Coulthard, E.J.; (2008) Cognitive control in visual neglect. Doctoral thesis , University of London.

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Abstract

Flexible behaviour in humans requires rapid choices between conflicting actions plans. Although much attention has focused on how prefrontal cortex guides action under situations of response conflict, little is understood about the contribution of parietal cortex. In this thesis I explore the role of the parietal lobe when action selection requires resolution of competition between conflicting motor programs. Competitive imbalance between motor programs, resulting in disparity between leftward and rightward action plans, could in theory lead to directional motor bias in patients with parietal damage and spatial neglect. The first three chapters reporting experimental findings examine directional motor performance in right-hemisphere stroke patients, with and without neglect. Using a modified Eriksen flanker task, we show that right parietal damage associated with leftward spatial neglect leads to paradoxical facilitation (speeding) of rightward movements in the presence of conflicting leftward response plans. These findings indicate a critical role for parietal regions in action planning when there is response competition. In contrast, patients with prefrontal damage have an augmented cost of conflict for both leftward and rightward movements. Subsequently, results from both masked prime and free choice tasks support a parietal role in directional motor processing. Two further chapters reporting experimental findings investigate firstly the asymmetric basis of motor programming in normal subjects and secondly motor neglect, showing that underutilization of the left arm is associated with failure to suppress unwanted right-sided action plans. Overall, the data presented in this thesis suggest that parietal cortex plays a key role in directional movement selection particularly when there is competition between potential action choices. Further, there is evidence for at least two independent systems, with right parietal cortex being a crucial site for automatic activation of competing motor plans and prefrontal regions acting in parallel to inhibit information irrelevant to current task goals.

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