Evaluating Speech Perception Across Speech Processing Strategy And Background Noise In Normal Hearing Adults Using A Vocoded, Cochlear Implant Simulation - Info and Reading Options

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Hearing loss is a reality for 1.5 billion individuals globally, of which 430 million have disabling hearing loss (WHO, 2024). As a leading cause of disability, it is considered a global health challenge (Looi et al., 2015). Thankfully, there are various solutions to this challenge. One is cochlear implants (CIs), which restore a sense of sound to those with disabling hearing loss. CIs are estimated to have restored hearing to approximately one million individuals to date (Zeng, 2022), with reviews and meta-analyses finding significant improvements in speech recognition and quality of life (Boisvert et al., 2020; McRackan et al., 2017). Studying CI function is thus a fruitful area of research, yielding benefits for potentially millions more. This study is one of many seeking to improve CI function. Specifically, it aims to investigate the performance of different CI speech processing strategies (SPSs) in different types of background noise, with the aim of identifying potential interactions effects of SPS and noise types on speech perception. We focus on background noise as speech perception remains challenging for CI recipients in conditions containing background noise. The majority of CIs use envelope-based speech processing strategies, in which the amplitude envelope of the bandpass-filtered speech signal is used to modulate the amplitude of pulse trains applied to each electrode. Envelope-based strategies either stimulate all (M) electrodes, as in the case of the Continuous Interleaved Sampling (CIS) strategy, or a subset (N-of-M) of electrodes, as in the case of the Advanced Combination Encoder (ACE) strategy, which selects the N electrodes corresponding to the frequency bands with the highest energy. Previous comparisons of speech-in-noise perception with ACE and CIS either found no significant difference between speech intelligibility with either strategy (Cucis et al. 2019; Hwang et al. 2012) or moderate improvements in speech intelligibility with ACE over CIS (Wong et al. 2008; Skinner et al. 2002; Kiefer et al. 2001). Notably, studies that found improved speech intelligibility with ACE tested intelligibility at positive target-to-masker ratios with relatively stationary maskers, where ACE likely selects target-dominated pulses for stimulation and may provide a benefit over a CIS strategy. Real-world listening conditions often contain non-stationary maskers, such as competing talkers, in which the instantaneous target-to-masker ratio fluctuates over time and frequency and where non-positive target-to-masker ratios may occur. The highest-energy criterion used by ACE may select masker-dominated pulses in situations where the energy of the masker dominates the energy of the target signal such that the perception of ACE-processed speech may show a greater deterioration with decreasing SNR with a non-stationary than a stationary masker. Whether the highest-energy criterion for channel selection interacts with the stationarity of the masker remains to be determined. As mentioned, this study investigates the possible interaction between strategy and masker type on speech intelligibility. To avoid confounds due to heterogeneity in processing parameters (e.g. channel stimulation rate or number of active channels), variable hearing outcomes across CI recipients, or familiarity with either processing strategy, the study tests typical-hearing listeners using a vocoder that simulates the spread of electrical current within the cochlea (Grange et al., 2017). Experiment 1 characterized psychometric functions of speech intelligibility by target-to-masker ratio for three maskers: speech-shaped noise, four-talker babble noise, and a single competing talker. It provided measures of the slopes of the psychometric functions for the three maskers, informing the design of experiment 2 such that the measurement precision of speech reception thresholds (SRTs) would be similar for the three masker types. Speech intelligibility was measured as the percent of words correctly identified in the closed-set UK matrix sentence test from a target-to-masker ratio of -12 dB to 9 dB. Psychometric functions were obtained using vocoded speech in two realistic levels of simulated current spread: -8 dB/octave and -16 dB/octave. Ten typical-hearing participants took part and all stimuli were vocoded with a 22-channel vocoder, simulating the CIS strategy in Cochlear™ devices. Experiment 2 then measured SRTs across the three maskers and two processing strategies (CIS and ACE). To simulate the ACE strategy, 8 of the 22 vocoder channels were selected for presentation within each temporal frame and gated using a 4-ms raised-cosine ramp. Twenty-four typical-hearing participants took part. Two measures of the SRT were measured adaptively for each strategy and masker combination using the UK matrix sentence test. We will discuss results from the ongoing listening experiments, revealing whether differences in speech intelligibility with either strategy depend on the temporal characteristics and level of the masker.

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