1.1 Past investigations into the neuroanatomical basis of language ability
In the 19th century, Wernicke introduced the classical neuroanatomical model for language organization. Based on postmortem investigations of individuals with acquired language deficits, a left hemisphere language system was identified, consisting of the posterior temporal lobe for speech recognition, and the inferior frontal lobe for speech production (Wernicke, 1874). Wernicke hypothesized that these two areas were connected, and that interruption of the pathway between them would lead to a specific deficit called conduction aphasia, where the patient could not repeat what they heard.
This syndrome was confirmed clinically when Lichtheim identified a patient with conduction aphasia (Lichtheim, 1885). Dejerine was the first to visually identify the connection between Broca’s and Wernicke’s area, and through post mortem dissections, Dejerine described a prominent fiber pathway originating in the temporal lobe and curving around the Sylvian fissure to project to the frontal lobe, now referred to as the arcuate fasciculus (Dejerine and Dejerine-Klumpke, 1895).
1.2 Current knowledge of the neuroanatomical basis of language ability Our understanding of the neurological basis of language ability has greatly
expanded since Wernicke proposed his model. Research in the fields of neuropsychology, psycholinguistics, and neuroimaging has lead to a better understanding of the brain-language relationship. Recent investigations have shown the classical Wernicke brain-language model fails to explain many findings. The model cannot account for the complex
variation in symptoms that are seen with aphasia, such as anomic aphasia (problems
recalling certain words or names) (Hickock and Poeppel, 2000). It has also become clear that the anatomical proclamations of the classical model are no longer accurate. It is now believed that certain aspects of language ability (e.g. speech awareness) are organized bilaterally, and not constrained to just the left hemisphere (Hickock and Poeppel, 2000).
It has also been shown that language regions proposed in the classical model are not anatomically or functionally isolated. Modern studies have identified several cortical and subcortical areas outside of the classical model that have been implicated in language processing, such as the anterior superior temporal lobe, middle temporal gyrus, basal ganglia, and several analogous structures in the right hemisphere (Dronkers et al., 2004;
Damasio et al., 2004).
1.3 Magnetic resonance imaging
A deeper understanding of language ability in the human brain has emerged with the invention of magnetic resonance imaging (MRI). While previous researchers like Wernicke were limited to post mortem brain examinations, MRI now allows for the safe assessment of human brain tissue in vivo, increasing the ability to study language in both diseased and healthy populations. MRI is a non-invasive tomographic imaging technique used to produce three-dimensional images of the human body. It uses a strong magnetic field and non-ionizing electromagnetic radiation (in the radiofrequency range) to generate a signal, typically from hydrogen protons.
MRI’s ability to produce high-resolution images, and its ability to study both the structure and function of the brain, has made it a powerful imaging tool for neurological investigations. The development of quantitative MRI methods has had a large impact on neuroanatomical language studies, and many different computational techniques now
allow for the investigation of the brain’s structural and functional relationships with language ability. Most MRI investigations of language have focused on adults and children older than 8 years of age. Very few imaging studies have investigated the relationship between language and brain structure or function in children younger than 5 years. This is because of the practical and technical challenges that research teams face when scanning children this young. These include procedural problems (e.g. patient anxiety, excessive movement), technical difficulties (e.g. availability of appropriate equipment or pediatric MR head coils), and the challenges of finding appropriate analysis methods for pediatric imaging data.
1.4 Rationale for the study
One of the most important developmental gains seen in young children is the acquisition of appropriate language skills. Before 5 years of age, there is a massive development of speech, language, and early reading behaviors that occurs (e.g.
phonological processing, letter knowledge) (Locke, 1996). In the preschool years,
vocabulary and grammatical knowledge grow quickly, with normally developing children being able to produce and comprehend multi-clause expressions before 5 years of age (Conti-Ramsden and Durkin, 2012). Language abilities gained in early childhood predict future reading success, which in turn can affect academic achievement, mental health, and future career prospects (Carroll et al., 2005). Although most children go on to develop fluent reading skills, 5-17% will present with dyslexia, a disorder characterized by reading problems that may persist throughout life (Shaywitz, 1998). Early diagnosis and phonological based interventions have shown to be effective at treating dyslexia
the third grade, when reading difficulty is clearly measureable (Gabrieli, 2009). There is general consensus that the roots of dyslexia begin before initial reading instruction, and through the use of early language testing, it is possible to identify children at risk for dyslexia before they start reading (Gabrieli, 2009). Due to the challenges of scanning young children, the relationships between brain structure, brain function, and language ability have not been fully explored at the preschool age. The goal of this study was to use advanced quantitative MRI techniques to investigate relationships between language ability and brain structure/function in children aged 3-5 years. Early identification of the structural and functional relationships between brain and language ability in typically developing children may help to better characterize the neurology of language associated disorders, which may result in earlier detection, and optimized interventions for children with language and reading difficulties.
1.5 Thesis overview
Chapter one contains the introduction to the work, and outlines the general background and rationale for the research. Chapter two provides information on the MRI methods used in the study. In addition, it reviews the literature on language ability and brain structure and function. Chapter’s three to five present the results of three separate MRI analyses of language ability. Chapter three focuses on diffusion tensor imaging and inhomogeneous magnetization transfer imaging, chapter four discusses the findings related to structural morphology, and chapter five presents the results on arterial spin labeling. Chapter six closes the thesis; limitations of the study, suggestions for future work, integration of all results, and overall conclusions are presented.
Chapter 2: Background Information