UC Berkeley

For disparate brain regions to communicate efficiently action potentials must be transmitted relatively long distances along myelinated axons, which make up the brain's "white matter". These connections are fundamental for the generation of complex cognition. A single cortical region is not sufficient--the human brain must work as a network, with the billions of neurons interacting together to perform the necessary computations. In my graduate work, presented in this dissertation, I sought to explain the variability in cognitive behavior with individual differences in white matter microstructure. The overarching goal was to further our understanding of the mechanisms through which structural connections in the brain underlie complex cognition in children and adults.

I used diffusion tensor imaging, a type of magnetic resonance imaging that is sensitive to white matter microstructure, to quantify in vivo the maturation of white matter in children and adults. I first motivate the need to study the individual differences in white matter integrity and their predictive value on cognitive behavior in chapter one. I then describe four experiments that investigate various aspects of this connection. In chapter two, I first describe the age related changes in brain and behavior through childhood and adolescence, and then use longitudinal data to highlight the additional explanatory power of how within-person measures of change (in brain and cognitive ability) over time are linked. In chapter three, I demonstrate the mediation of the relationship between white matter integrity (WMI) and reasoning ability by cognitive processing speed in typically developing children.

In chapter four, I show the WMI differences between two groups of adults who have experienced different perceptual environments through development (synesthetes and non-synesthetes) and demonstrate a correlation between vividness of visual imagery and the WMI of the synesthetic brain. Finally, in chapter five, I explain the effects of short term cognitive training on WMI . After only three months of undergoing an intensive reasoning training program we saw evidence of cellular reorganization in the white matter of healthy young adults. I conclude by summarizing the major contributions of the work in this dissertation and discuss possible future directions based on the results of my graduate research.