A review of the anatomy and function of the white matter tracts in language production and comprehension

Abstract

Much is known about the role of various cortical areas in language production and comprehension. The shift towards network approaches in recent years has highlighted the importance of uncovering the role of the white matter in connecting these areas. However, despite a relatively large body of research, the function of many of these tracts is still underdefined. In this paper, we present a comprehensive review of the empirical evidence on the role of eight major tracts that are hypothesized to be involved in language processing (inferior longitudinal fasciculus, inferior fronto-occipital fasciculus, uncinate fasciculus, the extreme capsule, middle longitudinal fasciculus, superior longitudinal fasciculus, arcuate fasciculus and frontal aslant tract). For each tract, we hypothesize the tract’s role based on the function of the cortical regions it connects. We then evaluate these hypotheses in light of data from three sources: studies in neurotypical individuals, neuropsychological data, and intraoperative stimulation studies. Finally, we summarize the conclusions supported by the data and highlight the areas in need of further investigation.

The role of the Frontal Aslant tract in bilingual language control

“…In line with this hypothesis, Budisavljević et al (2017) showed an association between the microstructural properties of the bilateral FAT and the hand kinematics in the grasping and reaching-to grasp movements, performed with the right hand. This proposal aligns well with the hypothesis that the FAT is primarily involved in the selection and control of the appropriate plan for motor action, consistently with what suggested by recent literature about the role of this tract (Dick et al, 2019; Shekari & Nozari, 2022). The fact that in BBs the inhibition of the signed modality involves both the right and the left FAT, while the inhibition of the spoken modality only involves the left FAT, further supports the idea that, on the left side, the FAT specializes for the control of speech actions (Dick et al, 2019).…”

“…All this evidence is consistent with the proposal that the FAT is involved in inhibitory control processes, also for those related to the regulation of the output of speech (Shekari & Nozari, 2022). However, the exact role of the FAT and whether its function is mainly related to domain-general or language-specific processes has not been fully understood.…”

“…Or results confirm the hypothesis that the FAT is mainly involved in the control of action. Specifically, this tract might have a role in resolving the competition among motor programs of language production, either by inhibiting the non-target action and/or selecting the appropriate one (Dick et al, 2019; Shekari & Nozari, 2022). We observed that the competition might occur even among actions performed with different effectors.…”

“…Within this network, the FAT would not be simply involved in motor processing, but it would perform a domain-general higher level function, i.e. resolving the conflict among competitor motor programs (see also Shekari & Nozari, 2022). In the same review, Dick and coll.…”