| Summary: | Left- and right-handers show functional and structural brain differences. However, the literature on the relationship between handedness and cognitive ability is inconsistent. Moreover, possible differences in the neuroanatomical correlates of cognitive ability, including regional grey matter (GM) volume, between left- and right-handers have not been explored. This thesis describes work with two main aims: (i) to explore differences in brain structure and function between left- and right-handers using MRI on a sample of left- (n=40) and right- (n=42) handers, and (ii) to explore the effect of handedness on the neuroanatomical correlates of cognitive capacity on the same cohort. The effect of sex and handedness on pars opercularis (PO) and pars triangularis (PTR) volume and the sulcal contours defining these regions are described in Chapter 5. PO volume asymmetry is leftward (left-greater-than-right) in right-handed males, non-asymmetrical in right-handed females and rightward in left-handed males and females. PTR volume is rightward in right-handers and non-asymmetrical in left-handers. The inferior frontal sulcus is discontinuous more often in the right than left hemisphere in right-handers and discontinuous more often in the left than right hemisphere in left-handers. The probability of presence of diagonal sulcus is higher in the right than the left hemisphere for left-handers. A second part to this study found a significant effect of handedness on foot preference for kicking and parental handedness. In Chapter 6 fractional anisotropy (FA) asymmetry across the whole brain is explored using voxel-wise statistics on FA maps obtained from diffusion weighted images: increased FA is found in right-handers, and FA asymmetry along the uncinate fasciculus and arcuate fasciculus in both groups. Chapter 7 shows significantly greater leftward language laterality in right-handers and greater activation in right IFG in response to a language production task in left- compared to right-handers. Working memory score is higher in right-handers is associated with increased leftward language laterality. Subjects with opposed language and spatial laterality perform better in tests assessing verbal comprehension and perceptual organization. Next, relationships between GM volume and cognitive ability is explored for fluid and crystallised intellectual functioning using voxel-based morphology (Chapter 8). Significant differences in the GM correlates of fluid and crystallised intelligence were found between the handedness groups. Lastly, Chapter 9 explores the relationship between prefrontal cortex (PFC) volume and intentionality in left- and right-handers using stereological volume estimates from T1-weighted MR images. Although no significant difference in intentionality score was found between the handedness groups, higher scores of intentionality were associated with larger orbital PFC volume in right-handers, but with larger dorsal PFC volume in left-handers. This research extends the literature demonstrating differences in brain structure and function between left- and right-handers. Overall, the results suggest that individuals may achieve similar cognitive ability scores with different brain designs. Future research should consider the effect of group differences in the population and how this might influence brain ‘design’ and cognitive ability.
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