoder (ACE) strategy. These strategies included Continuous Interleaved Sampling (CIS), Peak Derived Timing (PDT), Modulation Depth Enhancement (MDE), F0 Synchronized ACE (FOSync), and Multi-channel Envelope Modulation (MEM), the last four being experimental strategies. While pitch ranking results on average were poor compared to those expected for most normal hearing listeners, significantly higher scores were obtained using the MEM, MDE, and FOSync strategies compared to ACE. These strategies enhanced coding of temporal F0 cues by providing deeper modulation cues to F0 coincidentally in time across all activated electrodes. In the final study, speech recognition tests were also conducted using ACE, CIS, MDE, and MEM. Similar results among all strategies were obtained for word tests in quiet and between ACE and MEM for sentence tests in noise. These findings demonstrate that strategies such as MEM may aid perception of pitch and still adequately code segmental speech features as per existing coding strategies.

Binaural speech unmasking and localization in noise with bilateral cochlear implants using envelope and fine-timing based strategies

Four adult bilateral cochlear implant users, with good open-set sentence recognition, were tested with three different sound coding strategies for binaural speech unmasking and their ability to localize 100 and 500 Hz click trains in noise. Two of the strategies tested were envelope-based strategies that are clinically widely used. The third was a research strategy that additionally preserved fine-timing cues at low frequencies. Speech reception thresholds were determined in diotic noise for diotic and interaurally time-delayed speech using direct audio input to a bilateral research processor. Localization in noise was assessed in the free field. Overall results, for both speech and localization tests, were similar with all three strategies. None provided a binaural speech unmasking advantage due to the application of 700 micros interaural time delay to the speech signal, and localization results showed similar response patterns across strategies that were well accounted for by the use of broadband interaural level cues. The data from both experiments combined indicate that, in contrast to normal hearing, timing cues available from natural head-width delays do not offer binaural advantages with present methods of electrical stimulation, even when fine-timing cues are explicitly coded.

Emphasis of short-duration acoustic speech cues for cochlear implant users

A new speech-coding strategy for cochlear implant users, called the transient emphasis spectral maxima (TESM), was developed to aid perception of short-duration transient cues in speech. Speech-perception scores using the TESM strategy were compared to scores using the spectral maxima sound processor (SMSP) strategy in a group of eight adult users of the Nucleus 22 cochlear implant system. Significant improvements in mean speech-perception scores for the group were obtained on CNC open-set monosyllabic word tests in quiet (SMSP: 53.6% TESM: 61.3%, p<0.001), and on MUSL open-set sentence tests in multitalker noise (SMSP: 64.9% TESM: 70.6%, p<0.001). Significant increases were also shown for consonant scores in the word test (SMSP: 75.1% TESM: 80.6%, p<0.001) and for vowel scores in the word test (SMSP: 83.1% TESM: 85.7%, p<0.05). Analysis of consonant perception results from the CNC word tests showed that perception of nasal, stop, and fricative consonant discrimination was most improved. Information transmission analysis indicated that place of articulation was most improved, although improvements were also evident for manner of articulation. The increases in discrimination were shown to be related to improved coding of short-duration acoustic cues, particularly those of low intensity.

A Comparison of Speech Perception of Cochlear Implantees using the Spectral Maxima Sound Processor (SMSP) and the MSP(MULTIPEAK) Processor

The Spectral Maxima Sound Processor (SMSP) is a portable speech processor which has recently been developed at the University of Melbourne for use with multiple-electrode cochlear implants. In this processor, the six largest outputs (maxima) of 16 bandpass filters are used to stimulate the cochlea on a place basis at a constant rate. This speech processing strategy has been compared with the MSP(MULTIPEAK) strategy, in which four electrodes are selected for stimulation in every glottal pulse period. The study was undertaken on four postlinguistically deaf adults. The results show that, for this group of subjects, the performance of the SMSP processor was significantly better than that of the MSP(MULTIPEAK) processor for the recognition of closed-set vowels and consonants, open-set monosyllabic words, and open-set sentences in noise, when using electrical stimulation alone. The SMSP mean scores were: vowels 91.3%, consonants 74.9%, words 57.4%, and sentences in noise 78.7%. The MSP(MULTIPEAK) mean scores were: vowels 76.3%, consonants 59.4%, words 39.9%, and sentences in noise 50.0%.

A portable programmable digital sound processor for cochlear implant research

A programmable sound processor which utilizes digital signal processing has been developed for hearing prosthesis research. It incorporates a Motorola DSP56001 integrated circuit, 32 K words of memory, a 12 b analog-to-digital converter, and a data formatter and transmitter which conveys control codes to the receiver-stimulator of a cochlear implant. The processor is pocket-sized and battery powered. It has been programmed to emulate the Spectral Maxima Sound Processor for the University of Melbourne/Nucleus 22 electrode implant, and is currently being used by several implantees. In continuing research, speech processing programs are being improved, and other applications, including signal processing for binaural implants and advanced hearing aids, are being developed. >

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.

Parameter selection and programming recommendations for the ACE and CIS speech-processing strategies in the Nucleus 24 cochlear implant system

Abstract This study evaluated a range of programming parameters available for the ACE and CIS speech-processing strategies in the Nucleus 24 cochlear implant system. Specifically, the effect on speech perception of adjustments to the number of channels and the stimulation rate in the CIS strategy, and the effect of adjustments to the number of maxima in the ACE strategy were investigated in a group of adult subjects. Based on these findings, and the results of a previous study (Vandali et al., 2000), a number of recommendations for programming were identified that could improve efficiency in the clinical setting where time may be limited. The results suggest that speech perception benefits may be maximized for an individual cochlear implant recipient through concentration on selection of the appropriate stimulation rate. When using the CIS strategy, the number of channels should also be optimized. Adjustment to the number of maxima in the ACE strategy was found to be less likely to provide improvements in speech recognition for a given individual when the number of maxima parameter is set to eight or 12.

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.