Topography of the cortical representation of the visual field in humans with retinal lesions

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Age-related Macular Degeneration (AMD) affects the elderly and is the leading cause of blindness in the UK, Europe and North America. More rarely, younger patients can be affected similarly by other diseases that are grouped under the heading of Juvenile Macular Degeneration (JMD). As patients lose visual function in the macula, everyday tasks that require normal visual acuity become extremely difficult. Over time, some patients develop strategies to reduce the impact of their visual loss. One such adaptation is the development of the use of an eccentric region of retina for fixation, known as the Preferred Retinal Locus (PRL). It has been shown that the PRL is used as the oculomotor reference point for visual tasks and a stable and consistent PRL leads to improved reading. Although development of a PRL leads to improved visual performance, there is no information concerning the neural mechanisms that underlie this behavioural adaptation. One hypothesis is that the visual cortex reorganizes to confer additional resources on the representation of the retina that becomes the PRL. This hypothesis has been favoured because work on animal models revealed reorganization of primary visual cortex following induced central retinal lesions, with cortex originally devoted to representing central visual field being remapped to represent more eccentric locations. More recently fMRI evidence of similar cortical reorganization was found in human, although the retinal lesions (caused by the absence of cone photoreceptors) were present at birth. Our aim is to determine whether cortical reorganization occurs in patients with retinal lesions caused by AMD and JMD and whether there is a relationship between the acquisition of a PRL and cortical reorganization. We will use fMRI to perform retinotopic mapping experiments in patients with established PRLs to determine whether cortical reorganization is present, when in the disease process it occurs and whether its location is correlated with the position of the PRL. In a longitudinal study we will use fMRI to determine whether cortical reorganization is temporally correlated with the acquisition of a PRL. We aim therefore to evaluate the fundamental plasticity of the human visual cortex and how it relates to behavioural adaptations to blindness caused by disease of the macula. Evaluating the pattern of when and where reorganization occurs in the human brain and how it is related to behaviour is likely to aid the future design and implementation of rehabilitation for patients.