3-DIMENSIONAL ORIENTATION OF THE EYE ROTATION AXIS DURING THE PURKINJE SENSATION

The otolith-semicircular canal interaction during postrotatory nystagm us was studied in six normal human subjects by applying fast, short-la sting, passive head and body tilts (90 degrees in the roll or pitch pl ane) 2 s after sudden stop from a constant velocity rotation (100 degr ees/s) about the earth-vertical axis in yaw. Eye movements were measur ed with 3-D magnetic search coils. Following the head tilt, activity i n the semicircular canal primary afferents continues to reflect the po strotatory angular velocity vector in head-centered coordinates, where as otolith primary afferents signal a different orientation of the hea d relative to gravity. Pitch (roll) tilts away from upright during pos trotatory nystagmus after yaw rotation elicited a transient vertical ( torsional) VOR. Despite the change in head orientation relative to gra vity, postrotatory eye velocity decayed closely along the axis of semi circular canal stimulation (horizontal in head coordinates). These res ults suggest that postrotary nystagmus is largely organized in head-ce ntered rather than gravity-centered coordinates in humans as suggested by the Purkinje-sensation.