Cognitive correlates of dyslexia, dyscalculia and comorbid dyslexia/dyscalculia: Effects of numerical magnitude processing and phonological processing.
Dyscalculia is tied to weak spatial skills, not to faulty number size judgment, so screen space reasoning when math problems appear.
01Research in Context
What this study did
Peters et al. (2020) compared three groups of kids: pure dyslexia, pure dyscalculia, and kids with both. They gave each child tests of number sense, phonics, and spatial skills.
The goal was to see which thinking skill, if any, is the real weak spot for each learning disorder.
What they found
Kids with dyscalculia did NOT struggle with judging number size. Instead, they scored low on spatial tasks like mental rotation.
Kids with dyslexia still showed the classic phonics gap. The comorbid group had both problems added together.
How this fits with other research
Schwenk et al. (2017) pooled many studies and found that symbolic number comparison speed is slow in math difficulties. Lien’s null magnitude result seems to clash, but Christin’s meta mixed many diagnoses while Lien tested only pure dyscalculia.
Carmen et al. (2011, 2013) reported clear magnitude deficits in kids with mild intellectual disability. The contradiction fades once you see that ID brings broad delays, while specific dyscalculia does not touch the basic number line.
Bonifacci et al. (2026) later showed that bilingual heritage speakers with dyscalculia keep good number sense, backing the idea that language, not magnitude, is the moving part.
Why it matters
Stop assuming every child who counts poorly has a broken number sense. Add a quick spatial puzzle to your math assessment battery. If the child rotates objects poorly but judges number size fine, target spatial language and visual models before drilling magnitude games. This small shift can save minutes per eval and send you to the right intervention path faster.
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02At a glance
03Original abstract
Specific learning disorders (i.e., dyscalculia and dyslexia) are common, as is their comorbidity. It has been suggested that the core cognitive deficit in dyscalculia is an impairment in numerical magnitude processing; similarly, in dyslexia, phonological processing deficits are considered to be the main cognitive deficit. Cognitive theories on comorbid dyslexia/dyscalculia have suggested a number of hypotheses about which cognitive deficits underlie the comorbidity. However, few studies have thus far directly compared the abovementioned cognitive correlates of dyscalculia and dyslexia. In this study, we assessed symbolic and non-symbolic numerical magnitude and three subcomponents of phonological processing (phonological awareness, lexical access and verbal short-term memory). In addition, we investigated children's domain-general spatial and verbal skills. The effect of these cognitive correlates on dyscalculia, dyslexia and their comorbidity was explored. We did not find differences between children with and without dyscalculia on numerical magnitude processing. On the other hand, children with dyscalculia had significantly lower spatial skills compared to children without dyscalculia. Children with dyslexia performed significantly lower on all subcomponents of phonological processing. Finally, we found an additive effect for comorbid dyslexia/dyscalculia: impairments in children with co-occurring dyslexia and dyscalculia were similar to the sum of the impairments in the isolated dyslexia and isolated dyscalculia groups. The strongest unique predictor of isolated dyscalculia and comorbid dyslexia/dyscalculia was spatial skills, the strongest unique predictor of isolated dyslexia was phonological awareness. As only a limited number of cognitive variables were assessed in this study and the sample sizes were very small, we should be cautious when interpreting these results.
Research in developmental disabilities, 2020 · doi:10.1016/j.ridd.2020.103806