Chapter 6: Conclusions, Limitations, and Future Work
6.3 Integration of findings
Although it was beyond the scope of this thesis, several different methods were employed that attempted to combine the results from each of the three previously
presented investigations. We used the significant regions from both the cortical thickness study and CBF study as regions of interest in a DTI tractography analysis to isolate any white matter tracts that passed through them. No significant relationships were found between the diffusion measures from theses tracts and the language scores, nor was there any correlation between cortical thickness values and the diffusion measures, or CBF values and the diffusion measures. We also used the significant regions from the cortical thickness analysis to isolate measurements of CBF and vice versa. No relationships were found with the cortical thickness and CBF values and language scores, and no
correlations were found between the cortical thickness and CBF values.
The lack of findings from the integration of the MRI results might suggest that the relationships that white matter structure, cortical morphology, and cerebral blood each have with language ability are unrelated to each other. This makes sense when analyzing the findings from all MRI investigations that were used, as the overall results obtained for each method were very different. The relationship between white matter structure and language ability was very wide spread, and included a number of white matter pathways in the left and right hemisphere. The relationship between CBF and language ability was specific to the left hemisphere, and included inferior frontal and occipitotemporal brain regions. The relationship between cortical thickness and language ability was also specific to the left hemisphere, and included inferior parietal and lateral occipital brain regions.
For all analyses, we analyzed both phonological processing and speeded naming.
In older children dorsal white matter pathways and inferior frontal brain regions have been commonly associated with phonological processing, while ventral pathways and occipitotemporal brain regions are more typically associated with orthographic
processing (Jobard et al., 2003; Vigneau et al., 2006; Vandermosten et al., 2012a), the primary skill measure by speeded naming (De Jong, 2011). In our DTI analysis both dorsal and ventral brain areas were associated with phonological processing and speeded naming. Our cortical thickness analysis also found similar brain regions were related to phonological processing and speeded naming. From a structural viewpoint of the brain, this might suggest less differentiation among brain regions in preschool children for different types of language processing than in older children and adults. However, our CBF analysis found different brain regions were related to phonological processing (e.g.
inferior frontal gyrus) and speeded naming (e.g. fusiform gyrus). From a functional viewpoint of the brain, this finding might suggest that preschool-aged children and older children and adults have similar patterns of brain function for different types of language skills.
6.3 Conclusions
To our knowledge, no studies have investigated the relationships between language ability and brain structure and function in typically developing preschool-aged children using any of the MRI methods presented in this thesis. We have identified significant relationships between language ability and brain structure and function in a critical period of language development. We found structural and functional indicators of
language ability in several canonical left hemisphere language areas. Overall, our results suggest that young children with strong language skills possess a more mature pattern of brain structure and function. We utilized measures of phonological processing and
speeded naming ability because both measures have been shown to independently predict future reading ability (Puolakanaho et al., 2007). Our results have identified several neurological markers for children who are at risk for development of reading
impairments, including poorer myelination, decreased cortical thickness, and reduced perfusion of the left hemisphere. We find these structural and functional indicators in children who have not undergone any formal reading instruction, which supports the theory that altered brain structure may be a cause of poor reading, as opposed to a consequence of the poor or limited reading itself. By characterizing brain structure and function in children before reading begins, we can gain a better understanding of how the structure and function of the brain relates to language and reading impairments.
Ultimately, these results may assist in developing and optimizing interventions for children with reading and language difficulties.
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