Browsing by Author "Palo-oja, Peter"

Developmental dyslexia (DD) affects the accuracy and the fluency of reading without influencing the intelligence of an individual. Problems in phonological awareness (PA), the ability to manipulate the sound structure of words, has been proposed to be the key predictor of DD across languages. Diffusion tensor imaging (DTI) has been used to investigate white matter (WM) structure in DD. The DTI research has concentrated mainly on fractional anisotropy (FA) values, that measure the integrity of WM, and volume of the WM tracts, but also on lateralization differences. Structural alterations have been reported in multiple WM tracts, but left arcuate fasciculus (AF) have most consistently been associated with problems in phonological processing. Also, individuals with DD have been reported to have less prominent leftward lateralization in AF, and inferior fronto-occipital fasciculus (IFOF) compared to non-DD individuals. The participants consisted of 23 individuals with confirmed DD and 21 without DD. In this thesis the reading-related WM tracts were evaluated using deterministic tractography, and the goal was to 1) compare DD and non-DD participants in the FA and volume in reading circuitry, 2) study the associations between reading skills and FA and volume of the WM tracts, and 3) study the lateralization differences in FA and volume. The results of this thesis did not support the current view of the neuroanatomy of DD. Although groups did not differ in FA or in volume of the reading related WM tracts, the groups differed in the lateralization of the WM tracts. Both DD and non-DD participants manifested a rightward lateralization of WM volume in the AF, opposing the earlier findings. Furthermore, DD participants had a unique rightward lateralization of volume in uncinate fasciculus. Subthreshold results in correlations between reading skills and DTI indices also hint toward the heterogeneity found in the DTI research of DD, and do not confirm the role of the AF as the neural correlate of the phonological processing. It seems that no single abnormality in WM structure is responsible of DD. Rather, it seems that DD compiles a vast spectrum of symptoms, with possibly multiple trajectories, and individual compensatory mechanisms in adult samples.