• anatomy
• fiber dissection tecnique
• MRI tractography
• occipito-temporal connectivity

Objectives:
This study aimed to comprehensively examine the anatomical structure of occipito-temporal white matter connections in the human brain using a multimodal approach that integrated anatomical dissection and magnetic resonance imaging (MRI) tractography. Despite previous investigations, the precise definition and organization of these connections, particularly the inferior longitudinal fasciculus (ILF), have remained ambiguous and lack definitive consensus. A detailed understanding of these anatomical pathways is essential for interpreting higher-order brain functions such as language, reading, and vision, as well as for improving surgical strategies and elucidating disconnection syndromes as illustrated by a presented case report.
Methods:
Two primary techniques were employed: anatomical dissection and MRI tractography. Anatomical dissection, following the Klingler method, was performed on five postmortem human brains (10 hemispheres). Specimens were fixed in formalin and frozen to mechanically separate axonal fibers. After removing surface membranes and vessels, dissections were carried out under an operative microscope using wooden spatulas. The cortex was progressively peeled to visualize and classify U-fibers and long association bundles. MRI tractography was conducted in vivo on five healthy volunteers using a 3T scanner. Diffusion-weighted images were acquired with multiple gradient directions to estimate the diffusion tensor in each voxel. Deterministic tractography algorithms with enhanced strategies were employed, using manually defined seed regions of interest. Tractography data were co-registered with structural MRI sequences (T1-weighted, FLAIR) to improve anatomical accuracy. The integration of diffusion tensor imaging (DTI) with anatomical dissection was intended to improve the specificity and reliability of both modalities.
Results:
Both anatomical dissections and MRI tractography revealed that occipito-temporal connectivity consists of long direct tracts and shorter indirect U-fiber systems. The ILF was confirmed as a distinct associative tract connecting occipital and temporal cortices and was segmented into several components. These included the dorso-lateral occipital component (DLOC), connecting occipital gyri and the posterior superior parietal lobule with the superior, middle, and inferior temporal gyri; the cuneo-lingual component (CLC), connecting the calcarine cortex to the anterior parahippocampus, fusiform gyrus, and temporal pole; and the fusiform component (FC), linking the occipital region with the anterior third of the fusiform gyrus (fusiform anterior area; FAA). Additionally, a distinct direct tract, referred to as the occipito-fusiform fasciculus (OFF), was identified. The OFF originated from the anterior superior occipital gyrus and terminated in the FAA and anterior inferior temporal gyrus, following an anteroinferior course and lying superficial to the DLOC and sagittal stratum. A consistent system of short U-fibers interconnecting adjacent gyri along the occipito-temporal axis was also observed, forming an indirect system that supports the dual nature of the ILF, as proposed in previous anatomical and MRI studies. The close anatomical relationship between the middle longitudinal fasciculus (MdLF) and the DLOC suggests that these tracts may constitute parts of a shared association system within the most lateral portion of the sagittal stratum.
Conclusions:
Through the integration of anatomical dissection and MRI tractography, this study confirms the presence of both direct (ILF, OFF) and indirect (U-fiber) occipito-temporal white matter connections. U-fibers were consistently categorized across both techniques, revealing a coherent and reproducible organization. The ILF was anatomically redefined to consist of three distinct components, which are spatially separated by the optic radiation: the CLC and FC are located beneath it, while the DLOC lies superior to the optic radiation and lateral to the sagittal stratum. Notably, the DLOC appears to be more closely associated with the MdLF and the sagittal stratum than with the ILF itself. The OFF was identified as an independent, more superficial direct tract. The complex clinical manifestations observed in a case of left temporo-occipital glioblastoma included in the study, characterized by alexia, anomia, agraphia, and acalculia-support the involvement of occipito-temporal connections, such as the inferior longitudinal fasciculus (ILF), in these functions, potentially as disconnection syndromes. Furthermore, the functional recovery observed in this case highlights the need for precise anatomical understanding in the assessment and management of such lesions. This multimodal approach improves anatomical precision and methodological robustness, offering a refined framework for understanding complex neural networks, optimizing neurosurgical planning in the occipito-temporal region, and deepening insight into disconnection syndromes.