Two-microphone spatial filtering provides speech reception benefits for cochlear implant users in difficult acoustic environments

This article introduces and provides an assessment of a spatial-filtering algorithm based on two closely-spaced (∼1 cm) microphones in a behind-the-ear shell. The evaluated spatial-filtering algorithm used fast (∼10 ms) temporal-spectral analysis to determine the location of incoming sounds and to enhance sounds arriving from straight ahead of the listener. Speech reception thresholds (SRTs) were measured for eight cochlear implant (CI) users using consonant and vowel materials under three processing conditions: An omni-directional response, a dipole-directional response, and the spatial-filtering algorithm. The background noise condition used three simultaneous time-reversed speech signals as interferers located at 90°, 180°, and 270°. Results indicated that the spatial-filtering algorithm can provide speech reception benefits of 5.8 to 10.7 dB SRT compared to an omni-directional response in a reverberant room with multiple noise sources. Given the observed SRT benefits, coupled with an efficient design, the proposed algorithm is promising as a CI noise-reduction solution.National Institutes of Health (U.S.) (Grant R01 DC 000117)National Institutes of Health (U.S.) (Grant R01 DC DC7152)National Institutes of Health (U.S.) (Grant 2R44DC010524-02

Gain optimization for cochlear implant systems

Cochlear implant systems need Automatic Gain Control (AGC) to compress the large dynamic range (~120 dB) of the acoustic environment into the small dynamic range (< 20 dB) of electrical stimulation. This thesis is concerned with the design, implementation and evaluation of AGC systems for cochlear implants. It investigated the effects of AGC on the speech intelligibility of cochlear implant recipients. Various AGC configurations were evaluated with sentences presented over a wide range of levels at different Signal-to-Noise Ratios (SNR) to identify important factors affecting the performance. Signal metrics were developed to quantify the effects of AGC on the channel envelopes. The goal was to improve speech intelligibility in adverse listening conditions. The performance-intensity functions of cochlear implant recipients with no AGC and with a front-end compression limiter were measured in noise. With no AGC, the proportion of envelope clipping grew monotonically with presentation level. The front-end limiter substantially reduced envelope clipping yet gave little improvement in speech intelligibility. The recipients were highly tolerant of envelope clipping when the background noise was low. SNR degradation was identified as the main factor reducing speech intelligibility. A front-end limiter cannot guarantee zero envelope clipping. In contrast, the proposed envelope profile limiter eliminated envelope clipping and hence preserved the spectral profile. The two AGCs were evaluated, with two release times (75 and 625 ms). The shorter release time gave worse speech intelligibility because it caused more waveform distortion and output SNR reduction. For a given release time, preserving spectral envelope profile gave additional benefits. In a take-home experiment, cochlear implant recipients rated a program with the envelope profile limiter equivalent to their everyday program. A conventional cochlear implant signal path uses a predetermined input dynamic range, which is shifted up or down by the AGC. In contrast, the proposed Adaptive Loudness Growth Function (ALGF) continually optimized the input dynamic range by estimating the noise floor and peak level in each channel. The ALGF gave better Speech Reception Threshold (SRT) than the existing state-of-the-art AGC system at the high presentation level when evaluated with a newly developed roving-level SRT test at three presentation levels

The influence of channel and source degradations on intelligibility and physiological measurements of effort

Despite the fact that everyday listening is compromised by acoustic degradations, individuals show a remarkable ability to understand degraded speech. However, recent trends in speech perception research emphasise the cognitive load imposed by degraded speech on both normal-hearing and hearing-impaired listeners. The perception of degraded speech is often studied through channel degradations such as background noise. However, source degradations determined by talkers’ acoustic-phonetic characteristics have been studied to a lesser extent, especially in the context of listening effort models. Similarly, little attention has been given to speaking effort, i.e., effort experienced by talkers when producing speech under channel degradations. This thesis aims to provide a holistic understanding of communication effort, i.e., taking into account both listener and talker factors. Three pupillometry studies are presented. In the first study, speech was recorded for 16 Southern British English speakers and presented to normal-hearing listeners in quiet and in combination with three degradations: noise-vocoding, masking and time-compression. Results showed that acoustic-phonetic talker characteristics predicted intelligibility of degraded speech, but not listening effort, as likely indexed by pupil dilation. In the second study, older hearing-impaired listeners were presented fast time-compressed speech under simulated room acoustics. Intelligibility was kept at high levels. Results showed that both fast speech and reverberant speech were associated with higher listening effort, as suggested by pupillometry. Discrepancies between pupillometry and perceived effort ratings suggest that both methods should be employed in speech perception research to pinpoint processing effort. While findings from the first two studies support models of degraded speech perception, emphasising the relevance of source degradations, they also have methodological implications for pupillometry paradigms. In the third study, pupillometry was combined with a speech production task, aiming to establish an equivalent to listening effort for talkers: speaking effort. Normal-hearing participants were asked to read and produce speech in quiet or in the presence of different types of masking: stationary and modulated speech-shaped noise, and competing-talker masking. Results indicated that while talkers acoustically enhance their speech more under stationary masking, larger pupil dilation associated with competing-speaker masking reflected higher speaking effort. Results from all three studies are discussed in conjunction with models of degraded speech perception and production. Listening effort models are revisited to incorporate pupillometry results from speech production paradigms. Given the new approach of investigating source factors using pupillometry, methodological issues are discussed as well. The main insight provided by this thesis, i.e., the feasibility of applying pupillometry to situations involving listener and talker factors, is suggested to guide future research employing naturalistic conversations

A comparison of bilateral cochlear implantation and bimodal aiding in severely-profoundly hearing-impaired adults: head movements, clinical outcomes, and cost-effectiveness

Under current guidelines in the UK, eligible adults can receive a single cochlear implant through the National Health Service. Should they wish to aid their non-implanted ear they can either use an acoustic hearing aid and have ‘bimodal aiding’ or elect to pay for a second cochlear implant and have ‘bilateral cochlear implants’. The experiments reported in this thesis sought to inform this choice by establishing which option provides the greater benefit. It was found that both options offered listening and self-reported benefits over listening with a single cochlear implant. However a greater clinical benefit was found from bilateral cochlear implantation, with better localisation and speech perception in noise abilities. A series of experiments investigated whether head movements could improve listening performance. It was found that cochlear implant users made more complex head movements than normally hearing listeners. Whilst head movements by bilateral cochlear implant users improved localisation performance by reducing the number of front back confusions made, users of a single cochlear implant were unable to accurately locate sounds when head movements were permitted. Finally experiments demonstrated that current generic health related quality of life instruments are limited in their sensitivity to binaural hearing benefits. These instruments are used to inform the ‘effectiveness’ component in cost-effective analyses. Therefore a new questionnaire sensitive to benefits in binaural hearing was developed and its validity and sensitivity were demonstrated. Using this self-report instrument, bimodal aiding and bilateral cochlear implantation were shown to have the potential to be a cost-effective use of resources resulting in improvements to ‘hearing-related quality of life’ compared to a single cochlear implant