Nystagmus induced by circular head shaking in normal human subjects

We recorded three-dimensional eye and head movements during circular, horiz ontal, vertical, and torsional head shaking in six human subjects with norm al 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 nystag mus with slow phase eye velocity of about 20 degrees/s was observed upon ce ssation of circular head shaking. The three-dimensional eye movements expec ted from stimulation of the semicircular canals by the head-shaking maneuve rs were calculated. The predicted activation of the canals was determined b y projecting the head velocity (in head coordinates) into the canal planes and then processing the signal with the transfer function of the canals. Th e torsional eye velocity components predicted by the stimulation of the can als 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 horizon tal or vertical axis. The eye Velocities following the termination of head oscillations in the roll plane were small. The analysis methods developed f or this study may be useful in the investigation of eye movements elicited by other types of three-dimensional head movements.