Coding Strategies for Cochlear Implants Under Adverse Environments

Cochlear implants are electronic prosthetic devices that restores partial hearing in patients with severe to profound hearing loss. Although most coding strategies have significantly improved the perception of speech in quite listening conditions, there remains limitations on speech perception under adverse environments such as in background noise, reverberation and band-limited channels, and we propose strategies that improve the intelligibility of speech transmitted over the telephone networks, reverberated speech and speech in the presence of background noise. For telephone processed speech, we propose to examine the effects of adding low-frequency and high- frequency information to the band-limited telephone speech. Four listening conditions were designed to simulate the receiving frequency characteristics of telephone handsets. Results indicated improvement in cochlear implant and bimodal listening when telephone speech was augmented with high frequency information and therefore this study provides support for design of algorithms to extend the bandwidth towards higher frequencies. The results also indicated added benefit from hearing aids for bimodal listeners in all four types of listening conditions. Speech understanding in acoustically reverberant environments is always a difficult task for hearing impaired listeners. Reverberated sounds consists of direct sound, early reflections and late reflections. Late reflections are known to be detrimental to speech intelligibility. In this study, we propose a reverberation suppression strategy based on spectral subtraction to suppress the reverberant energies from late reflections. Results from listening tests for two reverberant conditions (RT60 = 0.3s and 1.0s) indicated significant improvement when stimuli was processed with SS strategy. The proposed strategy operates with little to no prior information on the signal and the room characteristics and therefore, can potentially be implemented in real-time CI speech processors. For speech in background noise, we propose a mechanism underlying the contribution of harmonics to the benefit of electroacoustic stimulations in cochlear implants. The proposed strategy is based on harmonic modeling and uses synthesis driven approach to synthesize the harmonics in voiced segments of speech. Based on objective measures, results indicated improvement in speech quality. This study warrants further work into development of algorithms to regenerate harmonics of voiced segments in the presence of noise

Pitch Perception, Production and Musical Development of Hearing Impaired Children

Children with cochlear hearing loss are offered a range of intervention devices to manage their hearing impairment. The most common devices fitted are hearing aids, cochlear implants or a combination of both (bimodal stimulation with a cochlear implant on one ear and hearing aid on the other). The main goal of these devices is to improve listening and communication for speech and language development. However in more recent years additional focus has been given to non-speech sounds such as music. Pitch is an important aspect of music because it carries the melody; however it is represented differently by the different devices used. The impact this has on children’s musical ability is not fully understood. This thesis explores this area and aims to determine if groups of hearing impaired children who use different intervention devices have a differential impact on pitch perception, singing and general musical ability. The primary research question addressed within the thesis was, do differences exist between different groups of hearing-impaired children who use different amplification devices for general musical ability, pitch perception and singing ability?.Fifty seven children aged between 4 and 9 years old (15 Cochlear implantees, 21 hearing aid users, 8 children with bimodal stimulation and 13 normally hearing children) were assessed for pitch perception and singing while their parents completed a questionnaire on their general musical ability. Results indicated that children using purely electrical stimulation (bilateral cochlear implants) performed more poorly for pitch perception, than children using acoustic information either through bilateral hearing aids or bimodal stimulation. This result was not demonstrated for singing competency, however a reduced comfortable singing range and greater voice irregularity was observed for the cochlear implantees when singing. Normally hearing children performed better with respect to pitch perception and singing competency but did not show a significantly better score for musical enjoyment or involvement in comparison to all three hearing impaired groups. The results indicate that the bimodal configuration could provide some benefits for pitch perception for hearing-impaired children that have useable residual hearing. This doesn’t however extend to pitch production in terms of singing competency. The findings derived from this research study are important not only to build on current research literature but also to inform future clinical practice

Exploring the use of speech in audiology: A mixed methods study

This thesis aims to advance the understanding of how speech testing is, and can be, used for hearing device users within the audiological test battery. To address this, I engaged with clinicians and patients to understand the current role that speech testing plays in audiological testing in the UK, and developed a new listening test, which combined speech testing with localisation judgments in a dual task design. Normal hearing listeners and hearing aid users were tested, and a series of technical measurements were made to understand how advanced hearing aid settings might determine task performance. A questionnaire was completed by public and private sector hearing healthcare professionals in the UK to explore the use of speech testing. Overall, results revealed this assessment tool was underutilised by UK clinicians, but there was a significantly greater use in the private sector. Through a focus group and semi structured interviews with hearing aid users I identified a mismatch between their common listening difficulties and the assessment tools used in audiology and highlighted a lack of deaf awareness in UK adult audiology. The Spatial Speech in Noise Test (SSiN) is a dual task paradigm to simultaneously assess relative localisation and word identification performance. Testing on normal hearing listeners to investigate the impact of the dual task design found the SSiN to increase cognitive load and therefore better reflect challenging listening situations. A comparison of relative localisation and word identification performance showed that hearing aid users benefitted less from spatially separating speech and noise in the SSiN than normal hearing listeners. To investigate how the SSiN could be used to assess advanced hearing aid features, a subset of hearing aid users were fitted with the same hearing aid type and completed the SSiN once with adaptive directionality and once with omnidirectionality. The SSiN results differed between conditions but a larger sample size is needed to confirm these effects. Hearing aid technical measurements were used to quantify how hearing aid output changed in response to the SSiN paradigm

The importance of "scaffolding" in clinical approach to deafness across the lifespan

Throughout the present work of thesis, the concept of scaffolding will be used as a fil rouge through the chapters. What I mean for “scaffolding approach”, therefore, is an integrated and multidisciplinary clinical and research methodology to hearing impairments that could take into account persons as a whole; an approach that needs to be continuously adapted and harmonized with the individuals, pursuant to their progress, their limits and resources, in consideration of their audiological, cognitive, emotional, personal, and social characteristics. The following studies of our research group will be presented: A study (2020) designed to assess the effects of parent training (PT) on enhancing children’s communication development (chapter two); Two studies of our research group (2016; 2020) concerning variables influencing comprehension of emotions and nuclear executive functions in deaf children with cochlear implant (chapter three and chapter four) In chapter five a presentation and description of our Mind-Active Communication program, main topics and aims, multidisciplinary organizations of group and individual sessions with a description of used materials and methodology is given. Finally, a preliminary evaluation to explore the use of this multidisciplinary rehabilitative program on quality of life, psychological wellbeing, and hearing abilities in a sample of cochlear implanted elderly persons is reported

Biophysical modeling of a cochlear implant system: progress on closed-loop design using a novel patient-specific evaluation platform

The modern cochlear implant is one of the most successful neural stimulation devices, which partially mimics the workings of the auditory periphery. In the last few decades it has created a paradigm shift in hearing restoration of the deaf population, which has led to more than 324,000 cochlear implant users today. Despite its great success there is great disparity in patient outcomes without clear understanding of the aetiology of this variance in implant performance. Furthermore speech recognition in adverse conditions or music appreciation is still not attainable with today's commercial technology. This motivates the research for the next generation of cochlear implants that takes advantage of recent developments in electronics, neuroscience, nanotechnology, micro-mechanics, polymer chemistry and molecular biology to deliver high fidelity sound. The main difficulties in determining the root of the problem in the cases where the cochlear implant does not perform well are two fold: first there is not a clear paradigm on how the electrical stimulation is perceived as sound by the brain, and second there is limited understanding on the plasticity effects, or learning, of the brain in response to electrical stimulation. These significant knowledge limitations impede the design of novel cochlear implant technologies, as the technical specifications that can lead to better performing implants remain undefined. The motivation of the work presented in this thesis is to compare and contrast the cochlear implant neural stimulation with the operation of the physiological healthy auditory periphery up to the level of the auditory nerve. As such design of novel cochlear implant systems can become feasible by gaining insight on the question `how well does a specific cochlear implant system approximate the healthy auditory periphery?' circumventing the necessity of complete understanding of the brain's comprehension of patterned electrical stimulation delivered from a generic cochlear implant device. A computational model, termed Digital Cochlea Stimulation and Evaluation Tool (‘DiCoStET’) has been developed to provide an objective estimate of cochlear implant performance based on neuronal activation measures, such as vector strength and average activation. A patient-specific cochlea 3D geometry is generated using a model derived by a single anatomical measurement from a patient, using non-invasive high resolution computed tomography (HRCT), and anatomically invariant human metrics and relations. Human measurements of the neuron route within the inner ear enable an innervation pattern to be modelled which joins the space from the organ of Corti to the spiral ganglion subsequently descending into the auditory nerve bundle. An electrode is inserted in the cochlea at a depth that is determined by the user of the tool. The geometric relation between the stimulation sites on the electrode and the spiral ganglion are used to estimate an activating function that will be unique for the specific patient's cochlear shape and electrode placement. This `transfer function', so to speak, between electrode and spiral ganglion serves as a `digital patient' for validating novel cochlear implant systems. The novel computational tool is intended for use by bioengineers, surgeons, audiologists and neuroscientists alike. In addition to ‘DiCoStET’ a second computational model is presented in this thesis aiming at enhancing the understanding of the physiological mechanisms of hearing, specifically the workings of the auditory synapse. The purpose of this model is to provide insight on the sound encoding mechanisms of the synapse. A hypothetical mechanism is suggested in the release of neurotransmitter vesicles that permits the auditory synapse to encode temporal patterns of sound separately from sound intensity. DiCoStET was used to examine the performance of two different types of filters used for spectral analysis in the cochlear implant system, the Gammatone type filter and the Butterworth type filter. The model outputs suggest that the Gammatone type filter performs better than the Butterworth type filter. Furthermore two stimulation strategies, the Continuous Interleaved Stimulation (CIS) and Asynchronous Interleaved Stimulation (AIS) have been compared. The estimated neuronal stimulation spatiotemporal patterns for each strategy suggest that the overall stimulation pattern is not greatly affected by the temporal sequence change. However the finer detail of neuronal activation is different between the two strategies, and when compared to healthy neuronal activation patterns the conjecture is made that the sequential stimulation of CIS hinders the transmission of sound fine structure information to the brain. The effect of the two models developed is the feasibility of collaborative work emanating from various disciplines; especially electrical engineering, auditory physiology and neuroscience for the development of novel cochlear implant systems. This is achieved by using the concept of a `digital patient' whose artificial neuronal activation is compared to a healthy scenario in a computationally efficient manner to allow practical simulation times.Open Acces

Understanding hearing aid sound quality for music-listening

To improve speech intelligibility for individuals with hearing loss, hearing aids amplify speech using gains derived from evidence-based prescriptive methods, in addition to other advanced signal processing mechanisms. While the evidence supports the use of hearing aid signal processing for speech intelligibility, these signal processing adjustments can also be detrimental to hearing aid sound quality, with poor hearing aid sound quality cited as a barrier to device adoption. Poor sound quality is also of concern for music-listening, in which intelligibility is likely not a consideration. A series of electroacoustic and behavioural studies were conducted to study sound quality issues in hearing aids, with a focus on music. An objective sound quality metric was validated for real hearing aid fittings, enabling researchers to predict sound quality impacts of signal processing adjustments. Qualitative interviews with hearing aid user musicians revealed that users’ primary concern was understanding the conductor’s speech during rehearsals, with hearing aid music sound quality issues a secondary concern. However, reported sound quality issues were consistent with music-listening sound quality complaints in the literature. Therefore, follow-up experiments focused on sound quality issues. An examination of different manufacturers’ hearing aids revealed significant music sound quality preferences for some devices over others. Electroacoustic measurements on these devices revealed that bass content varied more between devices than levels in other spectral ranges or nonlinearity, and increased bass levels were most associated with improved sound quality ratings. In a sound quality optimization study, listeners increased the bass and reduced the treble relative to typically-prescribed gains, for both speech and music. However, adjustments were smaller in magnitude for speech compared to music because they were also associated with a decline in speech intelligibility. These findings encourage the increase of bass and reduction of treble to improve hearing aid music sound quality, but only to the degree that speech intelligibility is not compromised. Future research is needed on the prediction of hearing aid music quality, the provision of low-frequency gain in open-fit hearing aids, genre-specific adjustments, hearing aid compression and music, and direct-to-consumer technology