Conversion from the SPEAK to the ACE strategy in children using the nucleus 24 cochlear implant system: speech perception and speech production outcomes.

OBJECTIVE: The main objective of this study was to assess whether speech perception and speech production in children using the Nucleus 24 cochlear implant system improved with a change in speech processing strategy from the SPEAK to the Advanced Combination Encoder (ACE) strategy. The major difference between the two strategies is that ACE uses a higher stimulation rate (in this study the stimulation rate was 900 Hz per channel) compared with the SPEAK strategy, where the stimulation rate is 250 Hz per channel. Information also was obtained regarding the adjustment period after conversion to the ACE strategy. DESIGN: An ABA experimental design was used where scores were initially obtained using the SPEAK strategy' (in the initial A time interval), and subsequently performance was assessed using the ACE strategy (B time interval) and then again with the SPEAK strategy (second A time interval). The duration of the B interval was 10 wk, and the duration for the second A interval was 4 wk. Seven children aged between 9 and 16 yr who had at least 6 mo experience with the SPEAK strategy participated. Open-set monosyllabic CNC word perception in quiet and Speech Intelligibility Test sentence perception in noise was evaluated at the end of each of the time intervals. Word perception was also monitored at fortnightly intervals during the B time interval. Speech production was assessed at the end of the initial A time interval and at the end of the B time interval. RESULTS: Mean word and phoneme scores for open-set words in quiet for the group of seven children were significantly higher with the ACE strategy as compared with the SPEAK strategy scores obtained in both of the A time intervals. For sentences in noise, mean scores using the ACE strategy as well as the SPEAK strategy at the second A evaluation point were significantly higher than the scores using the SPEAK strategy measured at the first A time interval. This suggests that learning effects may have influenced outcomes. For some subjects, an initial decrease in scores was found during the initial 2-wk period after fitting the ACE strategy; however, scores subsequently were found to be similar to or higher than those when using the initial SPEAK strategy. Analysis of speech production assessments showed an improvement in the medial consonant scores after using the ACE strategy. CONCLUSIONS: This study demonstrated that some children were able to benefit from the additional information provided by the ACE strategy as compared with the SPEAK strategy. However, the differences in overall performance between the two strategies appear to be relatively small.

Electrode impedance in adults and children using the Nucleus 24 cochlear implant system.

This study measured changes in electrode impedance over time in 19 adults and 29 children implanted with the Nucleus 24 cochlear implant system, using common ground and three monopolar modes of stimulation, over a series of time intervals. Impedances increased from the intraoperative to the initial fitting session, decreased during the initial fitting session and for the following week, and were then stable. Impedances were lowest for the common ground mode, while for the monopolar modes, impedances were related to the surface area of the return electrode. Impedances for children were higher than those for adults at the initial fitting session and for the following three weeks. The clinically measured impedance values increased in a basal-to-apical direction. When these data were corrected for differences in electrode surface area, impedances decreased in a basal-to-apical direction. Impedances were influenced by whether the electrodes were stimulated or unstimulated. Adults with higher electrode impedances tended to have lower hearing thresholds and comfortable listening levels, but this did not appear to clearly be the case for children.

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 Maxima 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.

Cochlear implant place psychophysics 1. Pitch estimation with deeply inserted electrodes.

Numerical estimation of pitch was performed by 8 adult subjects implanted with cochlear prostheses manufactured by Cochlear Limited. The electrode arrays had been inserted into the scale tympani to between one and one and a half turns of the cochlea. Using bipolar stimulation, the pitch estimates for each subjects showed an overall reduction with insertion depth of the stimulated electrode. However, for several subjects, after decreasing regularly for the more basal electrodes, pitch estimates showed an abrupt decrease, followed in some cases by a region of low pitch. Two of the subjects, implanted with a modified electrode array, the '20 + 2' which allowed monopolar in addition to bipolar stimulation, exhibited an abrupt decrease in pitch estimate with bipolar but not with monopolar stimulation. In these two subjects, for stimulating electrodes inserted more deeply than about three quarters of a turn, bipolar stimuli produced lower pitch sensations, and presumably more apical neural excitation patterns, than monopolar stimuli.

Speech perception in children using the advanced Speak speech-processing strategy.

The Speak speech-processing strategy, developed by the University of Melbourne and commercialized by Cochlear Pty Limited for use in the new Spectra 22 speech processor, has been shown to provide improved speech perception for adults in both quiet and noisy situations. The present study evaluated the ability of children experienced in the use of the Multipeak (Mpeak) speech-processing strategy (implemented in the Nucleus Minisystem-22 cochlear implant) to adapt to and benefit from the advanced Speak speech-processing strategy (implemented in the Nucleus Spectra 22 speech processor). Twelve children were assessed using Mpeak and Speak over a period of 8 months. All of the children had over 1 year's previous experience with Mpeak, and all were able to score significantly on open-set word and sentence tests using the cochlear implant alone. Children were assessed with both live-voice and recorded speech materials, including Consonant-Nucleus-Consonant monosyllabic words and Speech Intelligibility Test sentences. Assessments were made in both quiet and in noise. Assessments were made at 3-week intervals to investigate the ability of the children to adapt to the new speech-processing strategy. For most of the children, a significant advantage was evident when using the Speak strategy as compared with Mpeak. For 4 of the children, there was no decrement in speech perception scores immediately following fitting with Speak. Eight of the children showed a small (10% to 20%) decrement in speech perception scores for between 3 and 6 weeks following the changeover to Speak. After 24 weeks' experience with Speak, 11 of the children had shown a steady increase in speech perception scores, with final Speak scores higher than for Mpeak. Only 1 child showed a significant decrement in speech perception with Speak, which did not recover to original Mpeak levels.

Evaluation of the Nucleus Spectra 22 processor and new speech processing strategy (SPEAK) in postlinguistically deafened adults.

A new speech processing strategy (SPEAK) has been compared with the previous Multipeak (MPEAK) strategy in a study with 24 postlinguistically deafened adults. The results show that performance with the SPEAK coding strategy was significantly better for 58.31% of subjects on closed-set consonant identification, for 33.3% of subjects on closed-set vowel identification and open-set monosyllabic word recognition, and for 81.8% of subjects on open-set sentence recognition in quiet and in competing noise (+ 10 dB signal-to-noise ratio). By far the largest improvement observed was for sentence recognition in noise, with the mean score across subjects for the SPEAK strategy twice that obtained with MPEAK.

Evaluation of a new spectral peak coding strategy for the Nucleus 22 Channel Cochlear Implant System.

Sixty-three postlinguistically deaf adults from four English-speaking countries participated in a 17-week field study of performance with a new speech coding strategy, Spectral Peak (SPEAK), and the most widely used strategy, Multipeak (MPEAK), both of which are implemented on wearable speech processors of the Nucleus 22 Channel Cochlear Implant System; MPEAK is a feature-extraction strategy, whereas SPEAK is a filterbank strategy. Subjects' performance was evaluated with an experimental design in which use of each strategy was reversed and replicated (ABAB). Average scores for speech tests presented sound-only at 70 dB SPL were higher with the SPEAK strategy than with the MPEAK strategy. For tests in quiet, mean scores for medial vowels were 74.8 percent versus 70.1 percent; for medial consonants, 68.6 percent versus 56.6 percent; for monosyllabic words, 33.8 percent versus 24.6 percent; and for sentences, 77.5 percent versus 67.4 percent. For tests in noise, mean scores for Four-Choice Spondees at +10 and +5 dB signal-to-noise ratio (S/N) were 88.5 percent versus 73.6 percent and 80.1 percent versus 62.3 percent, respectively; and for sentences at +15 dB, +10, and +5 dB S/N, 66.5 percent versus 43.4 percent, 61.5 percent versus 37.1 percent, and 60.4 percent versus 31.7 percent, respectively. Subjects showed marked improvement in recognition of sentences in noise with the new SPEAK filterbank strategy. These results agree closely with subjects' responses to a questionnaire on which approximately 80 percent reported they heard best with the SPEAK strategy for everyday listening situations.

Pitch perception for different modes of stimulation using the cochlear multiple-electrode prosthesis.

Numerical estimations of pitch were obtained from nine postlinguistically deafened adults using the 22-electrode cochlear implant manufactured by Cochlear Pty. Limited. A series of electrodes on the array were stimulated using three modes of stimulation: Bipolar (BP), common ground (CG), and monopolar (MONO). In BP stimulation, an electric current was passed between two electrodes separated by one electrode for eight patients and two electrodes for one patient. In CG stimulation, a single electrode was activated and the other electrodes on the array were connected together to serve as the return path for the current. In MONO stimulation, an electric current was passed between a single electrode and the most basal electrode on the array. Pitch estimations were generally consistent with the tonotopic organization of the cochlea. There was a marked reversal in pitch for electrodes in the middle of the array using CG stimulation for three patients. A reduced range of pitch using MONO stimulation was recorded for patients where the most basal electrode was internal to the cochlea. There were also individual differences in pitch estimations between the three modes of stimulation for most patients. The current levels required to elicit threshold (T) and comfortable listening (C) levels were, in general, higher for BP stimulation than for CG stimulation and were lowest for MONO stimulation. For CG stimulation, there was a tendency for T and C levels to be higher for electrodes in the middle of the array than at the basal or apical ends. For MONO stimulation, T and C levels uniformly increased in an apical to basal direction for the majority of patients. There was no consistent pattern in T and C levels for BP stimulation. The size of the range of usable hearing using CG stimulation tended to be similar to that using BP stimulation and was usually higher than that using MONO stimulation.

Radiologic evaluation of multichannel intracochlear implant insertion depth.

Postoperative plain film x-rays are necessary in all multichannel cochlear implant patients to confirm intracochlear position, detect possible electrode kinking, and provide a reference if postoperative slippage occurs. In addition, precise documentation of multichannel intracochlear electrode insertion depths is necessary for comparison of speech recognition results among patients and may be of use for future speech processing strategies. In the present study, a method has been devised, using a modified Stenver's view, to more accurately document insertion depths of the electrode array and location of individual electrodes on 50 multichannel cochlear implant patients. Surgical estimates of insertion depth are shown to have great variability in regard to distance along the basilar membrane when compared with x-ray documentation. Additionally, there is preliminary evidence that insertion depth, as determined by x-ray studies, has a strong correlation with open-set speech discrimination.

Safety studies with the University of Melbourne multichannel electrotactile speech processor.

Results of safety investigations conducted as an integral part of the development of a multichannel electrotactile speech processor (Tickle Talker) are reported. Electrical parameters of the stimulus waveform, design of the electrode handset and cabling, and the electrical circuitry of the speech processor/stimulator and programming interface have been analyzed for potential risks. Constant current biphasic square pulses delivered to electrodes positioned on the skin surface over the digital nerve bundles were chosen to optimize the safety, comfort, and function of the electrotactile stimulus. The device was battery-powered, and the user circuit was isolated from earth-referenced sources. Each electrode was isolated by capacitive coupling, preventing DC leakage of current to the user circuit. Studies of finger temperature showed slight cooling of the skin on the fingers of both stimulated and unstimulated hands for individual subjects following electrotactile stimulation through the Tickle Talker. Subsequent analysis of finger and hand vascular circulation in five subjects showed slight reductions in hand blood flow in some individuals. The results did not demonstrate a significant mean decrease in hand or finger blood flow following electrotactile stimulation. No evidence of sympathetic involvement was found, nor were any changes in vascular structure of the hand such as those associated with Raynaud's disease found. Evidence suggests that the decrease in temperature found in the initial study may be due to a change in the ratio of blood flow between arteriovenous anastomoses and nutritive capillary beds. Studies of: 1) changes in mean threshold and comfortable pulse widths over time; and, 2) changes in tactual sensitivity as measured by hot/cold, sharp/dull, and two-point difference limen discrimination, did not detect any systematic change in peripheral nervous system function following electrotactile stimulation. Analysis of electroencephalogram (EEG) recordings taken during electrotactile stimulation, and after relatively long periods of experience with the device did not show any pathological changes which might be associated with epileptic foci. In summary, no contraindications to long-term use of the Tickle Talker were detected in the studies performed.

Role of a multichannel electrotactile speech processor in a cochlear implant program for profoundly hearing-impaired adults.

Four profoundly hearing-impaired adults who did not meet current selection criteria for implantation at the University of Melbourne were each fitted with a wearable multichannel electrotactile speech processor (Tickle Talker). The subjects were evaluated with a test battery of speech discrimination tests subsequent to training in use of the device. Thresholds for detection of pure tones were lower for the Tickle Talker than for hearing aids across the frequency range 250 to 4000 Hz. Mean speech detection thresholds for the Ling 5-sound test showed that all sounds were detected by users of the electrotactile device at normal conversational speech intensity levels. Mean speech discrimination scores were significantly higher (p less than 0.05) in the tactually aided condition as compared with the tactually unaided for identification of vowels and consonants, on open-set words, open-set sentences, and on connected discourse tracking. Mean scores increased by 20% for vowels, 19% for consonants, 30% for open-set words, and 25% for open-set sentences when the Tickle Talker was used in a multimodal combination with lipreading or lipreading and hearing aids. Speechtracking rates for three subjects showed increases of from 18 to 28 wpm when the tactile device was used. Comparison of tactually aided versus unaided tracking rates for two subjects with long-term experience shows continuing improvement with additional experience with the device. These results demonstrate that hearing-impaired adults not meeting selection criteria for cochlear implantation may benefit from use of an electrotactile speech processor, and highlight the potential benefits from integration of such devices into cochlear implant programs for profoundly hearing-impaired patients.

Multichannel cochlear implantation in children: a summary of current work at the University of Melbourne.

This paper summarizes research work relating to multichannel cochlear implantation in children at the University of Melbourne. Ongoing safety studies relating to the implantation of young children are discussed. Results of these studies suggest that special design considerations are necessary for a prosthesis to be implanted in children under the age of 2 years. Results of clinical assessment of implanted children and adolescents are also discussed in terms of speech perception, speech production, and language development, and some possible predictive factors are suggested. Preliminary data suggests that a high proportion of young children can achieve open-set speech perception with the cochlear implant given appropriate training and support. Initial results with adults using new speech processing hardware and a new coding scheme are also presented. These suggest that improved speech perception in quiet and competing noise is possible with the new system.

Speech feature recognition by profoundly hearing impaired children using a multiple-channel electrotactile speech processor and aided residual hearing.

A group of prelinguistically hearing impaired children, between 7 and 11 years of age, were trained in the perception of vowel duration and place, the fricative /s/, and manner of articulation (/m/ vs /b/ and /s/ vs /t/) distinctions, using information provided by a multiple-channel electrotactile aid (Tickle Talker), and through aided hearing. Training was provided in the tactile-plus-aided hearing (TA) and tactile (T) conditions. Speech feature recognition tests were conducted in the TA, T, and aided hearing (A) conditions, during pretraining, training, and post-training phases. Test scores in the TA and T conditions were significantly greater than scores in the A condition for all tests, suggesting that perception of these features was improved when the tactile aid was worn. Test scores in the training and post-training phases were significantly greater than in the pretraining phase, suggesting that the training provided was responsible for the improvement in feature perception. Statistical analyses demonstrated a significant interaction between the main effects of condition and phase, suggesting that training improved perception in the TA and T conditions, but not in the A condition. Post-training and training test scores were similar suggesting that the perceptual skills acquired during training were retained after the removal of training. Recognition of trained features improved for trained, as well as for untrained words.

Speech perception studies using a multichannel electrotactile speech processor, residual hearing, and lipreading.

Three studies are reported on the speech perception of normally hearing and hearing-impaired adults using combinations of visual, auditory, and tactile input. In study 1, mean scores for four normally hearing subjects showed that addition of tactile information, provided through the multichannel electrotactile speech processor, to either audition alone (300-Hz low-pass-filtered speech) or lipreading plus audition resulted in significant improvements in phoneme and word discrimination scores. Information transmission analyses demonstrated the effectiveness of the tactile aid in providing cues to duration, F1 and F2 features for vowels, and manner of articulation features for consonants, especially features requiring detection and discrimination of high-frequency information. In study 2, six different cutoff frequencies were used for a low-pass-filtered auditory signal. Mean scores for vowel and consonant identification were significantly higher with the addition of tactile input to audition alone at each cutoff frequency up to 1500 Hz. The mean speechtracking rate was also significantly increased by the additional tactile input up to 1500 Hz. Study 3 examined speech discrimination of three hearing-impaired adults. Additional information available through the tactile aid was shown to improve speech discrimination scores; however, the degree of increase was inversely related to the level of residual hearing. Results indicate that the electrotactile aid may be useful for patients with little residual hearing and for the severely to profoundly hearing impaired, who could benefit from the high-frequency information presented through the tactile modality, but unavailable through hearing aids.

Speech perception using combinations of auditory, visual, and tactile information.

Four normally-hearing subjects were trained and tested with all combinations of a highly-degraded auditory input, a visual input via lipreading, and a tactile input using a multichannel electrotactile speech processor. The speech perception of the subjects was assessed with closed sets of vowels, consonants, and multisyllabic words; with open sets of words and sentences, and with speech tracking. When the visual input was added to any combination of other inputs, a significant improvement occurred for every test. Similarly, the auditory input produced a significant improvement for all tests except closed-set vowel recognition. The tactile input produced scores that were significantly greater than chance in isolation, but combined less effectively with the other modalities. The addition of the tactile input did produce significant improvements for vowel recognition in the auditory-tactile condition, for consonant recognition in the auditory-tactile and visual-tactile conditions, and in open-set word recognition in the visual-tactile condition. Information transmission analysis of the features of vowels and consonants indicated that the information from auditory and visual inputs were integrated much more effectively than information from the tactile input. The less effective combination might be due to lack of training with the tactile input, or to more fundamental limitations in the processing of multimodal stimuli.