Magnitude Processing in Developmental Dyscalculia : A Heterogeneous Learning Disability with Different Cognitive Profiles

Developmental dyscalculia (DD) is a learning disability that is characterized by severe difficulties with acquiring age-appropriate mathematical skills that cannot be attributed to insufficient education, language skills, or motivation. The prevalence rate is estimated at 3-6%, meaning that a substantial portion of the population struggles to learn mathematics to such a large degree that it affects overall well-being and academic prospects. However, our understanding of the etiology of DD is incomplete and there are competing hypotheses regarding the characteristics of DD and its underlying causal factors. The purpose of the current thesis is to contribute to our understanding of DD from the perspective of cognitive psychology and cognitive neuroscience. To this end, we identify children with DD to identify the cognitive determinants of DD that hamper their ability to learn basic mathematics. It is believed that human beings are endowed with an innate ability to represent numerosities, an ability phylogenetically shared with other species. We investigate whether the purported innate number system plays a role in children with DD insofar as failures in this system may undermine the acquisition of symbolic representations of number. Although some researchers believe DD is a monolithic learning disability that is genetic and neurobiological in origin, the empirical support for various hypotheses suggests that DD may be shaped by heterogeneous characteristics and underlying causes. The present thesis, and the studies presented therein, provides support for the notion that DD is indeed heterogeneous. We identify at least two subtypes of DD that are characterized by specific deficits in number processing, and one subtype that could more aptly be labelled as a mathematical learning disability, the causal factors of which are likely limited to deficits in non-numerical abilities. In addition, we locate candidate neurocognitive correlates that may be dysfunctional in DD.