Understanding and rehabilitating visual deficits caused by stroke: neurochemical and neurophysiological markers of residual vision

Damage to the primary visual cortex, following an occipital stroke, results in a loss of conscious vision in the contralateral visual field. Despite this, there are currently no widely used treatments for such visual impairment, and when therapy is available it tends to focus on learning to compensate for the visual field loss. Many patients with visual field deficits post-stroke retain the ability to detect moving visual stimuli in their blind field. However, not all patients exhibit residual visual processing, and the reasons for this are still debated. This has led to interest in whether this residual visual processing can be improved by visual training. Recently, research has shown that training within the blind field can improve vision for both trained and untrained tasks. However, the efficacy of visual rehabilitation is highly variable among participants, and the reasons for this remain to be determined. This thesis aims to address these questions, and specifically investigates the neural mechanisms that underlie residual vision and rehabilitation efficacy. The primary aim was to investigate whether a six-month visual rehabilitation training programme could improve vision in twenty-four stroke survivors with visual field deficits. Moreover, if successful, it aimed to understand the neural mechanisms that underlie these improvements. In addition, it aimed to understand the neural mechanisms that predict residual visual processing at the baseline visit, as these will likely be important biomarkers for recovery. I found that lower inhibitory GABA and excitatory glutamate were strongly related to residual visual processing in the blind field at the baseline visit. Moreover, lower functional connectivity between ipsilesional and contralesional hMT+ and contralesional extrastriate ventral visual cortex was predictive of superior residual vision at the baseline visit. These studies indicate that hMT+ is involved in residual visual processing and that greater connectivity between the ipsilesional and contralesional hemisphere may negatively impact residual vision. Stroke survivors with V1-damage also show significant improvements for both trained and untrained tasks after six-months of motion discrimination training in the blind field. Moreover, patients show increased white matter integrity of the distal dLGN-hMT+ pathway in the damaged hemisphere after six- months of visual training. These results suggest that the integrity of the dLGN-hMT+ pathway supports visual rehabilitation post-stroke. Taken together, these results indicate the importance of hMT+ after occipital strokes for supporting residual vision and rehabilitation post-stroke. Thus, targeting hMT+ in future studies may be beneficial for enhancing residual vison and visual rehabilitation

Visual perception and attention and their neurochemical and microstructural brain correlates in healthy and pathological ageing

Visual perception and attention declines with normal ageing, however their neural and cognitive mechanisms in healthy and pathological ageing are yet to be fully understood. This thesis aimed to provide a characterisation of normal age-related differences across the visual perception and attention hierarchy, identify their underlying neural correlates, and assess how normal ageing contrasts with pathological ageing in Dementia with Lewy bodies (DLB)