7T Magnetic Resonance Imaging Probabilistic Tractography-Based Evidence of Decussation of the Fibers Between the Lateral Geniculate Nucleus and the Primary Visual Area

Abstract

Background and ObjectivesGeniculocalcarine fibers are thought to be exclusively ipsilateral. However, recent findings challenged this belief, revealing bilateral recruiting responses in occipito-temporo-parietal regions upon unilateral stimulation of the lateral geniculate nucleus (LGN) in humans. This raised the intriguing possibility of bilateral projections to primary visual areas (V1). This study sought to explore the hypothetical decussation of the geniculocalcarine tract. MethodsForty healthy individuals' 7T MR images from the Human Connectome Project were examined. Employing MRtrix3 software with the constrained spherical deconvolution algorithm, scans were processed. LGN served as the seed region, and contralateral regions of interest – ROIs (splenium of the corpus callosum – SCC, posterior commissure, LGN, V1, pulvinar, and superior colliculus) were defined to reconstruct the hypothetical decussated fibers. Tractography included contralateral V1 as the target region in all segmentations, excluding ipsilateral V1 to eliminate fibers leading to or originating from this area. Additionally, a segmentation of the tract originating from LGN and projecting to the ipsilateral V1 was performed. Mean fraction anisotropy and mean diffusivity metrics were extracted from the density maps. ResultsObservations revealed a substantial volume of decussated fibers between LGN and contralateral V1 via the SCC, albeit much smaller than ipsilateral fibers. The volume of ipsilateral fibers was similar in both sides. Left LGN-originating decussated fibers were more than double those originating from the right LGN. Tract segmentation to other ROIs yielded no fibers. ConclusionsThis study suggests a partial decussation of the fibers between LGN and V1, likely constituting the geniculocalcarine tract.