Head direction cells use a head-referenced dual-axis updating rule in 3D space
Head direction (HD) cell signalling may support horizontal orientation-encoding during 3D movement by using a "dual axis" (DA) rule that summates head rotation around a local axis and local-axis rotation around the gravity axis. Experimentally, we observed that HD cell firing conformed best to a DA rule with a head-referenced (not environment-referenced) local axis. Standard HD cell models thus need an additional rotational input referenced to head tilt.