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Yoichiro Masuda, Satoshi Nakadomari, Serge O. Dumoulin, Sing-Hang Cheung, Ayumu Furuta, Kenji Kitahara, Brian A. Wandell; The mechanism underlying large-scale reorganization in human macular degeneration patients. Journal of Vision 2007;7(9):230. doi: 10.1167/7.9.230.
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Purpose. There is currently a controversy about the conditions required for reorganization in human adults. Some studies describe large-scale human reorganization in human macular degeneration (MD) patients whereas others do not (Baker et al., 2005; Sunness et al., 2004). We explored the requirement for large-scale reorganization in human MD.
Methods. We report fMRI measurements from one JMD patient, who was diagnosed with binocular JMD thirteen years prior to the experiments. She has about a 40x50 degree diameter central absolute scotoma; she fixates using preferred retinal loci (PRL) in the left lower visual field. We also measured fMRI responses in three healthy control subjects using the same experimental setup and eccentric fixation. We used a wide array of visual stimuli that included faces, scrambled faces, moving gratings and checkerboards. Subjects either passively viewed the stimuli or performed a “one-back” task consisting of detecting consecutive repetitions of identical stimuli.
Results. In the JMD patient widespread responses were observed that extended beyond the representation of the PRL into the cortical representation of central vision (occipital pole). This widespread activation was only present in the JMD patient and not in the control subjects. Importantly, these activations were (a) present only when the JMD patient performed the one-back task and (b) independent of the stimuli. We did not observe any cortical activations beyond those expected from the PRL when using passive stimulus viewing.
Discussion. The widespread V1 activations in the JMD subject have been described as evidence for large-scale reorganization within V1. However, since the activation depends on the task but not the stimuli, we suggest that the reported large-scale human reorganization is mediated by feedback from cortical circuits. The strength of these circuit connections may be increased following retinal damage.
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