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

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.