Independent Neural Control Networks for Human Vestibulo-Ocular Reflex

Background: Patients with vestibular hypofunction experience blurred images during rapid head rotation due to damage of the Vestibulo-Ocular Reflex (VOR). The healthy VOR ensures that images remain still on the fovea of the retina during rapid head rotation. It is known that the brain has the neural circuitry for ipsilesional VOR adaption.

Objective: Examine whether the pathways from each VOR (right vs left) can be concurrently modified in opposing directions

Methods: Nine healthy subjects were tested while seated upright, 1-meter in front of a wall in complete dark. Eye and head rotation was recorded with the EyeSeeCam system. Each subject made active (self-generated) head impulse rotations for 15 min while viewing a gradually increasing amount of retinal slip. VOR training demand changed by 10% every 90 seconds. Three separate VOR adaptation sessions (1 per day) were randomized such that the VOR was driven: Up-bilaterally; Down-bilaterally; or Mixed (one side up, opposite side down). During VOR training, the subject’s task was to fixate a visible laser target that either moved opposite the head direction (Up), with the head direction (Down) or only in one direction (Mixed).

Results: VOR changed significantly for all but passive test of Mix training condition. Significant correlation between post training gain values and training condition was found (Pearson’s r=0.663). Moreover, a significant linear regression model indicate prediction of 51% to our post-tests results. We found no significant difference in the magnitude of VOR gain change between Up to Mixed-Up and Down to Mixed-Down training.

Conclusions: The human VOR can be simultaneously driven in opposite directions. The similar magnitude of VOR gain changes across training conditions suggests functionally independent VOR circuits for each side of head rotation, that mediate simultaneous and opposing VOR adaptations. These data may have a significant clinical relevance in developing gaze stability training paradigms.