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Melissa Saenz, Christof Koch, Ione Fine; Retinotopic Organization of Visual-Callosal Fibers in Humans. Journal of Vision 2010;10(7):907. doi: 10.1167/10.7.907.
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Introduction: The visual cortex in each hemisphere is linked to the opposite hemisphere by axonal projections that pass through the splenium of the corpus callosum. Earlier work suggests there may be retinotopic organization within the splenium: Dougherty and Wandell (2005) traced callosal fibers from the splenium to a broad region of visual cortex that included multiple extrastriate regions and found a ventral-dorsal mapping (upper vs. lower visual field) running from the anterior-inferior corner to the posterior-superior end of the splenium. However it is not clear whether these results are due to dorsal and ventral visual areas projecting to different regions of the splenium, or whether individual visual areas also show retinotopic organization within the splenium. Here, we demonstrate consistent retinotopic organization of V1 fibers within the human splenium. Methods: High-angular resolution diffusion-weighted MR imaging (HARDI, 72 diffusion directions) and probabilistic diffusion tractography were used to track fibers between seed points in the splenium and retinotopically-defined sub-regions of V1 in 6 human subjects with normal vision. V1 was divided into sub-regions (three eccentricity bands, upper vs. lower visual field representations) based on functional retinotopic mapping in each subject. Each tractography seed point within the splenium was then labeled according to its profile of connection probabilities to these V1 retinotopic sub-regions. Results: For all 12 hemispheres, we found retinotopic organization of V1 fibers within the splenium. The eccentricity mapping (of fovea to periphery) runs from the anterior-superior corner to the posterior-inferior end of the splenium. This runs orthogonal to a ventral-dorsal mapping (upper vs. lower visual field) running from the anterior-inferior corner to the posterior-superior end of the splenium. These results give a more detailed view of the structural organization of the human splenium than previously reported and offer new opportunities to study structural plasticity in the visual system.
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