Connected Discourse Tracking Research Articles

An underestimated issue: unsuspected decrease of sound processor microphone sensitivity, technical, and clinical evaluation

ObjectiveTo describe the rate of occurrence of unsuspected decrease in sensitivity of the sound processor microphone and to evaluate its effect on the patient's audiological performance in terms of reduction in speech recognition scores.DesignSpeech processor microphones were tested by connecting the speech processor acoustic monitor circuit to a hearing aid analyzer. The response curves were compared with those obtained from fully working microphones.During a 6-month investigation period, microphone response curves were measured from a group of cochlear implant recipients who had not reported any problems. Despite the absence of any subjective problem, some microphones were found to show a loss of sensitivity. Their users, aged between 4 and 67 years, were tested both with the defective and a working microphone in order to calculate the correlation between the degree of microphone failure and the decline in audiological performance.To quantify the effect of microphone failure, patients’ speech recognition skills were measured by live voice connected discourse tracking series administered in different conditions and by recorded sentences lists.ResultsA total of 120 apparently fully functioning sound processors were tested in the investigation: 33 (27.5%) were affected by a subjectively unreported sensitivity decrease. Speech-tracking scores correlated significantly with the loss of microphone sensitivity in all test conditions (r = 0.69–0.77, P < 0.05). A high degree of correlation was also found for speech audiometry tests (r = 0.70–0.73, P < 0.05). Microphone sensitivity loss affected speech recognition skills, especially without lip reading and in the presence of background noise.ConclusionThe results indicate that any reduction in sound processor microphone sensitivity causes a degree of hearing decline that negatively affects the cochlear recipient's clinical performance. Microphone faults are often unreported events, and their occurrence rate is underestimated. To establish that the microphone is providing correct input to the speech processor a standard control procedure, including technical and clinical checks, is needed in clinical practice.

Does Cause of Deafness Influence Outcome After Cochlear Implantation in Children?

The objective of this study was to evaluate long-term speech perception abilities of comparable groups of postmeningitic and congenitally deaf children after cochlear implantation. This prospective longitudinal study comprised 46 postmeningitic deaf children and 83 congenitally deaf children with age at implantation of < or = 5.6 years. Both groups were comparable with respect to educational setting and mode of communication and included children with additional disabilities. Both postmeningitic and congenitally deaf children showed significant progress after implantation. Most (73% and 77%, respectively) could understand conversation without lip-reading or use the telephone with a known speaker 5 years after implantation, whereas none could do so before implantation. At the same interval, the postmeningitic and congenitally deaf children scored a mean open-set speech perception score of 47 (range: 0-91) and 46 (range: 0-107) words per minute, respectively, on connected discourse tracking. The respective mean scores at the 3-year interval were 22 and 29 correct words per minute, respectively. None of these children could score a single correct word per minute before implantation. The progress in both groups was statistically significant. When the 2 groups were compared, there was no statistically significant difference. Postmeningitic and congenitally deaf children showed significant improvement in their auditory receptive abilities at the 3- and 5-year intervals after cochlear implantation. There was no statistically significant difference between the outcomes of the 2 groups, suggesting that, provided that children receive an implant early, cause of deafness has little influence on outcome. Although the prevalence of other disabilities was similar in both groups, for individual children, their presence may have profound impact. The study supports the concept of implantation early in life, irrespective of the cause of deafness.

Effects of the number of channels and speech-to-noise ratio on rate of connected discourse tracking through a simulated cochlear implant speech processor.

To investigate the effects of number of channels and speech-to-noise ratio on connected discourse tracking (CDT) through simulations of cochlear implant speech processing. Previous studies have used citation-form vowel and consonant materials or simple sentences. CDT rates were expected to be less likely to be limited by ceiling effects and more representative of everyday speech communication. Four normal-hearing subjects were presented with speech processed through a real-time sine-excited vocoder having three, four, eight, or 12 channels. Amplitude envelopes extracted from each band modulated sinusoidal carrier signals placed at each band center frequency. Speech-spectrum shaped noise was added to speech before vocoder processing at three signal to noise ratios based on real-time measurements of speech level (+7, +12, +17 dB). CDT rates increased significantly with number of channels up to eight in both quiet and noise, and decreased significantly with each increase in noise level from quiet. The effects on CDT rates of the number of channels and speech-to-noise ratio are highly correlated with intelligibility measures for Hearing in Noise Test (HINT) sentences, consonants and vowels. However, HINT sentence scores even in noise show ceiling effects that obscure the advantages of processors with eight or more channels. Moderate levels of noise that have only slight effects on other measures significantly affected CDT rate. CDT rates with three or four bands of spectral information were much lower than asymptotic rates, especially in the presence of noise.

Effects of the salience of pitch and periodicity information on the intelligibility of four-channel vocoded speech: implications for cochlear implants.

Recent simulations of continuous interleaved sampling (CIS) cochlear implant speech processors have used acoustic stimulation that provides only weak cues to pitch, periodicity, and aperiodicity, although these are regarded as important perceptual factors of speech. Four-channel vocoders simulating CIS processors have been constructed, in which the salience of speech-derived periodicity and pitch information was manipulated. The highest salience of pitch and periodicity was provided by an explicit encoding, using a pulse carrier following fundamental frequency for voiced speech, and a noise carrier during voiceless speech. Other processors included noise-excited vocoders with envelope cutoff frequencies of 32 and 400 Hz. The use of a pulse carrier following fundamental frequency gave substantially higher performance in identification of frequency glides than did vocoders using envelope-modulated noise carriers. The perception of consonant voicing information was improved by processors that preserved periodicity, and connected discourse tracking rates were slightly faster with noise carriers modulated by envelopes with a cutoff frequency of 400 Hz compared to 32 Hz. However, consonant and vowel identification, sentence intelligibility, and connected discourse tracking rates were generally similar through all of the processors. For these speech tasks, pitch and periodicity beyond the weak information available from 400 Hz envelope-modulated noise did not contribute substantially to performance.

Determinants of speech perception in children after cochlear implantation

Cochlear implants provide access to the speech signal in those profoundly deaf children who derive no material benefit from acoustic hearing aids. Speech perception after implantation can vary widely--we have analysed the contribution of several factors. We examined 40 children with mean age at implantation of 52 months who were either born deaf or became deaf before 3 years. All patients received the same multichannel implant system and were followed up for 5 years. We used connected discourse tracking (CDT) as the measure of speech perception. The effect of five potential predictors (age at implantation, number of inserted electrodes, origin of deafness, mode of communication, and socioeconomic group) on speech perception was analysed. The mean number of words per minute perceived increased from 0 before implantation to 44.8 (SD 24.3) 5 years after implantation. Repeated-measures ANOVA showed that children significantly progressed over time (p=0.001). Age at implantation was a significant covariate (p=0.01) and mode of communication was a significant between-individuals factor (p=0.04). Young age at intervention and oral communication mode are the most important known determinants of later speech perception in young children after cochlear implantation. Early identification of candidate children necessitates implementation of universal neonatal screening programmes for hearing impairment.

The Prognostic Value of Promontory Electric Auditory Brain Stem Response in Pediatric Cochlear Implantation

To test the hypothesis that children with clear promontory electrically evoked auditory brain stem responses (prom-EABRs) would outperform, after cochlear implantation, children who had no prom-EABR preoperatively. A prospective study was undertaken on 47 implanted children assigning them to two groups (group A: 35 children with a clear wave e-V in the preoperative prom-EABR and group B: 12 children with no prom-EABR). Speech perception and speech intelligibility were assessed annually up to 3 yr after implantation with the IOWA sentence test (level A and level B), Connected Discourse Tracking, Categories of Auditory Performance, and Speech Intelligibility Rating. t-test and Mann-Whitney U test were used to compare the above outcome measures in the two groups. There was no statistically significant difference between the two groups on any of the outcome measures at any interval. Moreover, the small differences observed showed no consistent trend toward either group of children. Further analysis revealed that the outcomes have not been affected by possible confounding factors (age at implantation, duration of deafness, preoperative unaided pure-tone thresholds, and number of inserted electrodes). The results suggest that children with no prom-EABR performed at levels comparable with children who had clear promontory responses preoperatively. The prognostic value of prom-EABR is limited and absence of a prom-EABR is not, by itself, a contraindication for cochlear implantation. However, in selected cases (congenital malformations, cochlear nerve dysplasia or suspected aplasia, narrow internal auditory canal, etc.) the presence of a prom-EABR is a positive finding in the assessment of candidates for cochlear implantation as it confirms the existence of intact auditory neurones.

Adaptation by normal listeners to upward spectral shifts of speech: implications for cochlear implants.

Multi-channel cochlear implants typically present spectral information to the wrong "place" in the auditory nerve array, because electrodes can only be inserted partway into the cochlea. Although such spectral shifts are known to cause large immediate decrements in performance in simulations, the extent to which listeners can adapt to such shifts has yet to be investigated. Here, the effects of a four-channel implant in normal listeners have been simulated, and performance tested with unshifted spectral information and with the equivalent of a 6.5-mm basalward shift on the basilar membrane (1.3-2.9 octaves, depending on frequency). As expected, the unshifted simulation led to relatively high levels of mean performance (e.g., 64% of words in sentences correctly identified) whereas the shifted simulation led to very poor results (e.g., 1% of words). However, after just nine 20-min sessions of connected discourse tracking with the shifted simulation, performance improved significantly for the identification of intervocalic consonants, medial vowels in monosyllables, and words in sentences (30% of words). Also, listeners were able to track connected discourse of shifted signals without lipreading at rates up to 40 words per minute. Although we do not know if complete adaptation to the shifted signals is possible, it is clear that short-term experiments seriously exaggerate the long-term consequences of such spectral shifts.

Outcomes from adult implantation, the first 100 patients

We present the outcome of implantation in the first 100 adult patients treated under the Midland Cochlear Implant Programme. All patients were post-lingually deaf with profound or total hearing loss. Performance was tested in lip-reading, implant only and combined lip-reading and implant modes using BKB sentences, connected discourse tracking (CDT) and environmental sound recognition. Assessments were made at nine and 18 months post-implant. The dominant aetiologies were idiopathic and meningitis. Meningitis was associated with the greatest numbers of ossified cochleas. Forty-three per cent of cases of partial ossification were identified only at surgery. Four per cent of patients became non-users of their devices, however the majority used their implants for more than 12 hours each day. The mean scores at nine months post-implant, in the implant only mode, were for environmental sound recognition 56.7 per cent, for BKB sentences 46.6 per cent (80 per cent of patients scored above 0 per cent) and for CDT 31.2 words per minute (w.p.m.) (62 per cent scored above zero per cent). In the combined lip reading and implant mode the mean scores, at nine months, were for BKB sentences 81.5 per cent and for CDT 65.8 w.p.m. All results were sustained at 18 months. Patients reported that implantation significantly reduced their hearing handicap. Pre-operative measures of depression were also significantly reduced at nine months post-implant. Results were sustained at 18 months. Post-operative audiological outcomes in the electrical stimulation only mode correlated significantly with length of profound deafness. Results suggest that performance outcome is also related to the number of active electrodes.

Evaluation of cochlear implantation in post-meningitic adults

Meningitis is an important cause of deafness and in some studies has been associated with poorer outcomes in adult patients following cochlear implantation. Of the first 100 adults implanted under the Midland Cochlear Implant Programme, 28 were deafened as a result of meningitis. We compare our experience with these patients with patients with a non-meningitic aetiology. A degree of cochlear ossification was a more common finding in the meningitic group. In six cases (four meningitis, two non-meningitis) ossification was encountered only during surgery, not being apparent on pre-operative radiology. The average scores achieved on auditory tests by the meningitic group were similar to those achieved by non-meningitic patients. At nine months, using only the implant, users were able to identify 54 per cent of common environmental sounds, achieved an average score of 30 words per minute on connected discourse tracking and identified an average of 42 per cent of words correctly in BKB sentences. Poorer outcomes were more commonly associated with cochlear ossification. In patients with cognitive and neurological sequelae, benefits with the implant were not always apparent in the early months, however, with intensive therapy these patients can obtain measurable sustained benefit from their implant.

Cochlear implantation: long-term functional outcomes

The aim of this study is to evaluate the speech perception of profoundly deaf children following cochlear implantation. Material and methods: A prospective study was undertaken on a consecutive group of 138 children. Fifty children (36.3%) were deafened by meningitis, 75 (54.3%) were congenitally deaf, and 13 children (9.4%) were deafened by other causes. All children had received the Nucleus-22 multi-channel cochlear implant and followed a standard rehabilitation programme. No child was lost to follow-up and there were no exclusions from the study. The Iowa Matrix Closed Set Sentence Test and Connected Discourse Tracking were used to assess closed and open set speech perception respectively. Results: On the Iowa Matrix Test, the median score was 0% before implantation, 0% at 1 year, 85% at 2 years, 99% at 3 years, and 100% at 4 and 5 years following cochlear implantation. On Connected Discourse Tracking the median score was 0 words per min (wpm) before implantation, 3 wpm at 1 year, 10 wpm at 2 years, 22 wpm at 3 years, 34 wpm at 4 years and 37 wpm at 5 years following cochlear implantation. Conclusion: A comprehensive rehabilitation programme can help profoundly deaf children develop significant closed set speech perception abilities in the following 2–3 years after cochlear implantation. However, open set speech perception abilities need significantly more time to develop and they do not reach a plateau even 5 years after cochlear implantation.

Comparative Effectiveness of Correction Strategies in Connected Discourse Tracking

The effectiveness of four correction strategies commonly used in connected discourse tracking was investigated in the present study. The strategies were 1) verbatim repetition of a word or phrase; 2) use of antonyms or synonyms as cues; 3) use of phonemic cues, with no whole word repetition; and 4) going back or ahead in the text, with no repetition of the missed segment. Four normal-hearing adults served as listeners. Live-voice presentation of text by two female talkers was employed for all conditions. Listeners were tested in two stimulus presentation modes, speechreading alone and speechreading plus a multichannel tactile aid. Results indicated that strategy 1, repetition of the missed segment, produced the highest tracking rates, significantly higher than any of the other strategies. Strategy 2 produced the lowest tracking rates. Strategies 1 and 3 yielded the lowest percentage of initially missed words, or blockages, and strategy 4 the highest percentage. Significantly higher tracking rates were found under the speechreading plus tactile aid presentation mode, compared with speechreading alone. Further, tracking rates increased significantly from the beginning to the end of training. Data were compared with a more typical CDT task, in which all correction strategies were operative, and results showed little difference in tracking rates between this task and the constrained CDT employing only strategy 1. Overall, results suggest that simple repetition of missed segments is an effective correction strategy for CDT and argue for its inclusion in computer-assisted tracking implementations.

Speech pattern hearing aids for the profoundly hearing impaired: speech perception and auditory abilities.

A family of prototype speech pattern hearing aids for the profoundly hearing impaired has been compared to amplification. These aids are designed to extract acoustic speech patterns that convey essential phonetic contrasts, and to match this information to residual receptive abilities. In the first study, the presentation of voice fundamental frequency information from a wearable SiVo (sinusoidal voice) aid was compared to amplification in 11 profoundly deafened adults. Intonation reception was often better, and never worse, with fundamental frequency information. Four subjects scored more highly in audio-visual consonant identification with fundamental frequency information, five performed better with amplified speech, and two performed similarly under these two conditions. Five of the 11 subjects continued use of the SiVo aid after the tests were complete. A second study examined a laboratory prototype compound speech pattern aid, which encoded voice fundamental frequency, amplitude envelope, and the presence of voiceless excitation. In five profoundly deafened adults, performance was better in consonant identification when additional speech patterns were present than with fundamental frequency alone; the main advantage was derived from amplitude information. In both consonant identification and connected discourse tracking, performance with appropriately matched compound speech pattern signals was better than with amplified speech in three subjects, and similar to performance with amplified speech in the other two. In nine subjects, frequency discrimination, gap detection, and frequency selectivity were measured, and were compared to speech receptive abilities with both amplification and fundamental frequency presentation. The subjects who showed the greatest advantage from fundamental frequency presentation showed the greatest average hearing losses, and the least degree of frequency selectivity. Compound speech pattern aids appear to be more effective for some profoundly hearing-impaired listeners than conventional amplifying aids, and may be a valuable alternative to cochlear implants.

Stimulus presentation formats in training paradigms for tactile aids.

Previous studies involving tactile aids for speech perception conducted in our laboratory have typically employed a training paradigm consisting of initial training with single-item speech stimuli in a citation-style live-voice presentation, followed by later training with connected discourse. This selection of training procedures was based on the assumption that for recognition of vibratory patterns corresponding to particular phonemes and/or syllables, such initial single-item presentation promoted rapid learning. However, given the sometimes substantial differences in the acoustic waveforms of items presented citation style or in the context of connected speech, it is possible that early initial training with citation-style stimuli would actually impair later performance with connected speech. Thus, although high levels of performance for users of tactile aids with connected speech have previously been obtained, it is possible that initial training with stimulus items in carrier phrases would be superior to citation-style presentation. In the present study, two groups of subjects were trained to use the Queen’s University tactile vocoder; one group was trained initially with single-item stimuli presented citation style, and the other group was trained initially with stimuli presented in carrier phrases. Both groups were later tested in a connected discourse tracking task. Preliminary analysis of the results suggests no differences between the two groups, suggesting that citation-style presentation and carrier-phrase presentation have equivalent effects on later connected speech performance. [Work supported by NIH.]

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.

Evaluation of a wearable multichannel tactile aid

In designing a tactile aid, the most important considerations are that a device provide useful information and that it be wearable. While electrotactile rather than vibrotactile devices are more easily made wearable, results with these devices to date have been disappointing. One exception to this has been the Tickle Talker (Univ. of Melbourne). The Tickle Talker is an eight‐channel device, worn as rings on the fingers of one hand, with a electrode placed on either side of each finger. The processor performs a formant extraction, displaying an estimated fundamental frequency, second formant frequency, and the amplitude of the speech signal. In an attempt to corroborate early promising results [Cowan et al., J. Acoust. Soc. Am. 85, 2593–2607 (1989)], this device was evaluated with three normally hearing adult subjects. In initial testing with closed set tasks done under a tactile aid alone condition, subjects on the average were able to identify the manner of production of stimuli 76% of the time, presence or absence of voicing 74% of the time, and medial vowel 81% of the time. Wearing the tactile aid in conjunction with lipreading raised subjects' scores on a subsequent closed set identification task from 57% lipreading alone to 72% with the device. Results to date with connected discourse tracking also show improvements with the device. [Work supported by NIH.]

Evaluation of two multichannel tactile aids for the hearing impaired.

Two multichannel tactile devices for the hearing impaired were compared in speech perception tasks of varying levels of complexity. Both devices implemented the "vocoder" principle in their stimulus processing: One device had a 16-element linear vibratory array worn on the forearm and displayed activity in 16 overlapping frequency channels; the other device delivered tactile stimulation to a linear array of 16 electrodes worn on the abdomen. Subjects were tested in several phoneme discrimination tasks, ranging from discrimination of pairs of words differing in only one phoneme under tactile aid alone conditions to identification of stimuli in a larger set under tactile aid alone, lipreading alone, and lipreading plus tactile aid conditions. Results showed both devices to be better transmitters of manner and voicing features of articulation than of place features, when tested in single-item tasks. No systematic differences in performance with the two devices were observed. However, in a connected discourse tracking task, the vibrotactile vocoder in conjunction with lipreading yielded much greater improvements over lipreading alone than did the electrotactile vocoder. One possible explanation for this difference in performance, the inclusion of a noise suppression circuit in the electrotactile aid, was evaluated, but did not appear to account for the differences observed. Results are discussed in terms of additional differences between the two devices that may influence performance.

Evaluation of a 32‐channel electrotactile vocoder

Previous evaluations of laboratory prototype multichannel tactile aids have shown them to provide useful speech information. However, few of these prototypes have been successfully converted into wearable devices. The present study evaluated a device that has recently become commercially available, the Audiotact (Sevrain‐Tech, Inc.), a 32‐channel electrotactile vocoder worn on the abdomen. Four subjects pacticipated in the study, two of whom had previous experience with other tactile aids. In addition, a visual display designed as a training aid for new users of the Audiotact was evaluated. This display provides a visual representation of the activity on the tactile array. Two of the subjects were instructed to use this visual information while wearing the Audiotact. Results of testing with minimal pairs and small set phoneme identification tasks showed that manner features were transmitted quite well. Subjects were also able to integrate information available through lipreading with information from the Audiotact. However, preliminary results for connected discourse tracking with the Audiotact showed only a small improvement over lipreading alone. The results of this study are discussed in terms of comparability to other multichannel tactile devices.

Effect of text complexity on connected discourse tracking rate.

The effect of three levels of text complexity upon connected discourse tracking rates was investigated in normal listeners who tracked by lipreading alone and by lipreading with auditorily presented voice pitch. Text complexity affected connected discourse tracking under both lipreading conditions, with tracking rates decreasing as the level of text complexity increased. The improvement in tracking rate with the addition of voice pitch information was found to be invariant over changes in text complexity when expressed as a simple difference between the two tracking rates.

Effects of number of channels on speech perception with tactile aids

Although there is considerable evidence that multichannel tactile aids for the hearing impaired provide more information about the speech signal to the wearer than do single‐channel tactile aids, it has not been determined how many channels are necessary for optimal transmission of speech information. In the present study, the 5‐channel Tactaid V (Audiological Engineering Corporation) and the 16‐channel Queen's University tactile vocoder were evaluated in a series of phoneme identification and connected speech tasks, to determine whether the additional channels present in the 16‐channel design lead to better speech perception performance. Results showed that, in relatively simple tasks, such as minimal pairs phoneme discrimination, subjects performed at similar levels with both devices. However, in more complex tasks, such as integration with lipreading, the 16‐channel device yielded significantly higher levels of performance. In connected discourse tracking, both devices showed significantly higher levels of performance in conjunction with lipreading than were found under lipreading alone conditions. Results are discussed with respect to the benefits provided by a tactile aid relative to the practical limitations of tactile aids that employ a large number of tactile transducers. [Work supported by NIH and NSF.]

Speech perception by four single-channel cochlear implant users.

Four profoundly deaf adults, each a recent recipient of a scala tympani implant, underwent auditory and auditory-visual speech comprehension evaluations. Two subjects had multiple-electrode auditory prostheses, and 2 had single-electrode implants. All subjects were tested preoperatively with a high-power hearing aid, and postoperatively with a single-channel wearable sound processor. Reported here are the results of the first formal speech recognition tests which were conducted during the 8 months after the sound processor fitting. Three subjects had used the single-channel processor on a daily basis for up to 8 months at the time of postoperative testing. The 4th subject was a nonuser. On listening tests, a comparison between pre- and post-implant scores revealed little difference for any subject. On postoperative speechreading tasks, all subjects identified medial consonant phonemes and 2-digit numerals better with stimulation than without. The 3 frequent users of the device experienced significant improvement on connected-discourse tracking, and their speechreading of videotaped and live voice CID Everyday Sentences (Davis & Silverman, 1978) was enhanced with the addition of stimulation. The nonuser was a very proficient speechreader at the outset and exhibited no significant difference on connected-discourse tracking with and without stimulation. Moreover her ability to speechread Everyday Sentences was hampered slightly by the addition of stimulation. This single-channel sound processor functioned as a sensory supplement for the 3 frequent users, but no subject was able to use the processor as a sensory substitute.