MODULATION OF HIGH-FREQUENCY VESTIBULOOCULAR REFLEX DURING VISUAL TRACKING IN HUMANS

1. Humans may visually track a moving object either when they are stat ionary or in motion. To investigate visual-vestibular interaction duri ng both conditions, we compared horizontal smooth pursuit (SP) and act ive combined eye-head tracking (CEHT) of a target moving sinusoidally at 0.4 Hz in four normal subjects while the subjects were either stati onary or vibrated in yaw at 2.8 Hz. We also measured the visually enha nced vestibuloocular reflex (VVOR) during vibration in yaw at 2.8 Hz o ver a peak head velocity range of 5-40 degrees/s. 2. We found that the gain of the VVOR at 2.8 Hz increased in all four subjects as peak hea d Velocity increased (P < 0.001), with minimal phase changes, such tha t mean retinal image slip was held below 5 degrees/s. However, no corr esponding modulation in vestibuloocular reflex gain occurred with incr easing peak head velocity during a control condition when subjects wer e rotated in darkness. 3. During both horizontal SP and CEHT, tracking gains were similar, and the mean slip speed of the target's image on the retina was held below 5.5 degrees/s whether subjects were stationa ry or being vibrated at 2.8 Hz. During both horizontal SP and CEHT of target motion at 0.4 Hz, while subjects were vibrated in yaw, VVOR gai n for the 2.8-Hz head rotations was similar to or higher than that ach ieved during fixation of a stationary target. This is in contrast to t he decrease of VVOR gain that is reported while stationary subjects pe rform CEHT. 4. In a control experiment in which subjects carried out v ertical SP and CEHT while they were vibrated in yaw at 2.8 Hz, we foun d that three of four subjects showed an increase in horizontal VVOR ga in at 2.8 Hz compared with that achieved during fixation of a stationa ry target; such an increased horizontal gain would not be required to reduce retinal image slip in the vertical plane. 5. On the basis of th ese findings, we draw the following conclusions. I) During sinusoidal oscillations at 2.8 Hz, the gain of the VVOR is adjusted in accordance with peak head velocity in order to hold retinal slip of the image of the visual target below similar to 5 degrees/s. 2) During visual trac king of a moving target while the subject is in motion, there are two potential sources of retinal image slip: imperfect Visual tracking and an inadequate VVOR. When tracking deteriorates, it becomes necessary to increase the gain of the VVOR to levels that prevent additional ret inal image slip, so that vision is not compromised. 3) The increase of horizontal VVOR gain that occurs during both horizontal and vertical visual tracking while subjects are in motion may not be wholly due to retinal slip per se, but may also involve a nonvisual mechanism that e ffectively constrains retinal image slip to levels that permit clear v ision.