Mapping of The Visual Fields on FMA Voxels Based on Single, Dual, and Triple Receptive Field Models.

(A) The mapping of the visual fields was performed based on single, dual, and triple pRF models, with expected probabilities or voxel counts of 6231 (blue) for single, 439 (red) for dual, and 77 (yellow) for triple pRF models. (B) Spatial distribution patterns show that voxels best explained by the single pRF model (blue) populate the core of the FMA fiber bundle, whereas the sparse population of integrative dual pRF voxels (red) clusters near the occipital gray-white matter interface. (C, D) Representative voxels for the single (blue) and dual (red) models. The left panels indicate the pRF center (dot) and size (circle) in degrees of visual angle (dva). The right panels display the time series fitting, where the model predictions (red solid lines) robustly capture the observed BOLD signal variance (black dashed lines), confirming the high fidelity of the functional data. EV: explained variance.

Parallel Contralateral–Ipsilateral Organization of the FMA.

(A) Population receptive field (pRF) parameter maps for the winning single pRF model, shown in selected axial (top rows) and coronal (bottom rows) views. From the center outwards, columns display the FMA mask, polar angle, eccentricity, pRF size, and explained variance (EV). Polar angle topographies demonstrate a striking parallel organization: dominant contralateral representations (red/orange for left visual field, cyan/blue for right) are spatially segregated from, yet run alongside, distinct ipsilateral streams (highlighted in red box). Eccentricity and pRF size maps further characterize these streams, showing a preferential representation of the visual periphery with large receptive fields. (B) Cross-methodological validation confirms this angular organization in a representative coronal slice. Functional gradient analysis (bottom) delineates the same parallel retinotopic structure observed in the Bayesian pRF estimates (top), with consistent segregation of left (red) and right (blue) visual field representations. All maps are overlaid on the ICBM 152 2009c standard brain template.

Deterministic fiber tracking demonstrating parallel information flow in the FMA.

(A, B) Tractography seeding strategy based on functional pRF maps. To isolate specific information channels, regions of interest (ROI, pink) and regions of avoidance (ROA, blue) were defined for the left (A) and right (B) visual field representations, respectively. (C) Whole-brain reconstructions in MNI space reveal that these functionally distinct streams correspond to two anatomically separate fiber bundles. (D) Cross-sectional coronal and axial views further demonstrate this topological organization, where fibers carrying left visual field information (pink) and right visual field information (blue) maintain a strict parallel, non-overlapping trajectory throughout the fiber bundle.

Laminar segregation of contralateral axons in the mouse visual corpus callosum.

This figure presents mesoscale anatomical evidence from the mouse brain that validates the principle of segregated functional pathways observed in human fMRI. (A) Pipeline of whole brain viral labeling and imaging: viral labeling, brain fixation with transcardial perfusion, brain embedding and slicing, tissue clearing, whole-brain imaging and 3D reconstruction. Scale bar: 1 mm. (B) Three-dimensional reconstruction and visualization of the axonal projections expressing rAAV-hSyn-EGFP (green) and rAAV-hSyn-tdTomato (red) in the whole brain. A: anterior; P: posterior; D: dorsal; V: ventral. (C) Confined viral expression in the respective V1 injection sites confirms the specific labeling of these two distinct hemispheric pathways. Scale bar: 300 μm. (D) The ROI showing axonal projections in the CC, with a volume of 3950 × 1440 × 1970 μm3. Scale bar: 500 μm. (E) Coronal sections reveal a dynamic organizational principle. At the midline of the CC, the axons from both hemispheres intermingle, producing a transient zone of overlap (yellow). As the bundles project laterally away from the midline, they resolve into clearly separated red and green populations. Scale bar: 500 μm. AP: anterior-posterior. (F, G) High-magnification views of the regions outlined in (E1-E4) reveal the precise internal organization of the callosal bundles. The main panels show coronal sections, and the corresponding sagittal sections (F1-F4, G1-G4) taken along the medial-lateral axis demonstrate a definitive laminar segregation. Axons from one hemisphere (green) are consistently positioned dorsal to axons from the other (red). Scale bars: 50 μm.