Gorgoraptis, N; (2013) Visual Working Memory in Health and Disease. Doctoral thesis , UCL (University College London).

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Abstract

The ability of the nervous system to retain, manipulate and use visual information which is no longer present in the external environment contributes to intelligent behaviour. A new approach to studying visual working memory has led to re-evaluation of the nature of its limitations in keeping with a finite memory resource which is flexibly distributed across space according to attentional priority. Using a novel behavioural paradigm to study visual working memory precision for sequentially presented items, I demonstrate how the resolution with which healthy subjects recall simple objects changes dynamically with each new item in the sequence. Stochastic modelling of the distribution of responses suggested that memory for earlier objects in the sequence was especially prone to failure in integration of visual features, such as orientation and colour, into complete objects. Next, I examined how memory precision was affected by attentional selection according to the relative behavioural relevance of objects in a sequence, and explored the limitations in this filtering process and their relationship with performance on standard measures of memory and intelligence. The role of updating of non-spatial visual working memory across time was then examined in patients with visual neglect following right hemisphere stroke, revealing a profound non-spatial impairment in WM and its voluntary attentional control in neglect, when compared to stroke patients without neglect and healthy control subjects. Lesion analysis identified separable neural correlates of these deficits. Dopaminergic activity in the prefrontal cortex and basal ganglia has a pivotal and complex role in mediating and controlling working memory and attentional processes. In a randomised, double-blind, placebo controlled study, employing a replicated ABA N-of-1 randomised design, I tested the hypothesis that the dopamine agonist rotigotine improves visual neglect following right-hemisphere stroke. Rotigotine was associated with significant improvement in visual search, an effect that appears to have been mediated by an enhancement of selective, goal-directed attention. The medial temporal lobe (MTL) has an established role in supporting long-term memory processes, but its involvement in working memory has been debated recently. I studied visual working memory for sequentially presentated objects in four patients with MTL lesions and found that short-term memory can be compromised in such individuals. Overall, this thesis explores how visual working memory is updated dynamically across time according to attentional priority in health, how these processes are affected in patients with visual neglect following right hemisphere stroke and in those with medial temporal lesions, and how a dopamine agonist might ameliorate visual neglect by modulating selective attention. The thesis concludes with a brief discussion suggesting further research directions.

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