Auditory comprehension: from the voice up to the single word level
Jones, Anna Barbara
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Auditory comprehension, the ability to understand spoken language, consists of a number of different auditory processing skills. In the five studies presented in this thesis I investigated both intact and impaired auditory comprehension at different levels: voice versus phoneme perception, as well as single word auditory comprehension in terms of phonemic and semantic content. In the first study, using sounds from different continua of ‘male’-/pæ/ to ‘female’-/tæ/ and ‘male’-/tæ/ to ‘female’-/pæ/, healthy participants (n=18) showed that phonemes are categorised faster than voice, in contradistinction with the common hypothesis that voice information is stripped away (or normalised) to access phonemic content. Furthermore, reverse correlation analysis suggests that gender and phoneme are processed on the basis of different perceptual representations. A follow-up study (same paradigm) in stroke patients (n=25, right or left hemispheric brain lesions, both with and without aphasia) showed that lesions of the right frontal cortex (likely ventral inferior frontal gyrus) leads to systematic voice perception deficits while left hemispheric lesions can elicit both voice and phoneme deficits. Together these results show that phoneme processing is lateralized while voice information processing requires both hemispheres. Furthermore, this suggests that commencing Speech and Language Therapy at a low level of acoustic processing/voice perception may be an appropriate method in the treatment of phoneme perception impairments. A longitudinal case study (CF) of crossed aphasia (rare acquired communication impairment secondary to lesion ipsilateral to the dominant hand) is then presented alongside a mini-review of the literature. Extensive clinical investigation showed that CF presented with word-finding difficulties related to impaired auditory phonological analysis, while functional Magnetic Resonance Imaging (fMRI) analyses showed right hemispheric lateralization of language functions (reading, repetition and verb generation). These results, together with the co-morbidity analysis from the mini-review, suggest that crossed aphasia can be explained by developmental disorders which cause partial right lateralization shift of language processes. Interestingly, in CF this process did not affect voice lateralization and information processing, suggesting partial segregation of voice and speech processing. In the last two studies, auditory comprehension was examined at the single word level using a word-picture matching task with congruent (correct target) and incongruent (semantic, phonological and unrelated foils) conditions. fMRI in healthy participants (n=16) revealed a key role of the pars triangularis (phonological processing), the left angular gyrus (semantic incongruency) and the left precuneus (semantic relatedness) in this task – regions typically associated via the arcuate fasciculus and often impaired in aphasia. Further investigation of stroke patients on the same task (n=15) suggested that the connections between the angular gyrus and the pars triangularis serve a fundamental role in semantic processing. The quality of a published word-picture matching task was also investigated, with results questioning the clinical relevance of this task as an assessment tool. Finally, a pilot study looking at the effect of a computer-assisted auditory comprehension therapy (React2©) in 6 stroke patients (vs. 6 healthy controls and 6 stroke patients without therapy) is presented. Results show that the more therapy patients carry out the more improvement is seen in the semantic processing of single nouns. However, these results need to be reproduced on a larger scale in order to generalise any outcomes. Overall, the findings from these studies present new insight into, as well as extending on, current cognitive and neuroanatomical models of voice perception, speech perception and single word auditory comprehension. A combinatorial approach to cognitive and neuroanatomical models is proposed in order to further research, and thus improve clinical care, into impaired auditory comprehension.