Reorganization of retinotopic maps after occipital lobe infarction
Abstract
We studied patient JS, who had a right occipital infarct that encroached on visual areas V1, V2v, and VP. When tested psychophysically, he was very impaired at detecting the direction of motion in random dot displays where a variable proportion of dots moving in one direction (signal) were embedded in masking motion noise (noise dots). The impairment on this motion coherence task was especially marked when the display was presented to the upper left (affected) visual quadrant, contralateral to his lesion. However, with extensive training, by 11 months his threshold fell to the level of healthy participants. Training on the motion coherence task generalized to another motion task, the motion discontinuity task, on which he had to detect the presence of an edge that was defined by the difference in the direction of the coherently moving dots (signal) within the display. He was much better at this task at 8 than 3 months, and this improvement was associated with an increase in the activation of the human MT complex (hMT(+)) and in the kinetic occipital region as shown by repeated fMRI scans. We also used fMRI to perform retinotopic mapping at 3, 8, and 11 months after the infarct. We quantified the retinotopy and areal shifts by measuring the distances between the center of mass of functionally defined areas, computed in spherical surface-based coordinates. The functionally defined retinotopic areas V1, V2v, V2d, and VP were initially smaller in the lesioned right hemisphere,…
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