Multi-modal, multi-scale imaging shows that long-association systems are made of short relay fibers

COEX 

Obtaining accurate connectional neuroanatomy across scales and modalities ex vivo is crucial to inform the interpretation of in-vivo diffusion MRI (dMRI) findings and advance our understanding of brain circuitry. Here we combine data across multiple modalities, scales, and species to show that low spatial resolution may result in artifactual long-range connections. We focus on the dorsal superior longitudinal fasciculus (SLF-I), a major fiber association system running within the superior frontal gyrus (SFG). Tracing studies in monkeys describe the SLF-I as connecting the postero-medial parietal regions (PGm, PE, PEc) to different frontal regions (6D, 8B, 9)[1]. Due to the complexity of the SFG, with shorter, superficial fibers running parallel to longer, association fibers, the morphology of the human SLF-I remains controversial. Tractography and post-mortem dissections have yielded conflicting results, some supporting direct, long connections, and others supporting shorter or no SLF-I fibers [2,3]. Here, we combine multi-scale, multi-species, multi-modality data to investigate the mesoscopic organization within the SLF-I fiber system.