Effects of asymmetric vergence on compensatory eye movements during active head rotation

The kinematics of the horizontal VOR for near fixation demand that VOR gain should change dependent on the target distance and the orientation of the head with respect to the target, or, equivalently, the amount of ocular vergence and the asymmetry of this vergence. Across two experiments, the gain of the VOR was measured in the right eye of humans who rotated their heads to the right or left while viewing a target placed either 22, 32.5 or 200 cm from the center of head rotation, in conditions with and without visual feedback. When the eye was in-line with the target, the measured VOR gain was up to 43% greater than when the eye was in an eccentric position. However, in the eccentric position, higher VOR gains were achieved with visual feedback of the target than without feedback, indicative of a visual component in the compensatory eye movement. Also, by changing the posture of the left eye but keeping the right eye constant, the VOR gain in the right eye was changed substantially during a subsequent head rotation. Hence, the positions of both eyes in their orbits determine the gain of the VOR in each eye.