Neural-Discursive Dynamics in Collaborative Spatial Problem Solving: Evidence from fNIRS Hyperscanning and Accountable Talk

High-quality collaboration cannot be inferred simply from how much students talk or how strongly their brain activity is synchronized during joint work. This study examined relations among accountable talk, inter-brain synchrony (IBS), and group task performance in fifth-grade students’ collaborative mathematical problem solving. Forty-nine students worked in 15 groups and completed two collaborative spatial design tasks, yielding 30 group × task records. Functional near-infrared spectroscopy (fNIRS) data and video recordings were collected simultaneously; discourse was coded using accountable talk indicators, and group design products were scored as task performance. Mixed-effects models showed that higher task performance was associated with more accountable talk turns, shorter mean turn duration, and higher accountable talk density, but not with longer total accountable talk duration. IBS showed channel-group-specific associations with task performance, suggesting that overall synchrony alone did not fully characterize collaborative activity. Temporal-window analyses showed that the within-window association between accountable talk density and IBS differed according to task performance level. These findings suggest that, within the present collaborative tasks, productive collaboration was associated with dense task-relevant accountable discourse and performance-dependent behavioral–neural associations rather than talk quantity or overall neural synchrony alone.