The Role of the Right Hemisphere White Matter Tracts in Chronic Aphasic Patients After Damage of the Language Tracts in the Left Hemisphere.

Ioannis Zalonis,Georgia Angelopoulou,Efstratios Karavasilis,Michael Petrides,Nikolaos Kelekis,Ioannis Evdokimidis,Dimitrios Kasselimis,Georgios Velonakis,Evie Kourtidou,Constantin Potagas

doi:10.3389/fnhum.2021.635750

Abstract

The involvement of the right hemisphere (RH) in language, and especially after aphasia resulting from left hemisphere (LH) lesions, has been recently highlighted. The present study investigates white matter structure in the right hemisphere of 25 chronic post-stroke aphasic patients after LH lesions in comparison with 24 healthy controls, focusing on the four cortico-cortical tracts that link posterior parietal and temporal language-related areas with Broca’s region in the inferior frontal gyrus of the LH: the Superior Longitudinal Fasciculi II and III (SLF II and SLF III), the Arcuate Fasciculus (AF), and the Temporo-Frontal extreme capsule Fasciculus (TFexcF). Additionally, the relationship of these RH white matter tracts to language performance was examined. The patients with post-stroke aphasia in the chronic phase and the healthy control participants underwent diffusion tensor imaging (DTI) examination. The aphasic patients were assessed with standard aphasia tests. The results demonstrated increased axial diffusivity in the RH tracts of the aphasic patients. Patients were then divided according to the extent of the left hemisphere white matter loss. Correlations of language performance with radial diffusivity (RD) in the right hemisphere homologs of the tracts examined were demonstrated for the TFexcF, SLF III, and AF in the subgroup with limited damage to the LH language networks and only with the TFexcF in the subgroup with extensive damage. The results argue in favor of compensatory roles of the right hemisphere tracts in language functions when the LH networks are disrupted.

Highlights

Research in the 19th century established that language function, such as speech production and comprehension, is critically dependent on specific areas of the left hemisphere (Broca, 1861; see Petrides, 2014)
The language production region known as Broca’s area (Brodmann areas 44 and 45) in the inferior frontal gyrus is bidirectionally linked with the posterior superior temporal region (Wernicke’s region) via the arcuate fasciculus (AF) and the adjacent supramarginal and angular gyri of the inferior parietal lobule via superior longitudinal fasciculus (SLF) superior longitudinal fasciculus III (III) and SLF superior longitudinal fasciculus II (II), respectively
The present study investigated the structural properties of the homologs of these four language-related fasciculi in the right hemisphere (RH) of chronic aphasic patients

Summary

Introduction

Research in the 19th century established that language function, such as speech production and comprehension, is critically dependent on specific areas of the left hemisphere (Broca, 1861; see Petrides, 2014). Brodmann areas 44 and 45 which occupy the cortex of the pars opercularis and pars triangularis of the inferior frontal gyrus, respectively, are often referred to as Broca’s area and play a critical role in the planning and execution of language production in coordination with specific posterior parietal and temporal areas via specific bi-directional fasciculi (Indefrey and Levelt, 2004; Petrides, 2015; Sarubbo et al, 2020) In this context, converging evidence from lesion, electrical stimulation, and functional magnetic resonance imaging (fMRI) studies suggests that the posterior superior temporal region provides language-related auditory information via the arcuate fasciculus (AF), the supramarginal gyrus of the inferior parietal lobule provides higher level orofacial information via the third branch of the superior longitudinal fasciculus (SLF III), and the angular gyrus visuospatial information via the second branch of the superior longitudinal fasciculus (SLF II). The posterior parietal region that has been related to spatial processing and is connected via the SLF II with Brodmann area 45 in the inferior frontal gyrus is forming a network that, in the left hemisphere, has been suggested to participate in working memory (Makris et al, 2007), action in space (Koch et al, 2010), and writing (Shinoura et al, 2013)

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