Complementary frontoparietal and corticothalamic contributions to relational reasoning

Complex reasoning depends on flexible coordination among frontal, parietal, and thalamic systems, but the circuit mechanisms that support increasing relational demands remain unclear. We combined EEG with biologically grounded corticothalamic neural field modelling while participants solved relational problems of graded complexity. Successful reasoning was associated with dissociable frontoparietal dynamics. Frontal regions showed increased theta-band power, whereas parietal regions showed reduced alpha- and beta-band power. Theta-band phase synchronisation across frontoparietal-network nodes increased with problem complexity but was not associated with performance. By contrast, stronger beta-band synchronisation across the same network was associated with slower and less accurate responses as demands approached the highest complexity, suggesting that stronger coordination is not uniformly beneficial. Neural field modelling indicated that these regional spectral dynamics reflected specific complexity-dependent circuit adaptations. Parietal regions showed modulation of intracortical and corticothalamic gains, intrathalamic inhibition, prolonged loop delays, and faster synaptic filtering, whereas frontal regions primarily adjusted intracortical gains to maintain local excitatory-inhibitory balance and supported longer temporal integration windows. Together, these empirical and model-derived findings reveal complementary frontoparietal and corticothalamic mechanisms for relational reasoning.