Why does vestibular stimulation stop when a person moves at a constant speed?

The vestibular system, located in the inner ear, is designed to detect changes in motion—specifically acceleration—rather than constant velocity.

The system relies on two primary structures to sense movement:

• Semicircular canals: These function as rotary accelerometers. When you begin to rotate, the inertia of the endolymph fluid causes it to lag behind, which deflects the cupula and stimulates the hair cells. Once you reach a constant speed, the fluid eventually moves at the same rate as the canals, the cupula returns to its resting position, and stimulation ceases.12
• Otolith organs: The utricle and saccule act as linear accelerometers. They contain dense otoliths that deflect hair cells during acceleration or tilting. At a constant velocity, these structures become stable, and vestibular stimulation stops.13

Because the system is sensitive to changes in speed, a person moving at a constant velocity must rely on other cues, such as visual observation of the passing scene, to perceive their motion.1