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The mismatch negativity (MMN) is a negative component of the event-related potential (ERP) that occurs usually 100-200ms after any discriminable change in some repetitive aspect of auditory stimulation. Furthermore, the MMN can be elicited irrespective of the subject's attention, which makes it a unique measure of auditory perception skills in a wide variety of populations. Recent studies have indicated that the MMN may reflect the automatic auditory processing of language-specific phonemes and words, which may involve the activation of long-term neural memory traces in the brain. However, there have been conflicting results reported in the literature regarding the robustness of the MMN response to speech stimuli in healthy adults. Furthermore, to date there have been no studies investigating the MMN response to language-specific phoneme and word stimuli in aphasic individuals. Therefore, the first aim of this thesis was to establish a useful speech stimulus paradigm with which the MMN could be robustly elicited in normal adults, and then applied to individuals with aphasia. Secondly, this thesis aimed to investigate the ability of the MMN to index the various levels of spoken word processing in individuals with aphasia. Firstly, the robustness of the MMN responses of 30 normal adults to simple tone stimuli (differing in frequency, duration or intensity) and speech stimuli (consonant-vowel (CV) non-word contrast /de:/ vs /ge:/ and CV word contrast /del/ vs /gel/) was investigated. Robust MMN responses were elicited by the simple tone stimuli, but not the speech stimuli. Possible explanations of the poor results include adaptation and lateral inhibition of the neural populations involved in generating the MMN response. In order to further investigate the possible adaptation and lateral inhibition effects on the attenuation of the MMN response to fine acoustic speech contrasts, the MMN responses in a further 10 normal adults to the same four speech stimuli were investigated, but within a multiple deviant paradigm. Robust MMN responses to the fine acoustic speech contrast [d/g] were still not obtained (e.g., /de:/ vs. /ge:/ and /del/ vs. /gel/). However, a larger MMN response was obtained at an earlier latency to the real word deviants amongst non-word standards with the same initial consonant (i.e., de->day, ge->gay), when compared with the reverse contrasts (day->de, gay->ge). This word-related enhancement may provide further evidence for the presence of long-term neural traces for words in the brain. The possible effect of the subtitled video distractor task on the MMN responses to the simple tones, CV non-words, or CV words used in the previous studies was then investigated. However, it was found that the presence of subtitles on the distracting silent video had no attenuating or enhancing effect on the amplitude of the MMN or P3a responses to any of the stimuli, and movement artifacts were significantly reduced by the presence of the video subtitles. The last study of this thesis aimed to investigate the ability of the MMN to index various levels of spoken word processing in individuals with aphasia, using speech stimuli with large acoustic deviances (/de:/ vs. /del/). However, given that this study was most likely going to involve a small sample of test subjects, and therefore individual data analysis, a reliable method by which the presence or absence of an MMN response could be determined on an individual basis, was first established (the "filtering method"). The MMN responses of six aphasic individuals to complex tone, non-word and word stimuli (/de:/ vs. /del/) were then investigated and correlated with their performance on a battery of language assessments, in order to determine which levels of spoken word processing may be reflected by the MMN response. The aphasic patients demonstrated attenuated MMN responses to tone duration deviance and speech stimulus deviance. The frequency, duration and real word deviant aphasic MMN responses strongly correlated with performance on the auditory comprehension section of the Western Aphasia Battery (WAB). Furthermore, a word-related enhancement demonstrated by the aphasics during the automatic MMN paradigm was found to correlate with the aphasics' attentional lexical decision skills, providing evidence to support the MMN's ability to reflect the activation of language-specific memory traces for words in the brain, and to reflect deficient spoken word processing skills. The series of studies presented in this thesis served to provide a comprehensive investigation into the ways in which the MMN can be used to index the accurate automatic processing of spoken words. The clinical implications of the findings are discussed and directions for future research are proposed.
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