Structural connectivity of the language network : evidence from patients with aphasia

Abstract

Language processing is an extraordinarily rapid and accurate system that relies heavily on its neuroanatomical basis. In the 19th century, neurologists had initially proposed that the structural connection was a central component for understanding the neural mechanism underlying higher cognitive functions of the human brain. Inspired by the pioneering work conducted by Carl Wernicke who not only identified one additional classic language territory in the temporal lobe but also indicated the possible existence of structural pathways which connecting brain regions involved in speech perception and articulation, researchers have been working tirelessly to uncover the mystery of the structural features of human language.

Our understanding of the structural connectivity of the language network has been extensively expanded in the past decades. Empirical evidence from neuropsychological and neuroimaging studies has demonstrated that the structural transmissions in the brain, which are mostly carried out by the white matter tracts, play a crucial role in supporting the general language processing. Namely, certain white matter pathways are directly involved in multiple linguistic domains including speech comprehension and production, reading, and writing by connecting the respective brain regions. White matter dysfunction results in both neurological language abnormalities such as aphasia and developmental language disorders such as dyslexia. In addition, the neuroplasticity of white matter has shown a strong positive impact on language performance at the recovery or improvement stage. The lesion-behavioral method, which uses data from patients with language impairments, has long provided insights into the neural mechanism of the neuroanatomical language network. To date, the most straightforward technique to investigate the structural characteristics and functional roles of a particular fiber tract remains as the examination of language performance in individuals with lesions in the white matter fiber tracts.

Although the significance of the white matter pathways that are relevant to language has been underlined from a variety of angles, it remains unclear to what extent the hypothesized fiber tracts are involved in language processing. In this thesis, we have adopted the most recent tensor-based algorithm to estimate the structural connectivity model of the target white matter tracts both in the group of aphasic patients and the group of healthy participants. Three experiments were conducted with structural and diffusion magnetic resonance imaging, combined with neuropsychological assessments. In study 1, we found strong associations between the integrity of white matter tracts and language deficits in speech production and reading comprehension. In study 2, we reconstructed the structural connectivity among identified language areas distributed in the parietal and temporal lobe, as well as the visual perception areas in the occipital lobe that contribute to typical reading from patients with acquired alexia. In study 3, we explored the neuroplasticity of language related white matter tracts in association with typical aphasia syndromes.

Taken together, the experimental results supported the classic dual-stream hypothesis of language processing and provided new evidence for the specific function of the language-related white matter tracts. Both conceptual argument for the structural connectivity of the language network and clinical application pertaining to neurological language deficits will benefit from the implications of the research.