Parallel Wires: A Conserved Principle of Contralateral-Ipsilateral Segregation in the Visual Corpus Callosum

Abstract How the brain maintains distinct information streams within dense white matter is a fundamental question. We investigated whether the visual corpus callosum transmits information via segregated “parallel wires” or mixed pathways. This distinction is critical: a mixed architecture would render the signal’s origin ambiguous, whereas a segregated arrangement implies that spatial position tracks the direction of information flow. Using high-field fMRI and Bayesian modeling in humans, we demonstrate a segregated architecture featuring distinct contralateral and ipsilateral channels. This functional segregation mirrors a precise anatomical arrangement in mice, where dual-color viral tracing and light-sheet microscopy reveal that callosal axons remain spatially segregated in distinct laminae after crossing the midline. Our findings establish a conserved “parallel wires” principle of callosal organization, providing a new framework to decode directional information flow and assess pathway-specific damage in neurological disease. Competing Interest Statement The authors have declared no competing interest. Footnotes The title and abstract have been updated to improve clarity and to emphasize the parallel wires framework for contralateral and ipsilateral segregation in the visual corpus callosum. The Introduction has been reorganized to sharpen the central question and refine the motivation for distinguishing mixed versus segregated callosal architectures. Major revisions have been made to the Results section. Quantitative values for polar angle distributions, size and eccentricity relationships, and variance explained have been updated. Several figures have been revised with corrected labels, refined color maps, clearer annotations of fiber trajectories, and improved visualization of single and dual pRF voxels. The section on mouse viral tracing has been expanded with clearer descriptions of midline overlap, laminar segregation, and quantitative indices of spatial separation. A methodological update was made to the split half analyses in the Supplementary Data. The previous version used a split based on both subjects and runs. This has been revised to a split based solely on subjects. This change was made to avoid mixing within subject temporal variance with across-subject variability and to ensure that the reliability assessment reflects independent sampling of individuals, which is more appropriate for cross-subject generalization. The Discussion has been rewritten to improve conceptual flow and strengthen links between human functional mapping and mouse mesoscale anatomy. Additional clarification has been added regarding implications for directional information flow and developmental mechanisms. The Methods section has been updated with clearer descriptions of preprocessing, registration using both T1 and FA images, ROI definitions, Bayesian pRF modeling, and confidence interval estimation. Terminology has been standardized. Author information has been updated. The author order of Wang Huan has been revised to reflect contributions more accurately. Minor textual corrections, formatting adjustments, and consistency edits have been applied throughout the manuscript.