Effects of problem size and arithmetic operation on brain activation during calculation in children with varying levels of arithmetical fluency

► Children with low arithmetical fluency show atypical activation in the right IPS. ► A typical IPS activity represents a neural correlate of fact retrieval difficulties. ► Problem size and arithmetic operation modulate brain activity during calculation. ► The left hippocampus is active during fact retrieval (addition and small problems). ► Large problems and subtractions activate a fronto-parietal network.