What causes physiologic nystagmus?
Normally the eyes remain in the position of primary gaze, even when a person is not deliberatedly focusing on any visual target directly ahead, because of balanced input from the left-and-right vestibular systems, which are finely tuned to respond to any movement of the head relative to the body. If a person turns his head to the side without making any deliberate attempt to fixate on any visual target, the eyes will remain centered in the neutral position of primary gaze as a result of the vestibulo-ocular reflex (a reflex mediated via the vestibular system), which ensures that the eyes respond promptly to any movement of the head relative to the body in order to keep the eyes centered in the socket. If that person spins around in a circle (like a ballerina), theoretically the eyes should follow the head at the same speed, so that an observer will notice that eyes remain centered in the head as the person swivels around. However, if that person spins too fast, vertigo will ensue if th
