White matter variability, cognition, and disorders: a systematic review

Abstract

Inter-individual differences can inform treatment procedures and - if accounted for - can significantly improve patient outcomes. However, when studying brain anatomy, these inter-individual variations are largely unaccounted for, despite reports of differences in gross anatomical features, cross-sectional and connectional anatomy. Brain connections are essential to mediate brain functional organisation and, when severed, cause functional impairments or complete loss of function. Hence the study of cerebral white matter may be an ideal compromise to capture inter-individual variability in structure and function. Here we reviewed the wealth of studies that associate functions and clinical symptoms with individual tracts using diffusion tractography. Our systematic review indicates that tractography has proven to be a sensitive method in neurology, psychiatry and healthy populations to identify variability and its functional correlates. However, the literature may be biased, as we identified that the most commonly reported tracts are not necessarily those with the highest sensitivity to cognitive functions and pathologies. Finally, we demonstrate that tracts, as we define them, are not usually correlated with only one, but rather multiple cognitive domains or pathologies. While our systematic review identified some methodological caveats, it also suggests that tract-function correlations might be a promising biomarker for precision medicine as it characterises variations in brain anatomy, differences in functional organisation and predict resilience or recovery in patients.

Preoperative transcranial magnetic stimulation for picture naming is reliable in mapping segments of the arcuate fasciculus

“…This rate may have been lower as speech errors could not be elicited so commonly in the temporal lobe (also identified in around half of patients). As a recent language model has indicated that the temporo-parietal segment of the arcuate fasciculus plays an important role in speech production, specifically in word repetition, we propose that this lower incidence of errors may be due to the paradigm used ( Forkel et al, 2020 a ). To name a picture, one must recognize the object, categorize it semantically, retrieve and select the target word and phonologically encode this, thus converting incoming visual information into articulatory output.…”

“…In fact, while the arcuate fasciculus was traditionally described solely in its fronto-temporal component ( Geschwind, 1965 ), more recently it has been shown to have parietal ‘indirect’ projections which have been incorporated into modern models of language organization ( Catani and Bambini, 2014 ; Tremblay and Dick, 2016 ; Poeppel and Assaneo, 2020 ). Interindividual differences in segments of the arcuate fasciculus have been linked to differences in language ability in a number of studies ( Forkel et al , 2020 a , b ). Terminations of the fronto-parietal anterior segment (also termed the superior longitudinal fascicle III) were identified with nTMS almost 90% of patients.…”

Cerebral White Matter Myelination and Relations to Age, Gender, and Cognition: A Selective Review

“…Diffusion tensor imaging is one of the most popular noninvasive indirect imaging methods used to assess microstructure and orientation of myelinated WM fibers ( Basser et al, 1994 ; Basser and Jones, 2002 ; Alexander et al, 2007 ). Diffusion MRI (dMRI) allows for investigation of the microstructure of the tissues by measuring diffusion of water molecules within the tissue considering their interaction with biological membranes and macromolecules and has been widely used to study individual differences in WM microstructure related to neurological and psychiatric conditions, as well as in healthy brains ( Forkel et al, 2020 ).…”

Hodology of the superior longitudinal system of the human brain: a historical perspective, the current controversies, and a proposal

“…The AF and the SLF are known to be mainly involved in the dorsal phono-articulatory language loop and in sensory-motor integration processes (Maldonado et al, 2011;Sarubbo et al, , 2015Thiebaut de Schotten, 2005). Still, as outlined in a recent review by Forkel et al (2020a) and as briefly addressed by the present work, it is not possible to trace a precise oneto-one correlation between a WM structure and the derived cognitive function across studies, being function a property arising from the interplay of different cortices and mediated by WM tracts. Similarly to the controversies described for the structural definition of the SLS, the adoption of different nomenclatures and the application of different partitionings within the system does not encourage comparisons between functional studies.…”