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Approximately 360 million people in the world suffer from a disabling hearing loss. A cochlear implant (CI) is a successful treatment for people with profound sensorineural hearing loss and evokes a hearing sensation by direct electrical stimulation of the auditory nerve fibers (ANFs). Tremendous technological advances in the last decade make it possible that a lot of CI users are able to understand speech very well in quiet listening conditions. However, the speech intelligibility (SI) performance decreases rapidly in adverse listening conditions with interfering background sounds. Speech enhancement algorithms are developed to try to overcome the deficit of poor speech perception in speech in noise scenarios.There are two general approaches for speech in noise enhancement. One approach is called speech enhancement strategies that try to improve SI by an intentional emphasis of speech cues important for speech perception. Another approach are the noise reduction (NR) algorithm that try to reduce as much noise as possible from the noisy mixture of the target signal and the interfering background sound to improve intelligibility. This dissertation deals with the development and analysis of both kinds of speech in noise enhancement strategies for the application in CIs.The first study focused on the development of a new physiologically motivated speech enhancement strategy called enhanced envelope (EE) strategy that mimics the adaptation characteristics of the hair-cell-to-neuron synapse at the onsets of the speech envelope. In a CI, the synapse is bypassed by the direct electrical stimulation of the ANFs. The potential and feasibility was assessed with normal-hearing (NH) subjects listening to noise vocoder CI simulations. A significant speech reception threshold (SRT) improvement was obtained in stationary speech-shaped noise (SSN) and with a competing talker when the onsets were enhanced with a priori knowledge of the target signal. The extension to real-time applications was investigated with a Wiener filter NR algorithm that was used in the onset extraction stage.In the second study, the potential of the EE strategy for the application in CI was investigated with CI users. A significant improvement in speech recognition was obtained in stationary SSN, with an interfering talker, and in a stop consonant recognition task. The loudness perception was not influenced by the processing. The results suggest that an enhancement of the onset cues of the speech envelope can lead to a better parsing of the auditory scene and, therefore, support the segregation of the desired speaker from the background sounds.In the third study, the potential of ideal time-frequency masking approaches in single-channel NR systems for SI and speech quality improvement was investigated with NH listeners. The ideal Wiener filter (IWF) as a representative of a mask with continuous weight was compared to the ideal binary mask (IBM) as a representative of a binary mask. The comparison was done in terms of SI under ideal and with corrupted parameter estimates for the mask pattern derivation and in terms of speech quality. It was shown that the IWF vastly outperformed in all tasks the IBM when the frequency resolution was chosen similar to state-of-the art hearing aids (HAs) and CIs. The results suggest that the IWF should be preferred over a hard-decision approach in the application of auditory prostheses.In the fourth study, the IBM and IWF were systematically compared for the specific application in CI. It was shown that for CI users, the choice of mask pattern was not crucial and that there is no clear preference in terms of SI and speech quality in most conditions. In the fifth study, the application in HAs of the IBM and IWF was investigated with hearing-impaired (HI) listeners. Both mask patterns were not able to restore perfect SI. However, SI of the IWF processed signals was significantly higher than of IBM processed signals. In terms of speech quality, there was no significant preference for one of the NR strategies. The results underline that the development of speech enhancement algorithms have to take the needs of the respective target group into account because results obtained with NH listeners do not translate in all cases to results with CI and HA users.In multiple studies in this thesis, the potential of single-channel speech enhancement strategies was exploited. It was shown that envelope enhancement and NR algorithms have the potential to vastly improve SI of CI users in adverse listening conditions.
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Developments in direct acoustic cochlear stimulation:Hearing problems have been associated with poor quality of life, cognitive decline and a source of frustration. Over the past years, an essential progress has been made in hearing aids improving the wearing comfort, speech processing in noise and wireless connectivity. In a subgroup of patients with a combination of an important sensorineural hearing loss and a conductive component, defined as severe to profound mixed hearing loss, hearing rehabilitation was lagging behind. Conventional middle ear surgery and/or hearing aids would not suffice to overcome the degree of hearing loss and often local anatomical issues were complicating the hearing aid use. This subgroup is growing in number because of an increased awareness for hearing with two ears and for communicating. Also because of an aging population, the sensorineural hearing loss component is becoming increasingly important.With the recent development of a new hearing system, a direct acoustic cochlear implant (DACI), the inner ear can be stimulated acoustically at the oval window. In this way, the amplified signal is provided directly to the cochlea and pathological external and middle ear structures are bypassed. This dissertation explores, for the first time, the functioning of direct acoustic cochlear stimulation in several domains. First, as guidance for evidence-based research, a systematic review of the literature on the hearing rehabilitation of mixed hearing loss was carried out. Next, the clinical application and thorough evaluation of DACI compared to the best current alternative are described. Finally, objective electrophysiological measures and new coupling strategies for DACI are developed.In the first study, the clinical outcome and safety of a whole range of acoustic hearing implants in adults with mixed hearing loss was assessed through a systematic review of the literature. It was concluded that acoustic hearing implants and the respective various coupling strategies were beneficial in terms of speech perception in quiet, patient-reported outcome measures and safety regarding residual hearing. Overall, the level of evidence and the quality of the included studies was judged to be moderate to low.The second study consisted of a prospective, multicenter evaluation of nineteen subjects with severe to profound mixed hearing loss due to several etiologies. The DACI treatment was compared to the best-aided preoperative treatment. The mean speech reception threshold (SRT), a measure for speech in noise understanding and representative for real-life communication, improved with 7.9 dB compared to the preoperative aided condition for the study cohort. A mean postoperative aided SRT of 2.6 dB SNR was noted. Patient-reported questionnaires indicated a significant global benefit in hearing ability and in quality of life. The DACI surgery was regarded as a safe and efficient treatment.In the third study, we focused on the development of an objective electrophysiological assessment of the devices coupling and stimulation of the entire auditory pathway. By investigating the feasibility of evoked auditory transient (ABR) and steady-state responses (ASSR), we opened the path for future intra-operative applications. Responses were recorded to click trains in the 40 Hz and 90 Hz range in three DACI subjects. A direct stimulation setup for reliable auditory response recording was developed in a first set of experiments. Next, by comparing amplitude growth function and phase delay in the same stimulus range, validity of the recorded responses was confirmed in DACI subjects. Electrophysiological stimulation thresholds could objectively be determined from the ABRs and ASSRs in all subjects and the relation with behavioral thresholds was made. Estimated latencies were in agreement with electrophysiological auditory pathway studies, with apparent latencies of about 40 and 25 ms for 40 and 90 Hz, respectively. For the first time, auditory evoked potentials could reliably be recorded and analyzed in patients with a digital speech processing DACI.The fourth study aimed to explore the coupling of a DACI transducer to an anatomically easy accessible inner ear site, being the lateral semicircular canal (LC). This could simplify and shorten the surgical procedure. Fresh-frozen human cadaver heads were implanted with the DACI device stimulating the LC in different coupling situations. The LC was kept intact, blue-lined (i.e. thinning but keeping the last shell of bone closed) or opened, respectively, and each condition was compared with standard oval window coupling. As a measure of the performance of the device and its coupling efficiency, the round window velocity was determined using a laser Doppler vibrometry setup. Pairwise comparisons in three frequency ranges showed that round window velocity was significantly lower in case of intact LC stimulation than in the standard oval window coupling condition, confirming the added value of direct inner ear stimulation. Equivalent output calculations showed a modest but significant added value of blue-lining over the intact condition. Opening the canal resulted in a significantly higher round window velocity than in the intact or blue-lined conditions for all frequency ranges, similar to the oval window coupling. Experimentally induced stapes footplate fixation did not impede the DACI performance when stimulating the opened LC.In the different studies in this project important steps have been made towards reliable hearing rehabilitation, even in difficult listening situations, objective electrophysiological measures and easier surgery in the challenging treatment of severe to profound mixed hearing loss.
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Besides the gold standard pure-tone audiometry, it is highly relevant to assess a persons supra-threshold speech perception skills, especially in background noise, as it approaches our functional hearing ability in a daily communicative environment. In case of a sensorineural hearing loss, an impairment leading to both the attenuation and distortion of incoming sounds, speech perception problems are highly likely to arise. A relationship thus exists between the speech reception threshold (SRT) in noise and the pure-tone thresholds (PTT), although not perfect. The main influencing factors are (1) individual differences in (peripheral or central) supra-threshold versus threshold auditory processing, (2) cognitive and linguistic skills, (3) the redundancy in speech samples, and (4) the measurement error on both behavioral measures. In this PhD project, it was investigated how different types of speech and noise material may influence the SRT-PTT relationship and may lead to new possibilities, new applications of a speech-in-noise test for efficient and sensitive hearing assessment.In the first part of this PhD thesis, including the first three studies, the efficiency and sensitivity for screening purposes was investigated. Both for detecting age-related hearing loss, which is slowly progressing and often undiagnosed, and for noise-induced hearing loss, where periodic screening in high-risk populations may prevent further damage, there is a need for a robust, simple, fast, and sensitive hearing screening technique. When conducting a supra-threshold speech-in-noise test, neither a sound-proof booth nor a high-quality, well-calibrated audiometer are requisite, and it can be implemented as a quick, automated self-test. This makes a speech-in-noise test highly suitable for screening purposes. Smits et al (2004) were the first to develop their Dutch Digit Triplet speech-in-noise self-screening test through home telephone. In the first study, a Digit Triplet test was developed in French, for both telephone and broadband headphone presentation. Due to the separate optimization of the speech material based on the intelligibility of the individual digits, a steep psychometric curve (17 and 27%/dB, respectively) and a low measurement error on the SRT (0.7 and 0.6 dB, respectively) were achieved for both versions. Furthermore, the telephone version was validated in hearing-impaired listeners and yielded a strong correlation of 0.77 with the pure-tone average at 0.5, 1, 2, and 4 kHz (PTA0.5,1,2,4). With 20 000 calls within one month after the nationwide release in France, the test also proves to fulfill the need for an easily accessible hearing screening test. In the second study, the sensitivity of the broadband Digit Triplet test for detecting (early-stage) high-frequency hearing loss was investigated in a group of 118 noise-exposed workers. The 84 Dutch-speaking participants showed a very strong linear relation between the SRT and PTA2,3,4,6 (R=0.86). The sensitivity and specificity to detect mild high-frequency hearing loss were 92% and 89%, respectively. The results of the 34 French-speaking participants showed a highly similar trend. Third, it was explored if a further improvement could be achieved by changing the speech items of the test (9 consonant-vowel-consonant (CVC-) words all with the same vowel) and/or by using a low-pass (LP) filtered version of the standard speech-shaped noise. Within the same group of noise-exposed listeners, the CVC test showed a higher measurement error and a lower between-subject variation than the Digit Triplet test, leading to a lower sensitivity. The CVC test in LP-noise, on the other hand, yielded a sensitivity improvement due to the large increase in between-subject spread. However, there was no improvement compared with the original Digit Triplet test. In the second part, three types of speech-in-noise tests, all developed with a different purpose, were compared to each other: a common everyday sentence test, a closed-set sentence test (Matrix), and the Digit Triplet test. Forty-nine normal-hearing and 69 mildly-to-moderately hearing-impaired listeners, recruited from four French-speaking regions in Belgium and France, participated. No regional effects on any of the three types were found. Mutual correlations between the three tests were high (R>0.81). The Digit Triplet and Matrix test, both with a low contextual redundancy, showed the highest discriminative power, especially between the normal-hearing and mildly hearing-impaired listeners.
Academic collection --- #KVHB:Audiometrie --- #KVHB:Spraakperceptie --- Theses
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Individuals with intellectual disabilities can experience language and/or speech difficulties, which can affect their self-determination, social interactions, and quality of life. To enhance communication, various strategies that augment or replace natural speech can be used: Augmentative and Alternative Communication (AAC) strategies. The present project concerns one strategy in particular, key word signing (KWS). Individuals who use this strategy augment their speech by simultaneously signing the key words as they are speaking. In Flanders, the KWS system Speaking with Support of Signs (SMOG: Spreken Met Ondersteuning van Gebaren) is used in half of the Flemish services for adults with intellectual disabilities. Even though KWS can support language and speech, its use may be challenging. The present research project starts from earlier findings that direct support staff inconsistently use AAC. At present, it is not clear how varying KWS input from staff may shape the KWS usage of adults with intellectual disabilities who have experience in using KWS. This finding warrants further research. The project’s objective is two-fold. First, we aim to examine the interaction between on the one hand environmental factors (e.g., team support and task load) and support staff characteristics (e.g., KWS attitudes), and on the other hand KWS usage of support staff. Second, we study the hypothesis that adults with intellectual disabilities who have KWS experience will use more KWS when they interact with a professional who consistently uses and imitates manual signing.
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