Rebound Relays and Inhibitory Vetoes Stabilize Sparse Sequential Activity in HVC

Brains build behavior by chaining actions and perceptions into precisely timed sequences, an ability central to speech, skilled movement, and memory, yet the circuit logic that propagates sequences remains unclear. Songbird HVC captures this problem: premotor HVCRA neurons burst once per motif, while basal ganglia-projecting HVCX neurons burst 2-4 times across the same motif. We developed a biophysically grounded HVC network as linked microcircuits encoding sub-syllabic segments. The model highlights inhibition not merely as suppressive but actively structuring sequence propagation and fidelity. Tonic inhibitory epochs prime HVCX by deinactivating T-type Ca2+ channels and recruiting Ih, so that release elicits precisely timed rebound bursts that recruit the next HVCRA ensemble. A complementary phasic inhibitory veto suppresses off-time activation, preventing pathological restarts while preserving HVCRA single-burst sparseness. More broadly, inhibitory timing can serve as the brains internal "clocked handshake," converting suppression into forward drive to advance sequences while enforcing error-corrected precision.