Understanding Degeneration and the Capacity for Reorganisation in the Adult Human Visual Cortex

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Loss of vision in infancy results in functional reorganisation of the visual cortex1-3;this shows how the brain can adapt to an altered sensory input early in life. Much lessis known about how the adult brain reorganises in response to abnormal visual inputor how it might adapt to treatments that try to restore visual function. Since inheritedretinal diseases are the leading cause of blindness in the developed world, the demandfor therapy will increase in the next decade. Even where therapy is available, the brainneeds to be able to interpret the restored sensory input, and it is not clear whether thiswill be possible following prolonged loss of input. In order to optimally restorefunction it is vital to understand the principles of how the adult brain responds to aloss of sensory input.Stargardt disease (STGD) is a type of retinal disease in which central vision is mostaffected. This project will measure the longitudinal changes in the visual system ofpatients with STGD and will be completed under the supervision of Professor HollyBridge.Aims1) Cortical effects of retinal degeneration in STGDPrevious research has investigated structural and functional cortical changes in retinaldiseases4, 5. 25 STGD patients and 25 age-matched controls will be recruited toundertake a cross-sectional investigation of the cortical changes associated withSTGD. This will be measured using Magnetic resonance spectroscopy imaging(MRSI), structural MRI, diffusion-weighted imaging and population receptive field(pRF) mapping. Clinical measures of retinal function will then be compared to thesemeasures.2) Longitudinal investigation of reorganisation due to degeneration in STGDSince retinal degeneration can be rapid in STGD, many patients show a decrease invisual function after two years. To quantify the effects of this loss of input andfunction on the visual cortex, patients will be rescanned after two years. By measuringdifferences in the same MRI quantities, I will be able to directly demonstrate theeffects of retinal degeneration on the brain.3) Assessing the potential for boosting plasticity in the healthy visual systemIncreasing excitability in the brain is thought to boost the amount of reorganisationthat can take place; studies in adult animals have shown that this change can occurwhen manipulating the neurochemical environment6. In order to maximise visualfunction following visual restoration it may be necessary to boost the ability of thecortex to respond to restored input. In the motor system tDCS has been used as anadjunct procedure to increase behavioural effects of training or rehabilitation7, muchless is known of its effects in the visual system. 20 volunteers will be scanned usingMRSI and fMRI (7T) to investigate changes that occur following tDCS applicationfirstly to the occipital pole, and secondly to hMT+.To summarise, this project will determine the direct consequences of visual loss dueto retinal degeneration, specifically the changes that occur in individual patients overtime. Additionally, this project will assess the potential to boost plasticity in the visualsystem using tDCS, which may be important for optimising visual function.