Mapping intralobar fiber connections in the human occipital lobe by tracer electrophoresis

The cellular connectivity of the human visual cortex remains largely uncharted, with most insights derived from animal models. A custom-designed tracer electrophoresis setup for polar lipophilic tracers enabled the reconstruction of fiber tracts in fixed postmortem human brains, visualizing their trajectories across previously unmapped distances, from cellular origins to axonal terminations. Applied to the occipital lobe, four pathways of the calcarine cortex were demonstrated: the stratum calcarinum, stratum proprium cunei, fasciculus transversus cunei, and the fasciculus transversus gyri lingualis. Specifically, axonal projections from granular and supragranular layers of the calcarine cortex were traced to the granular and supragranular layers of the cuneus, inferior, middle, and superior occipital, lingual, and fusiform gyri. Additionally, infra- and supragranular feedback projections from the prestriate cortex to the supragranular layer of the calcarine cortex were identified. These results extend previous descriptions by offering the first cellular-level evidence for intrahemispheric pathways in the human occipital lobe.