Experiential Effects on the Neural Substrates of Visual Word and Number Processing.

Abstract

Visual processing of words and numbers is a uniquely human cognitive ability. Evidence suggests that a region in left occipitotemporal cortex, the so-called visual word form area (VWFA), is crucially involved in this ability, particularly in the visual recognition of words. In this dissertation, I present a methodological study and two empirical studies to investigate the role of experience in shaping the VWFA, to explore ways to estimate the amount of this experiential influence, and to examine how the neural substrates of visual number recognition are different from those of visual letter recognition.

In the first study, I develop a novel statistical method to efficiently estimate correlation between paired spatial processes, and hence heritability in patterns of activation in neuroimaging datasets. The results demonstrate that the proposed method provides a better estimate of correlation and heritability than conventional voxelwise or region of interest methods.

The second study applies this method in a monozygotic twin sample to explore the role of unique environmental effects in shaping VWFA activation. The results demonstrate that there are greater unique environmental effects for neural activity associated with familiar word recognition than with unfamiliar word recognition.

The last study investigates whether the VWFA is also the crucial site for visual number recognition or whether number recognition is neurally dissociable from word recognition. I demonstrate letter-selective activation in left occipitotemporal cortex and number-selective activation in right lateral occipital cortex, thus establishing double dissociation. Furthermore, I show that individual differences in the laterality of visual number activation can be explained by individual differences in the laterality of numerical processing activation in parietal cortex.

In sum, this dissertation investigates experiential effects on the neural substrates of visual word and number processing. In a methodological study, I present a more powerful statistical method to estimate correlation and heritability in neuroimaging datasets. The findings from the two empirical studies suggest a critical role of environment in shaping the VWFA, demonstrate a novel double dissociation between the neural substrates of letter and number recognition, and provide evidence that top-down influences give rise to the functional neural organization for visual number recognition.