Abstract
We combined the neuroanatomists’ approach of defining a fascicle as all fibers passing through its compact stem with diffusion-weighted tractography to investigate the cortical terminations of two association tracts, the inferior fronto-occipital fasciculus (IFOF) and the uncinate fasciculus (UF), which have recently been implicated in the ventral language circuitry. The aim was to provide a detailed and quantitative description of their terminations in 60 healthy subjects and to do so to apply an anatomical stem-based virtual dissection, mimicking classical post-mortem dissection, to extract with minimal a priori the IFOF and UF from tractography datasets. In both tracts, we consistently observed more extensive termination territories than their conventional definitions, within the middle and superior frontal, superior parietal and angular gyri for the IFOF and the middle frontal gyrus and superior, middle and inferior temporal gyri beyond the temporal pole for the UF. We revealed new insights regarding the internal organization of these tracts by investigating for the first time the frequency, distribution and hemispheric asymmetry of their terminations. Interestingly, we observed a dissociation between the lateral right-lateralized and medial left-lateralized fronto-occipital branches of the IFOF. In the UF, we observed a rightward lateralization of the orbito-frontal and temporal branches. We revealed a more detailed map of the terminations of these fiber pathways that will enable greater specificity for correlating with diseased populations and other behavioral measures. The limitations of the diffusion tensor model in this study are also discussed. We conclude that anatomical stem-based virtual dissection with diffusion tractography is a fruitful method for studying the structural anatomy of the human white matter pathways.
Highlights
Both inferior fronto-occipital (IFOF) and uncinate (UF) fasciculi are crucial for the ventral intra-hemispheric transfer of information between the frontal cortex and the occipital, temporal and parietal cortices, and knowing their cortical terminations is fundamental for understanding their role in mediating language semantics (Turken and Dronkers, 2011; Duffau, 2015)
The results of the inferior fronto-occipital fasciculus (IFOF) and uncinate fasciculus (UF) stems drawn by one operator (GP) are presented
The ANOVA with Tract (IFOF, UF), Hemisphere (Left, Right) and their interaction (Tract × Hemisphere) as within-subject factors and Sex as a between-subjects factor only revealed an Hemisphere effect with both IFOF and UF stems significantly larger in the right hemisphere (F = 40.9, p < 0.0001)
Summary
Both inferior fronto-occipital (IFOF) and uncinate (UF) fasciculi are crucial for the ventral intra-hemispheric transfer of information between the frontal cortex and the occipital, temporal and parietal cortices, and knowing their cortical terminations is fundamental for understanding their role in mediating language semantics (Turken and Dronkers, 2011; Duffau, 2015). Less recent, Ebeling and von Cramon (1992) meticulous dissection study on the anatomy of the uncinate fasciculus provides a detailed description of its anatomical projections within the frontal and temporomesial areas that elaborates on earlier descriptions. While these dissection studies confirmed the conventional definition of the respective tracts, they revealed termination territories in more detail and beyond those previously described, suggesting more expansive definitions are needed. Previous dissection work indicates that there is some inter-individual variability in the tract projections (Martino et al, 2010) This no doubt contributes to the inconsistencies in the cortical terminations observed in the IFOF and to a limited extent the UF. There has been no detailed study of the UF terminations using tractography (Von Der Heide et al, 2013)
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