VERTICAL EYE POSITION CONTROL IN DARKNESS - ORBITAL POSITION AND BODYORIENTATION INTERACT TO MODULATE DRIFT VELOCITY

How stable is vertical eye-in-head position control in darkness when n o visual targets are present? We evaluated this while varying both bod y-in-space orientation and eye-in-orbit position in six subjects who w ere free from oculomotor/vestibular disease. Vertical eye movements we re monitored using a CCD-video tracking system, and results were confi rmed on one subject with the magnetic search coil, Three body orientat ions were used: (1) seated upright; (2) supine; and (3) prone, In each of these body orientations starting eye-in-orbit position was varied in quasi-random order from -20 to +20 deg, while vertical eye drift wa s monitored for a 90 sec period at each position. Subjects were instru cted to hold their eyes as steady as possible. The relationship betwee n body orientation/eye position and vertical eye drift velocity was ex amined using a linear regression technique. In contrast to prior clini cal reports, normals exhibit a vertical nystagmus/drift in darkness, M oreover, slow-phase eye velocity was found to be dependent on eye-in-o rbit position in the upright and supine body orientations, This patter n of eye drift mirrors Alexander's Law, with significantly increased d rift velocities when subjects looked in the direction of their re-cent ering saccades (P < 0.05 or better), Body-in-space orientation also mo dulated the eye drift velocity, with significant differences in rate o f eye drift (P < 0.05 or better) between extremes of body orientation (supine and prone) for five out of six subjects, The stability of the vertical oculomotor control system in the absence of visual input is s trongly affected by body-in-space orientation and eye-in-orbit positio n: manipulating either of these variables results in non-random patter ns of drift. These results are discussed using a multiple-input model of vertical eye-in-head position control. (C) 1997 Elsevier Science Lt d.