Medical Bionics - Theses

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    Rate modulation and speech perception with cochlear implants
    Brochier, Tim ( 2018)
    While cochlear implants (CI) have been successful in restoring a sense of hearing to people with severe to profound sensorineural hearing loss, there is still a wide variance in speech outcomes for CI users. Psychophysical experiments have shown that some of the variance can be explained by sensitivity to temporal modulations. If poor outcomes are partly caused by limited access to temporal speech cues, then improved transmission of those cues may provide perceptual benefits to CI users. The broad aim of the research presented in this thesis was to improve speech outcomes for CI users through better transmission of temporal speech information. The first study investigated the effect of stimulation rate and presentation level on speech perception and temporal modulation detection for CI users, in order to identify stimulation rates that provide a perceptual advantage in certain listening conditions. Speech perception (in quiet and in noise) and amplitude modulation detection thresholds (AMDTs) were measured at different presentation levels and stimulation rates. The reduction in speech perception due to added noise was significantly greater at higher rates. Speech perception was also significantly affected by presentation level, with scores increasing as the level increased. In contrast, AMDTs, as measured via acoustic input to the speech processor, exhibited no effect of rate or level. Correlations were found between AMDTs and speech perception for both the low-rate and high-rate processors. Therefore, while AMDTs explained inter-subject variability in speech perception, they did not explain within subject variability across stimulation rates and presentation levels. The second study evaluated the perception of amplitude modulation (AM) and rate modulation (RM), providing fundamental information about the temporal processing abilities of CI users and the perceptual mechanism underlying those abilities. The study was the first psychophysical evaluation of AM and RM detection in the same CI users. AM and RM detection thresholds were correlated and exhibited similar effects of modulation frequency and presentation level, indicating that AM and RM may be perceived by a common perceptual mechanism that involves central temporal integration. In the final study, a novel speech processing strategy called ARTmod (Amplitude and Rate Temporal modulation) was developed and tested. The ARTmod strategy encoded speech with simultaneous AM and RM, in order to observe whether RM can be used to enhance the perception of temporal envelopes of speech signals. In the experiment, the amount of AM was fixed and the amount of RM was varied, and speech perception was measured for the different RM amounts. A significant effect of RM amount was found, with speech scores improving as the RM amount was increased. The results indicated that RM can constructively combine with AM to enhance the perception of temporal speech envelopes and improve speech perception for CI users.
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    Neural network models of pitch perception in normal and implanted ears
    Pitch is the perceptual correlate of sound frequency and is important for using speech prosody, understanding tonal languages, and music appreciation. According to pitch perception theories, pitch is encoded by the place along the cochlea that has the maximum rate of excitation and the timing of the neural activity caused by cochlear excitation. As one of the most successful neural prosthesis, cochlear implants (CIs) have enabled most recipients to achieve good speech perception in favourable listening conditions. Perceiving a precise pitch, however, is still a challenge for many CI users. The goal of this study is to develop computational models of normal and CI hearing to advance our understanding of the mechanisms of pitch perception in both cases and to investigate the factors that affect pitch perception in electrical hearing. Based on the two possible neural codes for pitch perception, two models of pitch perception were developed: the place model and the integrated model. The models simulated a common psychophysical experiment usually referred to as pitch ranking − where subjects are asked to decide which of the two sequentially presented sounds has a higher pitch – by receiving two stimuli and generating two outputs, the higher-amplitude of which would indicate the higher-pitched stimulus. Synthesised vowels with defined pitches were the sound stimuli used in this study. An artificial neural network (ANN) constituted the core of the models. Inputs to the ANN were place pitch information for the place model and both place and temporal pitch information for the integrated model. Applying the error back-propagation algorithm, the ANN was trained on a training set of pitch pairs. The performance of the pitch perception model was measured using a previously unseen test set of pitch pairs. Place code for pitch perception was extracted from simulated auditory peripheral outputs by averaging the rate of activity in the auditory nerve (AN) over time. An acoustical and an electrical model of the auditory periphery were used to simulate the activity of the AN in normal and CI hearing, respectively. The activity of the AN was further processed through a spiking neural network (SNN) to extract the temporal code of pitch. Synaptic connections in the SNN were modified by spike-timing-dependent-plasticity (STDP) to generate pitch-related precisely-timed neural activities in the SNN output neurons. Pooled inter-spike-interval histogram (ISIH) across the SNN output neurons was found to be indicative of pitch. Validation of both pitch perception models was performed by comparing their performance with psychophysical results. The place model was applied to investigate the impact of stimulation field spread on pitch perception in CI hearing using two commercial sound processing strategies. Simulation results showed that 1) the model could replicate the performance of normal and average-performing CI listeners and 2) providing focused stimulation fields in CI hearing can be beneficial, depending on the type of sound processing strategy. The integrated model was used to explore the role of and interaction between place and temporal cues in performing simulated pitch ranking tasks. Simulation results associated with the integrated model revealed that 1) temporal cues for pitch perception compensated for missing place cues in listening conditions such as a telephone conversation where low-frequency content of the signal was suppressed and 2) new strategies with improved temporal information can improve pitch perception in CI hearing, provided temporal and place information are consistent. Although drawing general conclusions about auditory perception would eventually require psychophysical experiments, computational models of auditory perception such as this work assists in focusing human testing upon factors that demonstrate the strongest impact on the auditory performance of normal and CI listeners. This would therefore lead to the more rapid development of CI sound processing strategies.