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Zusammenfassung:
Damage of the primary visual cortex (V1) and/or its inputs leads to a visual field loss (scotoma) in the corresponding, homonymous, region of the contralateral visual hemifield. The resulting visual field defect often involves the whole hemifield (hemianopia) or one quadrant (quadrantanopia). However, some patients have been found to retain a small amount of residual visual sensitivity within the blind field, a phenomenon termed blindsight, suggesting that there are alternate pathways that effectively bypass V1 and transmit visual information directly to extrastriate visual cortex. Blindsight has been associated with activity observed in the middle temporal area complex (V5/MT+) following V1 lesions. An important issue is how the organization of area V5/MT+, including how it covers the visual field, changes following V1 lesions. We used the population receptive field (pRF) method (Dumoulin SO, Wandell BA, Population receptive field estimates in human visual cortex, Neuroimage 39, 2008) to study the organization of human area V5/MT+ after V1 injury in adulthood. Functional magnetic resonance imaging (fMRI) measurements were obtained during the presentation of a moving bar stimulus while the subjects were fixating. We mapped the retinotopic organization of area V5/MT+ in 5 patients with approximate quadrantanopia and compared them to control subjects stimulated with matching “artificial scotomas”. We investigated i) whether area hV5/MT+ remains responsive following the V1 lesion, ii) whether it retains its retinotopic organization, and iii) whether hV5/MT+ organization changes by recruiting inputs from intact portions of area V1, or from the contralateral hV5/MT+. In three patients we found responses in hV5/MT+ arising inside the scotoma, independent of area V1 input, suggesting the existence of a functional alternate pathway bypassing area V1. hV5/MT+ of the other patients responded only to locations outside of the perceptual scotoma. The retinotopic organization of V5/MT+ for all 5 patients was different than that seen in controls stimulated with the “artificial scotoma.” Specifically, the pRF center distribution was shifted across the horizontal meridian towards locations outside the visual field scotoma for all patients. PRF size distributions differed between patients and controls under the artificial scotoma condition, with some subjects having larger pRFs on average, while others smaller. Finally, changes were observed in the retinotopic organization of the contra-lesional area hV5/MT+, likely mediated via trans-callosal connections.
