Landerl, K.
Although deficits in reading, spelling, and arithmetic can occur in isolation (i.e. in the context of otherwise typical development), recent prevalence studies clearly show co-morbidity between these problems. Nevertheless, dyslexia and dyscalculia are associated with distinct neuro-cognitive profiles (dyslexia: phonological deficit; dyscalculia: numerical processing deficit), with additive deficits in children with comorbid dyslexia + dyscalculia. Interestingly, recent empirical evidence suggests that deficits in reading or spelling can dissociate. Many children show specifically poor spelling in combination with age-adequate reading skills or specifically poor reading in combination with age-adequate spelling. Early deficits in phonological awareness are observed in children who later on develop problems with orthographic spelling, however, they are not evident among isolated poor readers. Dysfluent reading in the absence of spelling problems is probably best explained by deficits in the fast access from visual symbols to phonological representations (indicated by a marked and persistent deficit in naming speed). Theories of learning disorders need to explain associations as well as dissociations between disorders.