Ipsitorsional Quick Phases with Head Tilt in a Normal Subject

This is a demonstration of ocular counterroll, which can be seen when the head is tilted to the right or to the left. For example, when the head is tilted to the right, the top poles of both eyes should rotate toward the left ear to keep the top poles oriented with earth vertical. This is part of the physiologic ocular tilt reaction. However, if the head is tilted far enough, ipsitorsional quick phases will be seen, which are generated by the ipsilateral rostral interstitial medial longitudinal fasciculus (riMLF). These quick phases, which are seen in normal patients during the roll VOR (tilting the head), are lost when a patient suffers injury to the riMLF. For example, if the patient has a stroke that involves the right riMLF, when the head is tilted toward the right, these ipsitorsional (rightward) quick phases cannot be generated. This is because the riMLF does not only contain the vertical burst neurons, but also the neurons responsible for generating torsional quick phases.