Excitatory delay-coupling explains in-phase and antiphase functional connectivity

Coordinated oscillatory activity between brain regions underpins cognition, yet the phase relationships governing this coordination remain poorly understood. Using scalp EEG from 31 participants performing a motor learning task, we show that inter-site phase clustering occurs predominantly at in-phase or antiphase relationships, with the transition between them governed by conduction delay. Homologous interhemispheric pairs remain in-phase despite long distances, consistent with faster callosal conduction. A minimal model of two delay-coupled excitatory populations reproduces these features without parameter tuning; stability analysis shows that in-phase and antiphase oscillations arise from competing instabilities, with delay determining which dominates. Task performance shifts a frontoparietal network toward in-phase connectivity, a modulation captured by the Phase Relationship Index (PRI) but missed by conventional clustering metrics. Preliminary evidence from independent datasets suggests these patterns generalise. These findings challenge the assumption that zero-lag connectivity reflects volume conduction, offer a mechanistic account linking conduction delay to phase organisation, and introduce PRI as a metric sensitive to functionally relevant connectivity changes invisible to existing measures.