Phillips, Jacqueline Anne; (2002) An investigation of the functional neuroanatomy of action ideation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis uses PET to investigate the neural systems mediating praxis and language. These are cognitive functions that can be dissociated in brain damaged patients, but more commonly they are both compromised in the same patient - suggesting that either their neural correlates are anatomically contiguous, or that they share cognitive and neural sub-processes. The five experiments comprising this thesis, attempt to characterise the neural correlates of the ideational component of praxis and how it is related to language, in particular, semantic memory. Experiments 1-3 attempted to distil the cognitive sub-components of praxis by segregating the neural systems mediating semantic and visuo-spatial strategies for action. Subjects were asked to retrieve actions cued by either words (object names) or pictures of objects. This involved deciding whether the stimulus should be 'twisted' (e.g. a cork-screw) or 'poured' (e.g. a jug). Activation was revealed in left anterior temporal cortex (BA 38 and 20) for semantically driven action, and left middle occipital cortex (BA 19) for actions cued by visual structure. The left inferior frontal (BA 45/44) and posterior middle temporal (BA 21/37) cortices were activated for retrieving actions regardless of stimulus type. Experiment 4 examined the relationship between praxis and semantic memory by comparing the neural regions involved in processing tool and fruit stimuli, and in retrieving action and object size. A region of activation common to action retrieval and tool processing was revealed in the left posterior middle temporal gyrus (BA 21/37), supporting the theory that tool-like objects are semantically categorised on the basis of their propensity for action. Experiment 5 compared the neural activation for retrieving vocal and manual learned actions. It was hypothesised that the inferior frontal gyrus (BA 44 and 45) would be involved in retrieving both newly-learned and well-learned, vocal and manual actions. The results suggested that only newly-learned action retrieval involves amodal neural regions, with activation in the inferior frontal sulcus and left anterior insula. In contrast, well-learned actions activated regions that were specific to either the vocal (inferior frontal gyrus, BA 44) or manual (inferior parietal sulcus) modalities. Thus Experiments 1-4 suggested neural correlates for semantic and visual routes to action, and revealed the left posterior middle temporal gyrus as the focus of the collaboration between the semantic system and the action system. Experiment 5 suggested that the intraparietal sulcus, revealed for manual well-learned actions, is involved when action retrieval is not verbally mediated. This thesis uses these results to frame the cognitive models of praxis in anatomical terms.

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