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D. A. Poggel, L. J. Toth, D.-S. Kim, J. F. Rizzo; Qualitative and Quantitative Comparison of Functional MRI in Normally-Sighted Subjects and Patients With Outer Retinal Diseases. Invest. Ophthalmol. Vis. Sci. 2008;49(13):899.
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Functional MRI studies on patients with age-related macular degeneration (ARMD) showed contradicting results with respect to the qualitative Gestalt of visual cortex activation maps: either silencing of lesion projection zones (LPZ) or activation of the LPZ by stimulation of peripheral intact areas. Our goal was to test the effects of blindness on brain activation by performing a qualitative and quantitative comparison of fMRI signals between normally-sighted subjects and patients with ARMD and retinitis pigmentosa (RP).
We performed retinotopic mapping following a block-design approach. For the polar angle dimension, a rotating wedge stimulus (flickering black-and-white checkerboard) was used; for eccentricity mapping an expanding ring stimulus was used. Data from 7 ARMD patients, 8 RP patients, and 9 healthy subjects were acquired with a 3T Philips Intera MR Scanner. Data were analyzed with Brain Voyager software (General Linear Model), signal amplitudes and locations of peak activation were determined for each stimulus segment. Qualitative comparison was based on linear correlation maps.
Normally-sighted subjects showed a robust activation of early visual cortical areas. In ARMD patients, the LPZ was silenced. In contrast, the LPZ in RP patients showed widespread activation in response to stimulation of intact central visual field areas. Quantitative analysis of retinotopy showed lower overall signal intensity and smaller activated areas in both groups of patients vs. healthy subjects, and eccentricity-dependent differences among all three groups with respect to the distribution of brain signal intensity.
Partial blindness changes the topography of visual cortex activation. Silencing in ARMD and spreading activation in RP patients suggest different processes of plasticity after central vs. peripheral vision loss.
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