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ItemLanguage processing in cochlear implant users using fNIRSZhou, Xin ( 2018)Cochlear implant (CI) users differ in their auditory speech understanding ability. This variability is partly due to variability in deafness history and pathology, and partly due to functional brain changes that are likely to occur during deafness and after implantation. By measuring cortical activity in CI users, a relation between functional changes in language associated regions of their brain and speech understanding may be revealed. However, when investigating cortical activity in CI users, commonly-used neuroimaging techniques have limitations. For example, EEG and fMRI may suffer from magnetic or electrical artefacts, and PET imaging is invasive for participants. The studies described in this thesis used a non-invasive technique – functional near-infrared spectroscopy (fNIRS) – to investigate cortical activity in CI users related to speech understanding and the integration of audio-visual speech cues. Compared to fMRI, fNIRS also has the advantages of being quiet (not suffering from the loud magnetic scanning noise) thus suitable for auditory-related tasks, and more tolerant of body movement. The first study determined whether fNIRS measures of cortical activity in post-lingually deafened CI users when listening to or watching speech are correlated with their auditory speech understanding. The fNIRS results showed that speech-evoked cortical activity in CI users that was not only different from normal-hearing listeners but also was negatively correlated with the speech understanding ability. That is, CI users who had poorer auditory speech understanding ability showed higher fNIRS activation in certain brain regions of interest when listening to or watching speech. The increased brain responses might be related to brain functional changes that occurred in CI users during deafness and after implantation for visual speech processing or more listening effort and more neural responses that were used by CI users to process auditory speech. The second study determined whether audio-visual (AV) integration of speech cues in post-lingually deafened CI users is different from that in their similar-aged normal-hearing adults. Participants’ reaction times, response accuracy, and cortical activity were measured when performing different speech identification tasks. A novel method was proposed that combined a probability model and a cue integration model to quantify the amount of AV integration based on response accuracy measures. Consistently, behavioural results using response accuracy and reaction time measures did not show better AV integration in CI users compared to people who had normal hearing. In addition, fNIRS measures of cortical activity did not show AV integration in either CI users or normal-hearing adults. The third study determined whether aging affects AV integration in people who have normal hearing when responding to speech using the same behavioural and fNIRS measures as in the second study. Again, fNIRS results did not show AV integration in either younger or older participants. Behavioural results found no significant difference in AV integration between the older and young participants using both reaction time and response accuracy measures. This thesis integrates knowledge from multisensory neuroscience and psychophysics and uses a novel brain imaging technique to measure cortical activity in CI users for language processing. Results in this thesis showed that this novel imaging technique – fNIRS – could be implemented to examine the variances in auditory speech understanding among CI users. It makes a new advance in the way that multisensory abilities are measured behaviourally, by combining models of optimal and minimum integration. Results in this thesis found that there was no significant difference between CI users and normal-hearing adults in the integration of audio-visual speech cues. Neither was there a significant effect of aging on AV integration.
ItemInvestigating the effect of focused multipolar stimulation for cochlear implants: preclinical studiesSam George, Shefin ( 2016)Multichannel cochlear implants have been well accepted as an effective and safe treatment for severe to profound sensorineural hearing loss through electrical stimulation of residual spiral ganglion neurons. However, speech intelligibility with existing cochlear implants is thought to be limited by poor spatial selectivity and interactions between channels caused by overlapping activation with contemporary stimulation strategies such as monopolar (MP) stimulation. Focused intracochlear stimulation, resulting in an increase in the number of truly independent stimulating channels available for simultaneous activation, may enable better speech and pitch recognition and also improve temporal resolution. Various current focusing stimulation strategies such as tripolar (TP) stimulation have been reported to produce sharper excitation patterns and reduced channel interactions compared to MP stimulation at the cost of higher stimulation current levels. Focused multipolar (FMP) stimulation is another such focusing technique; utilizing simultaneous stimulation of multiple channels to create focused electrical fields. FMP stimulation has been validated in a small group of cochlear implant recipients showing that focusing can be achieved, however this was at the expense of higher stimulation currents compared to MP stimulation. There have been no previous attempts to systematically compare the efficacy of FMP stimulation against TP stimulation or to determine whether factors such as neural survival and the electrode position within the cochlea would affect the performance of FMP stimulation. Controlled preclinical studies in experimental animals can reduce the possible confounding effects of neural survival in human studies. It is also important to test if FMP produces non-auditory sensations since the simultaneous nature of the stimuli would be expected to require greater charge to evoke neural responses. The primary objectives of this thesis were to determine the efficacy of FMP stimulation, compared to both MP and TP stimulation, by evaluating a) the spatial extent of neural activation b) interactions between cochlear implant channels and c) modulation sensitivity to sinusoidal amplitude-modulated pulse trains. The effects of factors such as degeneration of spiral ganglion neurons, induced by long-term deafness, and the electrode position within the cochlea on the effectiveness of FMP, TP and MP stimulation were also examined. These objectives were achieved by implanting a multichannel cochlear implant into cats and guinea pigs, and recording the neural responses in the inferior colliculus in acute electrophysiological experiments. Neural thresholds and the spread of activation along the tonotopic gradient were measured. In summary, the main results of this thesis showed that FMP and TP stimulation resulted in more restricted neural activation and reduced channel interaction compared to MP stimulation and these advantages were maintained in cochleae with significant neural degeneration. Moreover, these effects were not adversely affected by the position of the electrode array within the scala tympani. Although greater charge was required to achieve threshold levels, no evidence of ectopic stimulation of non-auditory neurons was observed with FMP or TP stimulation. Systematically varying the degree of current focusing lowered threshold levels for FMP stimulation while still maintaining a selectivity advantage. Modulation detection of MP was found to be significantly better than FMP and TP stimulation at low stimulation levels, but similar at high stimulation levels. Importantly, there was no benefit in terms of restricted neural activation, reduced channel interaction or better modulation sensitivity for FMP compared to TP stimulation. The greater spatial selectivity, reduced channel interactions and the ability to convey modulation using FMP and TP stimulation would be expected to result in improved clinical performance. The insights into current focusing described in this thesis may also be helpful in other neural prostheses such as deep brain stimulation devices and visual prostheses, when more selective stimulation is desired.