The goal of this research is to establish the extent to which the adult brain is plastic even in adulthood and to determine the functional consequences of such plasticity. Over the past year we have continued to focus on plasticity following the loss of input to particular regions of the brain, including visual and somatosensory cortex. People with macular degeneration lose central vision due to damage of the retina. But what happens to the parts of the brain that are specifically involved in processing central visual stimuli? Do those parts of the brain simply become inactive or can they take on new function? Using functional magnetic resonance imaging (fMRI), we have previously shown that the parts of the brain deprived of input start to respond to visual stimuli in other regions of the visual field, as if they are taking on new function. But what drives this reorganization of visual processing? We investigated whether the observed plasticity is use-dependent (affecting only the peripheral part of the retina, the preferred retinal locus or PRL, used for fixation in patients with macular degeneration), or whether it reflects a more passive process that affects input through all parts of the retina. We found that the part of the brain deprived of visual input responded to visual stimuli both at PRL and non-PRL locations, suggesting passive rather than use-dependent plasticity. We have also investigated the functional consequences of deprivation of visual input, by patching one eye in participants with full vision. We found that depriving visual input produced very rapid perceptual distortions, suggesting that changes in visual processing, such as those observed in patients with macular degeneration, may reflect the unmasking of exisiting connections rather than any new anatomical changes. Following limb amputation, the majority of people report phantom sensations in their missing limb, often painful sensations. One current theory is that the phantom limb pain arises as a result of cortical reorganization. We are continuing to recruit participants and to monitor brain activation with fMRI in unilateral limb amputees over a period of four weeks while they undergo therapy to treat the phantom limb pain. The therapy being used is mirror therapy: each day the participants view their intact limb in a mirror giving the impression of their amputated limb moving. In particular we are trying to establish whether the presence of phantom limb pain correlates with cortical reorganization in the somatosensory cortex (similar to that observed in our participants with macular degeneration) and whether the therapy works by reducing the extent of cortical reorganization. Establishing the nature, degree and consequences of plasticity in the adult cortex provides important insights into the potential for rehabilitative brain therapies following damage to the nervous system.
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