Ocular responses to head rotations during mirror viewing

The gain of the human vestibuloocular reflex (VOR) is influenced by the pro ximity of the object of regard. In six human subjects, we measured the eye rotations induced by passive, sinusoidal, horizontal head rotations at 2.0 Hz during binocular fixation of a stationary far target at 7 m; a stationar y target close to the subject's near point of fixation (<15 cm); and the br idge of the subject's own nose, viewed through a mirror positioned so that, for each subject, the angle of vergence was similar to that during viewing of the near target. The median gain of compensatory eye movements for the group of subjects during far viewing was 0.99 (range 0.80-1.04), during nea r viewing was 1.21 (range 0.88-1.47), and during mirror viewing was 0.85 (r ange 0.71-1.01). The gain during near and mirror viewing was significantly different for each subject (P<0.001) even though the vergence angles were s imilar. The lower gain values during mirror viewing can be attributed to th e geometric relationship between the head rotation, the position of the eye s in the head, and the movement of the subject's virtual image in the mirro r. To determine whether visually mediated eye movements were responsible fo r the observed gain values, we conducted a control experiment in which subj ects were rotated using a sum-of-sines stimulus that minimized the effects of predictive visual tracking; differences of gain values between near- and mirror-viewing conditions were similar to those during rotation at 2 Hz. W e conclude that, in these experiments, target proximity and vergence angle were not the key determinants of gain of the visuo-vestibular response duri ng head rotation while viewing a near target but that contextual cues from motion vision were more important in generating the appropriate response.