A study by Karin Landerl et al. (2009) investigates whether dyslexia and dyscalculia—two common learning disorders—stem from shared or separate cognitive deficits.
Testing the domain-specific deficit hypothesis
The authors examined 8- to 10-year-old children across four groups: controls, dyslexia only, dyscalculia only, and comorbid dyslexia/dyscalculia. The goal was to assess whether phonological processing and number sense impairments are disorder-specific or shared across both.
Findings strongly support a domain-specific deficit model. Dyslexia was associated with phonological deficits, including poor phoneme deletion and slow rapid automatized naming (RAN). These difficulties were not observed in the dyscalculia-only group, indicating that phonological impairments are specific to dyslexia. Conversely, children with dyscalculia showed deficits in number processing, such as difficulty comparing numerical magnitudes and using number lines—tasks that were unaffected in the dyslexia-only group.
Comorbidity: Additive, not interactive
The comorbid group displayed both types of impairments, and these were additive rather than interactive. This suggests that the two disorders have independent cognitive origins and do not arise from a shared underlying deficit. Interestingly, short-term and working memory (ST/WM) deficits were most evident in the comorbid group, while the single-disorder groups did not differ significantly from controls in these areas. This may indicate that ST/WM issues exacerbate challenges when both disorders co-occur, though they are not core deficits.
The study used a variety of tasks to assess abilities, including phonological awareness tests (like phoneme deletion), memory spans (digit, nonword, and Corsi blocks), RAN tasks, symbolic and nonsymbolic magnitude comparisons, and number line estimation. In symbolic number tasks, dyscalculic children responded more slowly, particularly with two-digit numbers and on the number line task—suggesting a “fuzzy” or imprecise number representation. However, they still exhibited expected developmental effects (like the numerical distance effect), indicating partially intact processing systems.
Implications for diagnosis and intervention
Overall, the results reinforce the view that dyslexia is primarily a language-based disorder, while dyscalculia is grounded in numerical cognition impairments. The lack of significant overlap in core deficits highlights the importance of targeted interventions for each disorder. Comorbidity appears to be a result of the coexistence of two distinct problems rather than a single shared cause. These findings align with brain imaging evidence showing different activation patterns in each disorder and support the continued exploration of distinct neurocognitive pathways in learning disabilities.
