| Summary: | The present thesis investigates the neural mechanisms supporting working memory (WM) guidance of visual attention, focusing on the role of the thalamus. Chapter 1 is a review of the relevant literature and sets-up the specific research aims. Chapters 2 and 3 explore the role of the thalamus on guidance of attention by WM contents. Stroke patients with focal-brain lesions performed a WM-guided search task. In valid conditions, the colour of the search target was pre-cued by the WM cue while on neutral conditions there was no cue prior to search. In invalid conditions, the WM cue specified the colour of a search distracter and the target appeared elsewhere. First, it was hypothesized that lesions to the thalamus could lead to deficits in attentional control (e.g. failing to separate irrelevant memory contents with relevant target information and leading to increased capture from WM-like distracters during invalid search conditions). An alternative hypothesis was that the thalamus may support the capture of attention by WM contents, hence thalamic patients would display little bias of attention from the WM contents, despite those contents are being maintained in memory. It was found that patients with focal-thalamic lesions especially in the ventrolateral nucleus, showed no search benefit from the valid cues on search as opposed to a control group of patients with lesions outside the thalamus and non-stroke patients. In the invalid condition, thalamic patients showed no capture by the irrelevant search item that matched the WM cue, whereas a group of healthy age-match controls exhibited the normal effect of capture by irrelevant contents held in WM. These observations suggest that lesions to the ventrolateral nucleus of the thalamus impair the capture of attention from WM contents. In Chapter 4, I aimed to establish the role of cortical structures that are known to be structurally connected with the ventrolateral nucleus of the thalamus (i.e. superior frontal gyrus) in WM guidance of attention. To do this, I investigated the effects of transcranial direct current stimulation (tDCS) of the dorsal frontal cortex in WM guidance of attention under distinct WM loads. I found that despite the effect of WM guidance of attention decreasing as WM load increased, frontal-tDCS modulated WM guidance in these conditions. We suggest that the dorsal frontal cortex forms part of a network alongside the thalamus in supporting WM guidance of attention. Finally, I conducted a functional Magnetic Resonance Imaging (fMRI) experiment (Chapter 5) with healthy volunteers to test the hypothesis that the thalamus plays a role in WM guidance when learning of abstract cue-target feature associations needs to take place for guidance of behaviour to emerge. I used four Japanese ideograms as WM cues, each associated with the colour surrounding the sought after target in the subsequent search display (valid trials). In the neutral condition, four different Japanese ideograms were presented that did not predict the colour of the target. Hence, for WM to guide attention the association between the abstract cue and the colour that surrounded the search target needed to be learned. I found that responses in the thalamus and the frontoparietal cortex displayed sensitivity to the predictiveness of the ideogram cues as WM guidance of attention emerged during learning. The findings reported in this thesis demonstrate the pivotal role of the thalamus in WM guidance of attention.
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