Individual differences in brain systems supporting declarative and procedural memory during grammatical error processing in children.

Learning and memory rely on two interacting long-term memory systems: declarative memory (DM) and procedural memory (PM). The Declarative-Procedural (DP) model proposes that rule-based grammar is primarily supported by PM, but DM may also contribute-especially when tasks are explicit, unfamiliar, or when procedural representations are still undergoing consolidation. Although age-related differences in how these brain systems support grammatical processing have been identified, how this varies at the individual level among children has yet to be characterized. We used mixed-effects models of functional magnetic resonance imaging (fMRI) data to capture individual variability in brain activity among 7-year-old children in DM- and PM-linked regions during an auditory grammaticality judgment task. PM activity was selectively heightened for more difficult grammatical errors relative to correct sentences, whereas DM regions showed similarly elevated activation across conditions. Accuracy for identifying the most challenging errors was positively associated with both DM and PM activity, with comparable interindividual variability in association size across systems. These findings provide neuroscientific evidence for partially differentiated but similarly important contributions of DM and PM to grammatical error detection in childhood. They also highlight substantial individual variability in how long-term memory systems are recruited during explicit language tasks at early stages of formal language learning.