HEAD POSITION-DEPENDENT ADJUSTMENT OF THE 3-DIMENSIONAL HUMAN VESTIBULOOCULAR REFLEX

We studied the influence of static head position relative to the rotat ion axis on the three-dimensional (3-D) vestibuloocular reflex (VOR). By means of a 3-D rotating chair and a 3-D magnetic search-coil system , eye movements were recorded in 6 normal human subjects during sinuso idal rotation about an earth-vertical axis (yaw) with the head positio ned at different pitch angles and about an earth-horizontal axis (pitc h) with the head positioned at different yaw angles. We confirmed that torsional VOR gain is about 2/3 of horizontal and vertical VOR gain l eading to non-collinear axes of eye and head rotation whenever torsion al head movement components were present with eye movement axes tiltin g away from the torsional direction. Despite this non-collinearity of stimulus and eye movement rotation axes there was close to perfect adj ustment of the different VOR components depending on the angle between gaze direction and stimulation axis, indicating a vectorial summation of gains obtained for rotations in the major body axes (roll, pitch a nd yaw). The horizontal and vertical eye movement components followed a cosine function of the angle between the plane of rotation and gaze direction. The torsional component followed a sine function.