The contribution of corpus callosum to lateralization of the resting state language network
Language is viewed as one of the most lateralized human brain functions. Left hemisphere dominance for language has been consistently confirmed in clinical and experimental settings and constitutes one of the main axioms of neurology and neuroscience. However, functional neuroimaging studies are finding that the right hemisphere also plays a role in diverse language functions. Critically, the right hemisphere may also compensate for the loss or degradation of language functions following extensive stroke-induced damage to the left hemisphere. Here, we review studies that focus on our ability to choose words as we speak. Although fluidly performed in people with intact language, this process is routinely compromised in aphasic patients. We suggest that parceling word retrieval into its sub-processes – lexical activation and lexical selection – and examining which of these can be compensated for after left-hemisphere stroke can advance the understanding of the lateralization of word retrieval in speech production. In particular, the domain-general nature of the brain regions associated with each process may be a helpful indicator of the right hemisphere's propensity for compensation.
Background and Purpose Despite the continuing efforts in multimodal assessment of the motor system after stroke, conclusive findings on the complementarity of functional and structural metrics of the corticospinal tract (CST) integrity and the role of the contralesional hemisphere are still missing. The aim of this work was to find the best combination of the motor system parameters, allowing classification of patients into three predefined groups of upper limb motor recovery.
Methods 35 chronic ischemic stroke patients (47 [26–66] y.o., 29 [6–58] months post-stroke) with only supratentorial lesion and unilateral upper extremity weakness were enrolled. Patients were divided into three groups depending on the upper limb motor recovery. Non-parametric statistical tests and regression analysis were used to investigate the relationships among structural and functional motor system parameters, probed by diffusion tensor imaging (DTI) and transcranial magnetic stimulation (TMS). In addition, stratification rules were tested, using a decision tree classifier to identify parameters explaining motor recovery.
Results Fractional anisotropy (FA) ratio in the internal capsule (IC) and absence/presence of motor evoked potentials (MEPs), were equally discriminative of the worst motor outcome group (96% accuracy). MEP presence diverged for two investigated hand muscles. Concurrently, for the three recovery groups’ classification, the best parameter combination was: IC FA ratio and Fréchet distance between the contralesional and ipsilesional CST FA profiles (91% accuracy). No other metrics had any additional value for patients’ classification.
Conclusions This study demonstrates that IC FA ratio and MEPs absence are equally important markers for poor recovery. Importantly, we found that MEPs should be controlled in more than one hand muscle. Finally, we show that better separation between different motor recovery groups may be achieved when considering the whole CST FA profile.
The mechanisms of lateralization of language processing are still not fully understood by neurolinguistics today. The current study aims to study the relation between language lateralization and such factors as individual handedness, familial sinistrality and tractography metrics of the corpus callosum (CC). We collected fMRI and DTI data, as well as information about individual handedness and familial sinistrality in 50 neurologically healthy Russian speakers. According to the results, language lateralization is related to the volume and fractional anisotropy of CC, as well as individual handedness. Specifically, people with greater right-hand preference and people with a larger volume and higher fractional anisotropy of CC have greater lateralization of language-related activation to the left hemisphere of a brain.
Handedness is the most prominent trait of functional asymmetry in humans, associated with lateralized cognitive functions and considered in relation to mental disorders. However, the neuroanatomical correlates of handedness are still unclear. It has been hypothesized that the structural properties of sub-regions of the corpus callosum (CC) are linked to handedness. Nevertheless, tractography studies of the relation between directly measured structural properties of CC subregions and handedness are lacking. The Constrained Spherical Deconvolution (CSD) approach enables full reconstruction of the sub-regions of the CC. The current study aimed to investigate the relation between the structural properties of the CC, such as volume and the CSD metric, referred to as hindrance modulated orientational anisotropy (HMOA), and handedness. Handedness was considered in two dimensions: direction (right-handed, ambidextrous, left-handed) and degree (the absolute values of Handedness quotient). We found no association between 1) volume or HMOA as a proxy of microstructural properties, namely the axonal diameter and fiber dispersion, of each sub-region and 2) either the direction or the degree of handedness. These findings suggest the absence of a direct relation between sub-regions of the CC and handedness, demonstrating the necessity of future tractography studies.