190304.qxd

Abstract

Abstract We recorded three-dimensional eye and head movements during circular, horizontal, vertical, and torsional head shaking in six human subjects with normal vestibular function. With circular head shaking, the stimulation of the canals by the termination of the head movement is similar to that following a step in velocity about the naso-occipital axis. A large torsional nystagmus with slow phase eye velocity of about 20°/s was observed upon cessation of circular head shaking. The three-dimensional eye movements expected from stimulation of the semicircular canals by the head-shaking maneuvers were calculated. The predicted activation of the canals was determined by projecting the head velocity (in head coordinates) into the canal planes and then processing the signal with the transfer function of the canals. The torsional eye velocity components predicted by the stimulation of the canals matched the recorded ones. We observed small horizontal eye velocities that could not be predicted by the stimulation of the canals alone. No eye movements were observed after the end of head shaking about a fixed horizontal or vertical axis. The eye velocities following the termination of head oscillations in the roll plane were small. The analysis methods developed for this study may be useful in the investigation of eye movements elicited by other types of three-dimensional head movements. Key words Eye movements · Active head movements · Three-dimensional · Semicircular canals · Vestibulo-ocular reflex Introduction Steps in angular head velocity about an earth-vertical axis evoke a horizontal nystagmus that declines in amplitude with a time constant determined by the mechanical properties of the semicircular canals and by velocity storage. A nystagmus with a similar time course, but oppositely directed, is noted at the termination of the rotation. These per-and postrotatory responses have been used to determine the dynamics of the horizontal vestibulo-ocular reflex (VOR) and to identify asymmetries in labyrinthine function. Such stimuli can be readily administered in the yaw plane with standard rotatory testing equipment. It has not been possible to produce comparable rotational stimuli in other planes with devices that are commonly available. Recent studies have extended the types of rotational stimuli employed in investigations of vestibular function through the use of active head movements Nystagmus following head shaking about a fixed horizontal or vertical axis is commonly seen in patients with unilateral vestibular hypofunctio