A Quantitative Tractography Study Into the Connectivity, Segmentation and Laterality of the Human Inferior Longitudinal Fasciculus.

Sandip S Panesar,Fang-Cheng Yeh,William Hula,Juan C Fernandez-Miranda,Timothée Jacquesson

doi:10.3389/fnana.2018.00047

Abstract

The human inferior longitudinal fasciculus (ILF) is a ventral, temporo-occipital association tract. Though described in early neuroanatomical works, its existence was later questioned. Application of in vivo tractography to the neuroanatomical study of the ILF has generally confirmed its existence, however, consensus is lacking regarding its subdivision, laterality and connectivity. Further, there is a paucity of detailed neuroanatomic data pertaining to the exact anatomy of the ILF. Generalized Q-Sampling imaging (GQI) is a non-tensor tractographic modality permitting high resolution imaging of white-matter structures. As it is a non-tensor modality, it permits visualization of crossing fibers and accurate delineation of close-proximity fiber-systems. We applied deterministic GQI tractography to data from 30 healthy subjects and a large-volume, averaged diffusion atlas, to delineate ILF anatomy. Post-mortem white matter dissection was also carried out in three cadaveric specimens for further validation. The ILF was found in all 60 hemispheres. At its occipital extremity, ILF fascicles demonstrated a bifurcated, ventral-dorsal morphological termination pattern, which we used to further subdivide the bundle for detailed analysis. These divisions were consistent across the subject set and within the atlas. We applied quantitative techniques to study connectivity strength of the ILF at its anterior and posterior extremities. Overall, both morphological divisions, and the un-separated ILF, demonstrated strong leftward-lateralized connectivity patterns. Leftward-lateralization was also found for ILF volumes across the subject set. Due to connective and volumetric leftward-dominance and ventral location, we postulate the ILFs role in the semantic system. Further, our results are in agreement with functional and lesion-based postulations pertaining to the ILFs role in facial recognition.

Highlights

The human inferior longitudinal fasciculus (ILF) is a ventral, temporo-occipital association tract
The authors proposed a rightward-lateralized ILF arrangement derived from its posterior connectivity profile: dorsally, distinct ILF subfasciculi originated from the cuneate and lateral occipital lobes, while ventrally, subfasciculi originated from the fusiform and lingual gyri (Latini, 2015; Latini et al, 2017)
The ‘connectivity matrix’ function in DSI studio was used to generate matrices representing the number of fibers terminating within regions of a specially modified, per-subject aligned version of the Automated Anatomical Labeling (AAL) atlas

Summary

Introduction

The human inferior longitudinal fasciculus (ILF) is a ventral, temporo-occipital association tract. It was described as connecting the superior, middle, inferior and fusiform gyri, to the lingual, cuneate, lateral-occipital and occipito-polar cortices. Radioisotopic tracer studies in non-human primates questioned the existence of a robust temporo-occipital fasciculus, proposing instead a series of cortico-cortical U-fibers, traveling along the antero-posterior temporal distance (Tusa and Ungerleider, 1985; Catani et al, 2003). The direct subfascicle demonstrated a tri-pronged posterior, and bi-pronged anterior connectivity profile This connective and structural description remained largely unchanged until a recent dissection and DTI series by Latini (2015) and Latini et al (2017). The authors proposed a rightward-lateralized ILF arrangement derived from its posterior connectivity profile: dorsally, distinct ILF subfasciculi originated from the cuneate and lateral occipital lobes, while ventrally, subfasciculi originated from the fusiform and lingual gyri (Latini, 2015; Latini et al, 2017)

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Discussion

Conclusion