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

Mathematics learning disorders (MD) and reading learning disorders (RD) are persistent conditions that interfere with success in academic and daily-life tasks, and cannot be attributed to intellectual disabilities, sensory deficits, or environmental factors. Prevalence rates of MD and RD are estimated at 5-10 % of school-age children, and their comorbidity (MDRD) is highly prevalent, with around 40 % of children with MD also experiencing RD. Despite this high comorbidity rate, research on MDRD has received less attention compared to isolated conditions, leaving its neurocognitive mechanisms unclear. In this study, we review behavioral, neuroimaging, and genetic MDRD research and discuss how they support current MDRD models, including the: (1) additive model, which proposes that MDRD results from the addition of neurocognitive deficits unique to MD or RD, (2) domain-general deficits model, which proposes that MDRD stems from executive function deficits, and (3) increased risk model, which proposes that MDRD emerges from phonological deficits characteristic of RD. Further, we recommend updating models of MDRD by integrating the multiple deficit and dimensional models to build a unified framework for research and diagnosis that considers multiple dimensions of mathematics, reading, and domain-general skills. This unified framework highlights the importance of a holistic, functional diagnosis.