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Victor M. Borges, Helen Danesh-Meyer, Joanna Black, Benjamin Thompson; Functional Magnetic Resonance Imaging of the Visual Cortex in Unilateral Open-Angle Glaucoma. Invest. Ophthalmol. Vis. Sci. 2012;53(14):732.
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It has been suggested that loss of input to regions of the visual cortex can result in neural plasticity. We used functional magnetic resonance imaging (fMRI) to investigate the response of the visual cortex to unilateral primary open-angle glaucoma (POAG). We assessed whether regions of V1 and V2 with lost input from the glaucomatous eye had a greater response to input from the non-affected eye than regions receiving input from both eyes. Such an increase in response could be indicative of ocular dominance plasticity and/or a reduction of inhibition which would be conducive to plasticity.
5 patients (mean age 72 ± 3.6 years) with unilateral POAG participated in 2 fMRI sessions. Session 1 involved monocular retinotopic mapping of the visual cortex. The maps for each eye were compared to identify regions of V1 and V2 that had lost input from the glaucomatous eye. Mirror-symmetrical areas in the unaffected region of the visual field were then identified as control regions. Session 2 involved monocular viewing of full field counterphasing (8Hz) checkerboards at 5%, 25% and 80% contrast. General linear modelling and event-related averaging were used to assess the response of glaucoma-effected and control regions to these checkerboards.
We found no evidence for an increased response to the healthy eye in glaucoma-effected regions of cortex. There was however, a pronounced loss of response within the glaucoma-effected regions of both V1 and V2 for glaucomatous eye viewing. This loss was greatest for the high contrast stimuli (V1 t4 = -3.8, p = 0.02; V2 t4 = -4.3, p = 0.01) and involved a reduction in contrast gain. A loss of response to the glaucomatous eye was also evident for control regions of V1. This loss was most pronounced for the low contrast stimuli (V1 t4 = 3.3, p = 0.03).
We did not find evidence for extensive plasticity of the visual cortex in unilateral POAG. This could be due to inputs from the non-glaucomatous eye inhibiting plasticity mechanisms and/or trans-synaptic neurodegeneration. Our data indicate that fMRI is sensitive to the visual loss associated with unilateral POAG, that the functional loss is characterised by a reduction in contrast gain and that losses to low contrast stimuli can be detected in unaffected regions of the visual field within the primary visual cortex.
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