In addition, the inner ear contains rotational accelerometers, known as the semicircular canals, which provide information to the lower brain on rotational accelerations in the pitch, roll and yaw axes. Changes in angular velocity are detected from the relative motion between the fluid in the canals and the canal itself, which is fixed to the head; because of inertia, the fluid in the canals tends to lag when the head moves, signaling a rotational acceleration. However, semicircular canal output ceases after prolonged rotation (beyond ) as the fluid has now been entrained into motion through friction, matching the motion of the head. If the rotation is then stopped, the perceived motion signal from the inner ear indicates the aviator is now turning in the opposite direction from actual travel, as the fluid continues to move while the canal has stopped. In addition, the inertia of the fluid means the detection threshold of rotational acceleration is limited to approximately 2°/sec; angular accelerations below this value cannot be detected. Specific common somatogyral illusions induced by the semicircular canals are the Leans, Graveyard Spin, Graveyard Spiral, and Coriolis.