Beyond the sum of their parts: a multi-dimensional approach to dyscalculia-dyslexia comorbidity integrating studies of the brain, behavior, and genetics

Dyscalculia, often referred to as mathematical learning difficulties (MD), and Dyslexia, often referred to as reading learning difficulties (RD), are specific learning disorders with independent prevalence rates estimated to be 5-10% of school-age children. However, their comorbidity (MDRD) is highly prevalent, with around 40% of children with MD also experiencing RD. Despite frequent co-occurrence, MDRD receives less attention than isolated MD and RD. As a result, the neurocognitive mechanisms of MDRD remain unclear. In the literature, three main models of MDRD have been drawn: (1) additive model, which proposes that MDRD arises from the addition of neurocognitive deficits unique to MD or RD, (2) domain-general deficit model, which proposes that MDRD stems from executive function deficits, and (3) increased-risk model, which proposes that MDRD emerges from deficits in phonological processing characteristic of RD. We review behavioral, neuroimaging, and genetic investigations of MDRD, discussing each of these models. Further, we recommend new research directions considering MDRD as a part of multiple dimensions of mathematics, reading, and domain-general skills, and highlight the importance of a holistic, functional diagnosis. A better understanding of the neurocognitive foundations of MDRD can enhance diagnosis sensitivity and specificity and facilitate tailored interventions for children with MDRD.