Whole number interference in rational number processing: Neuroimaging evidence from decimal comparison

Relative to fractions, decimal numbers are thought to be easier for students to learn because they employ the base-10 system as whole numbers. However, unlike whole numbers, larger decimals can have fewer digits, leading to worse performance when comparing Inconsistent decimal pairs, like 0.8 vs. 0.26, than Consistent pairs like 0.86 vs. 0.2. Students may be applying the whole number rule: “more digits = larger number” or they could be ignoring the decimal points and comparing 8 vs. 26. This study used neuroimaging and our specially designed stimulus set to distinguish between these possibilities. We focused on the intraparietal sulcus (IPS), implicated in numerical magnitude processing, and the anterior cingulate cortex (ACC) and insula, implicated in inhibitory control. We found no neural differences between Consistent and Inconsistent comparisons, suggesting that the number of digits does not drive brain responses in skilled adults (n=21). Instead, for Consistent comparisons, we found that the IPS was sensitive to the actual distance between the decimals, while the ACC showed this pattern for Inconsistent comparisons. Crucially, we also examined the effect of the distance between the decimal pairs when ignoring the decimal point. Here, we found sensitivity to this distance among Inconsistent comparisons in the IPS and insula, suggesting that whole number referents are automatically processed during decimal comparison and require engagement of cognitive control regions to counteract. More broadly, our results underscore the unique challenges of decimal notation, revealing the need for educational practices that emphasize differences to whole numbers rather than highlighting similarities.