Noppeney, Uta; (2004) Functional imaging of the semantic system. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This PhD thesis investigates how conceptual knowledge is represented and processed in the human brain using functional imaging methods (PET, fMRI). In particular, the feature-based model of semantic organisation is evaluated, which postulates a specialization of brain regions for processing different types of semantic featrues. For this, subjects were presented with semantic stimuli while the following factors were manipulated (i) stimulus modality (e.g. pictures vs. words), (ii) semantic content (e.g. action vs. visual features), (iii) task (e.g. explicit vs. implicit) and (iv) visual experience. This thesis focuses on action/tools and abstract/verbally-learnt knowledge: Tool stimuli and action features elicited increased responses in a left lateralized visuo-motor action system encompassing ventral pre-motor, A IP (anterior intraparietal) and LPMT (left posterior middle temporal) areas. Critically, these responses were observed irrespective of stimulus modality but only during explicit semantic tasks. In contrast, ventral occipito-temporal regions exhibited category-selective responses to animals and tools. These effects were found irrespective of task but only when the stimuli were pictures. Collectively, these studies demonstrate that semantic responses are context-sensitive and lie in the interaction of semantic content with either (i) stimulus-bound factors such as modality or (ii) task. In terms of neural mechanisms, effective connectivity analyses demonstrate that they emerge from distinct interaction patterns among brain regions. Next, early blindness was used as a lesion model to investigate whether perturbation of early visual experience that alters the sensory- motor system also modifies the semantic system. Surprisingly, the action-selective LPMT response was retained in early blind subjects suggesting a considerable degree of innate and epigenetic specification of the semantic system. Finally, abstract semantic concepts were investigated. Previous behavioural research has highlighted the importance of sentences for specifying the meaning of abstract concepts. In line with this, abstract concepts activated the same left fronto-temporal system as sentence processing. In summary, this thesis contributes to our understanding of the neural systems engaged in representing and processing action and verbally-learnt knowledge.

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