Abstract
The extended frontal aslant tract (exFAT) is a tractography-based extension of the frontal aslant tract (FAT) which has been shown to be related with language and working memory performance in healthy human adults, but whether those functional implications map to structurally separate regions along its trajectory is still an open question. We present a tractography-informed Voxel-Based Morphometry procedure capable of detecting local tract-specific structural differences in white matter regions and apply it in two maximum variation sampling studies by comparing local differences in diffusion-derived microstructural parameters and fiber density along the exFAT territory between top performers and bottom performers in language and working memory tasks. In the right hemisphere we were able to detect, without prior constraints, a vertical frontal aslant component approximating the original FAT trajectory whose fiber density was significantly correlated with language (but not working memory) performance and an anterior cluster component corresponding to a distinct anterior frontal aslant component whose fiber density was significantly correlated with working memory (but not language) performance. The reported sub-division of the exFAT territory describes a set of frontal connections that are compatible with previously reported results on the Broca’s territory and frontal cortex hierarchical organization along an anterior-posterior gradient, suggesting that the exFAT could be part of a common neuroanatomical scaffold where language and working memory functions are integrated in the healthy human brain.
Highlights
The frontal aslant tract (FAT) is a recently discovered left-lateralized bilateral tract described in virtual dissection studies (Catani et al, 2012) as a connection between Broca’s territory and the supplementary and pre-supplementary motor areas of the superior frontal gyrus (SFG)
The axial diffusivity (AD)-based voxel-based morphometry (VBM) study detected, in the right hemisphere, a significant cluster located in the posterior region of the extended frontal aslant tract (exFAT) territory in the language extreme group for both the correlation test (p = 0.012) and the unpaired t-test (p = 0.015)
The mean diffusivity (MD)-based VBM study detected, in the right hemisphere, a significant cluster located in the posterior region of the exFAT territory in the language extreme group for both the correlation test (p = 0.013) and the unpaired t-test (p = 0.015)
Summary
The frontal aslant tract (FAT) is a recently discovered left-lateralized bilateral tract described in virtual dissection studies (Catani et al, 2012) as a connection between Broca’s territory and the supplementary and pre-supplementary motor areas of the superior frontal gyrus (SFG). Several diffusion-derived parameters, such as fractional anisotropy (FA) and other simpler lambda-derived diffusivity parameters, such as axial diffusivity (AD), radial diffusivity (RD) and mean diffusivity (MD), have been used to study the structural properties of the FAT territory: in the seminal article approaching the tractography study of the FAT (Catani et al, 2013), FA and RD were used as surrogate measures of white matter spatial organization, myelination and axonal integrity, and were found to be correlated with mean length of utterance and words per minute scores in patients with primary progressive aphasia (PPA). A significant group difference in the average MD of the entire tract was detected (Kronfeld-Duenias et al, 2016) In this line, another study focused on the FA values in the FAT in patients with PPA, arguing that the lambda diffusivity values might introduce systematic biases, and found significant correlations between longitudinal white matter changes and FA in the left FAT (Mandelli et al, 2016). A study with children between ages 5 and 8 showed that the microstructural properties of the FAT, as measured by AD, RD, MD and FA, remained mostly stable (Broce et al, 2015), a later study with a larger sample and more powerful methods showed that the FAT presents changes in anisotropy measures until adulthood, and that microstructural properties of the right FAT are associated with increased reports of attention problems in children (Garic et al, 2018)
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