Structure, asymmetry, and connectivity of the human temporo-parietal aslant and vertical occipital fasciculi

Abstract

We previously proposed a bipartite 'dorsal-ventral' model of human arcuate fasciculus (AF) morphology. This model does not, however, account for the 'vertical,' temporo-parietal subdivision of the AF described in earlier dissection and tractographic studies. In an effort to address the absence of the vertical AF (VAF) within 'dorsal-ventral' nomenclature, we conducted a dedicated tractographic and white-matter dissection study of this tract and another short, vertical, posterior-hemispheric fascicle: the vertical occipital fasciculus (VOF). We conducted atlas-based, non-tensor, deterministic tractography in 30 single subjects from the Human Connectome Project database and verified our results using an average diffusion atlas compiled from 842 separate normal subjects. We also performed white-matter dissection in four post-mortem specimens. Our tractography results demonstrate that the VAF is, in fact, a bipartite system connecting the ventral parietal and temporal regions, with variable connective, and no volumetric lateralization. The VOF is a non-lateralized, non-segmented system connecting lateral occipital areas with basal-temporal regions. Importantly, the VOF was spatially dissociated from the VAF. As the VAF demonstrates no overall connective or volumetric lateralization, we postulate its distinction from the AF system and propose its re-naming to the 'temporo-parietal aslant tract,' (TPAT), with unique dorsal and ventral subdivisions. Our tractography results were supported by diffusion atlas and white-matter dissection findings.

1 A-D

1A Sagittal view of the left hemisphere using the AAL atlas in the HCP 842 atlas demonstrating the seeding method for both the VAF (TPAT) and VOF. The ROA and ROI were the same for both bundles. For TPAT supramarginal gyrus, dorsal and ventral portions (dSmG; vSmG), angular gyrus (AG), superior parietal lobule (SPL) and precuneus (PC). For the VOF, the superior occipital (SO; not visible), middle occipital (MO) and cuneus (Cu; not visible) were used. 1B Oblique, superior-posterior view of the seeding method for the VAF and VOF within the HCP 842 atlas. The ROA and ROI were the same for both bundles. For VAF dorsal (dSmG) and ventral (vSmG) supramarginal gyrus, angular gyrus (AG), superior, superior parietal lobule (SPL) and precuneus (PC). For the VOF, the superior occipital (SO; not visible), middle occipital (MO) and cuneus (Cu; not visible) were used. 1C Posterior-oblique view of left hemisphere in a single subject (Subject 1) from the subject-specific analysis showing the left VAF. Tract colors are directionally assigned. The wedge in the middle (red arrow) demonstrates the plane used to separate the tracts for further analysis. Also indicated are the ventral and dorsal parietal bifurcations of this tract in the parietal lobe. 1D Posterior-oblique view of left hemisphere of Subject 1. The two divisions of the VAF have been colored separately (light orange color for ventral, pink for dorsal) following division, whilst the VOF is also visible (dark orange) and clearly separated from the VAF. 1E Posterior-oblique view of right hemisphere of Subject 1 showing the left VAF. Tract colors are directionally assigned. The wedge in the middle (red arrow) demonstrates the plane used to separate the tracts for further analysis. Also indicated are the ventral and dorsal parietal bifurcations of this tract in the parietal lobe. 1F Posterior-oblique view of right hemisphere of Subject 1. The two divisions of the VAF have been colored separately (light orange color for ventral, pink for dorsal) following division, whilst the VOF is also visible and labeled (dark orange), clearly separated from the VAF.

2 A-F

2A Sagittal view of the left hemisphere of a single subject. The AAL atlas regions corresponding to the dorsal (dSmG) and ventral (vSmG) portions of the supramarginal gyrus, angular gyrus (AG), middle temporal lobe (MTL) and inferior temporal lobe (ITL) are all labelled. Also apparent within the translucent, separate atlas regions are the dorsal and ventral connections of the left VAF throughout the parietal and temporal gyri. 2B Sagittal view of the right hemisphere of a single subject. The AAL atlas regions corresponding to the relevant connections are highlighted, and are the same as in 3A. In comparison with 3A the figure shows more ventral and segregated nature of the parietal VAF connections. There are dense connections visible at the AG, and posterior ventral SmG (vSmG), with negligible connectivity to the dorsal SmG (dSmG). 2C White fiber dissection picture of a subjects’ left hemisphere, following removal of gray matter, U-fibers and exposure of the VAF. The red arrow indicates the angular demarcation between the AF proper and the VAF, as it passes to the dorsal aspect of the sylvian fissure (i.e. inferior parietal areas). 2D A magnified view of 3C. Orientation markers are provided at edges of the picture. The dorsal extremity of the VAF is marked by a yellow dashed-line ellipse. The ventral extremity is marked by a blue dashed-line ellipse. 2E A sagittal view of the left hemisphere of the HCP-842 atlas demonstrating both the VAF and the VOF. These individual tracts are clearly separated from each other, spatially. Tract color assignment is directional. 2F A sagittal view of the right hemisphere of the HCP-842 atlas demonstrating both the VAF and the VOF. These individual tracts are clearly separated from each other, spatially. Tract color assignment is directional.

3 A-D

3A A supero-lateral view of a post-mortem specimen. The left hemispheric cortex and superficial white matter have been removed to expose the VAF and AF. The parietal diversion of AF and VAF are readily apparent and are demarcated by a red arrow. 3B A magnified view of 5A. Orientation guides are included at the borders of the picture: Anterior (ant.), posterior (post.), ventral (vent.) and dorsal (dors.). The dorsal (parietal) terminations of the VAF are visible and highlighted by a yellow broken-ellipse. The ventral (temporal) terminations of the VAF are visible and highlighted by a blue broken-ellipse. 3C A tractographic representation showing the specific relationship between the AF and the VAF. Clearly visible is the VAF (dorsal – pink; ventral – light orange) lying superficial to the AF proper (green). The temporal terminations of the VAF are obscured by the upslanting of the AFs temporal terminations. 3D A magnified view of the left-hemisphere from another post-mortem specimen. The parietal divergence of the AF (purple broken-lines) and VAF (red broken-lines) is indicated by colored broken-lines.