Alvarez Ferreira, IA; (2017) Functional and structural organisation of the visual system in human albinism. Doctoral thesis , UCL (University College London).

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Abstract

Albinism is a developmental disorder which involves the misrouting of optic nerve projections, leading to an abnormally organised visual system. Despite the aberrant input, people with albinism have relatively normal vision and experience the world in much the same way as their peers. This thesis explores the functional and structural organisation of visual cortex in human albinism using magnetic resonance imaging (MRI) techniques. This thesis covers four main experiments. Experiment 1 deals with stimulus optimisation for population receptive field (pRF) mapping in healthy adults. pRF mapping is a functional MRI technique for estimating cortical receptive field characteristics non-invasively. In this experiment, a stimulus configuration optimized for short acquisition time requirements is presented and implemented in subsequent experiments. Experiment 2 combines the pRF approach for retinotopic localization with diffusion MRI tractography in healthy adults to show evidence for direct extrastriate connections of the human optic radiation. The optic radiation is the principal white matter pathway for relaying retinal input to visual cortex, and typically considered a projection to primary visual cortex. In this experiment, independent pathways of the optic radiation to visual areas V2 and V3 are identified, and functional-structural methodologies developed for Experiment 4. Experiment 3 investigates the functional organisation of visual cortex in participants with albinism. A pRF mapping approach was implemented, identifying abnormal retinotopic organisation and altered receptive field properties in extrastriate visual cortex. In addition, we explore evidence for and against a dual receptive field model of visual field representation in albinism. Experiment 4 examines the structural and functional connectivity of early visual system in human albinism. In particular, differences in white matter microstructure and inter- hemispheric visual map connectivity are found between participants with albinism and a cohort of healthy controls. This thesis highlights the capacity and limitations of developmental plasticity in human albinism.

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