 Gaze Stability Test: A custom computer application was developed for this purpose. This consists of tracking a bright visual target moving horizontally left and right on a computer display at fixed amplitude 80 of the monitor width at a frequency of 0.4 Hz. A 70 m monitor was used, positioned at eye-level, 100 cm away from the participant. This results in head horizontal rotations of between 30 to 40 degrees, left and right of center. The test procedure involves a closed loop tracking task with respect to head. In this task two cursors of different colors appear on the monitor. One is the target cursor and the 2nd cursor is slaved to head rotation via the head mounted motion mouse. The task goal is to overlap the two cursors during the cyclic left-to-right cursor motion. In this task foveation is necessary to determine the amount of overlap error between the target cursor and the head cursor. 