Developmental Changes in Nonsymbolic and Symbolic Fractions Processing: A Cross-Sectional fMRI study

Abstract

A substantial body of research has found that human and nonhuman animals are capable of processing the magnitudes of nonsymbolic ratios. Lewis, Mathews and Hubbard (2015) hypothesized that this ability may depend on a neurocognitive architecture called the ratio processing system (RPS). They further hypothesized 1) that the RPS might serve as a neurocognitive startup tool—an evolutionarily conserved cognitive architecture—and 2) that it can be recycled to support the acquisition of symbolic fractions knowledge. We tested these two predictions of the RPS account by comparing neural signatures of the RPS in 2nd-graders, who have not yet received formal symbolic fraction instruction, and 5th-graders, who have. During fMRI scanning, children performed ratio comparison tasks in which they determined which of two ratios or symbolic fractions was larger. Both cohorts showed behavioral and neural evidence of processing symbolic and nonsymbolic fractions magnitudes, with performance modulated by the numerical distance between stimuli. Consistent with our predictions, 2nd-grade children reliably recruited a right parietal-frontal network for nonsymbolic ratio comparisons but not symbolic fractions, and 5th-grade children recruited a bilateral parietal-frontal network for both nonsymbolic and symbolic fractions that overlapped with, but extended beyond, that found for 2nd-graders. These results present the first neuroimaging evidence that neural substrates for nonsymbolic ratios exist prior to formal learning and that this nonsymbolic foundation may be recycled to process symbolic fractions. These findings open the door for pedagogical strategies that focus on supporting this recycling process to improve students’ understanding of symbolic fractions.