Contralateral-Ipsilateral Segregation as a Conserved Principle of the Visual Corpus Callosum

How the corpus callosum (CC) integrates the left and right visual fields to create a unified perceptual experience is a fundamental unresolved question. It remains unresolved whether the CC transmits this information via segregated parallel channels or through mixed, integrated pathways. Here, we resolve this by creating a multimodal functional blueprint of the forceps major (FMA), the principal component of the visual callosum, in humans and mice. By combining ultra-high-field 7T fMRI with Bayesian population receptive field (pRF) modeling in humans, we directly test the competing hypotheses: segregated streams should manifest as voxels dominated by single pRFs, whereas integrated signals would produce voxels with multiple pRFs. Our results reveal a functionally segregated architecture dominated by single pRFs (expected probability 0.92), forming parallel streams that represent the contralateral (87.7%) and distinct ipsilateral (12.3%) visual field. In contrast, integrative dual pRFs are sparse and localized to the cortical boundary, reflecting the localized convergence of bilateral axons. This organizational principle is anatomically corroborated at the mesoscale in the mouse, where dual-color neuronal viral tracing and whole-brain light sheet microscopy imaging reveal a complementary laminar segregation of callosal fibers. Collectively, these findings establish that the visual callosum operates as a set of parallel, segregated information conduits, providing a new framework for understanding unified visual perception and for investigating callosal disruptions in neurological disorders.