Is transition from failing to passing False Belief Task reflected in the social brain? Insights from an fNIRS study with preschoolers

The preschool years are crucial for developing theory of mind (ToM) - understanding others' mental states. During this period, children transition from failing to passing explicit False Belief Tasks (eFBT), but the neural mechanisms behind this shift remain unclear. While the adult ToM brain network is well-studied, neuroimaging of children under six is limited. Most studies treat eFBT as a single process, overlooking distinct neurocognitive mechanisms in forming mental state representations (belief formation phase) and applying them to complete tasks (explicit answer phase). To address this gap, we investigated brain activity using functional near-infrared spectroscopy (fNIRS) during eFBT completion in preschoolers. Results show that the key brain regions critical for mentalizing (temporo-parietal junction (TPJ) and inferior frontal gyrus (IFG) activate in children regardless of their eFBT performance. This pattern was observed during the belief formation phase, when the discrepancy between the child's belief and the protagonist's belief is introduced. In addition, greater activity in the right IFG was observed in Passers compared to Non Passers during the explicit answer phase, regardless of condition. This suggests that Passers exerted more attention, possibly becoming more effective at applying belief-related knowledge during response selection. Findings align with developmental continuity approach to ToM development.