Understanding dissociations in dyscalculia - A brain imaging study of the impact of number size on the cerebral networks for exact and approximate calculation

Neuropsychological studies have revealed different subtypes of dyscalculia, including dissociations between exact calculation and approximation abilit ies, and an impact of number size on performance. To understand the origins of these effects, we measured cerebral activity with functional MRI at 3 T esla and event-related potentials while healthy volunteers performed exact and approximate calculation tasks with small and large numbers. Bilateral i ntraparietal, precentral, dorsolateral and superior prefrontal regions show ed greater activation during approximation, while the left inferior prefron tal cortex and the bilateral angular regions were more activated during exa ct calculation. Increasing number size during exact calculation led to incr eased activation in the same bilateral intraparietal regions as during appr oximation, as well the left inferior and superior frontal gyri, Event-relat ed potentials gave access to the temporal dynamics of calculation processes , showing that effects of task and of number size could be found as early a s 200-300 ms following problem presentation. Altogether, the results reveal two cerebral networks for number processing. Rote arithmetic operations wi th small numbers have a greater reliance on left-lateralized regions, presu mably encoding numbers in verbal format, Approximation and exact calculatio n with large numbers, however, put heavier emphasis on the left and right p arietal cortices, which may encode numbers in a nonverbal quantity format, Subtypes of dyscalculia can be explained by lesions disproportionately affe cting only one of these networks.