Occipitotemporal connections: redefining the inferior longitudinal fasciculus through fiber dissection and diffusion...

The inferior longitudinal fasciculus (ILF) is a major occipitotemporal white matter bundle involved in higher-order visual and cognitive functions. However, its anatomical consistency, structural organization, and distinction from adjacent tracts remain controversial. The aim of this study was to address these debates by investigating the dual nature of occipitotemporal connections, refining the classification of ILF subcomponents, and reassessing the inclusion of the dorsolateral occipital cortex component (DLOCC) in the ILF.

Cadaveric dissection was performed on 10 hemispheres from 5 neurologically healthy donors. Specimens were fixed, frozen, and dissected under an operating microscope according to Klingler's technique. The superficial U-fibers were peeled away to expose long associative fibers, and their cortical origins, trajectories, and relationships with surrounding structures were documented. Complementary diffusion tensor imaging (DTI) was performed in 5 subjects with no evidence of neurological disease and 20 subjects from the Human Connectome Project database. DTI data were acquired and analyzed with deterministic fiber tracking. Manual region-of-interest placement, length-based filtering, and shape analysis enabled reconstruction of short and long fiber tracts, corresponding to those identified in anatomical dissections. The integration of ex vivo and in vivo findings allowed detailed mapping and classification of occipitotemporal pathways.

The investigation revealed consistent indirect U-fiber chains and direct long-range fascicles. The vertical occipital fasciculus and a distinct occipito-fusiform fasciculus (OFF) were identified, the latter of which connected the superior occipital gyrus to the anterior fusiform and inferior temporal gyrus. The fusiform component was found to be a thin intragyral tract with selective termination in the anterior fusiform gyrus. Furthermore, the cuneolingual component, which combines the lingual and cuneal fibers, was defined and showed common temporal terminations and overlapping anatomy. The DLOCC, while anatomically consistent, exhibited greater similarity and overlap with the middle longitudinal fasciculus (MdLF) than with the core ILF. Tractography confirmed these findings, showing diverging courses and terminations.

These findings support a dual-model framework of occipitotemporal connectivity, comprising both direct and indirect fibers. The ILF should be redefined to include only the fusiform and cuneolingual components, with the DLOCC more accurately attributed to a distinct parietotemporal system alongside the MdLF. The OFF represents a separate underrecognized tract. This refined anatomical framework enhances the understanding of occipitotemporal pathways and might inform future functional and clinical studies.