Vertical eye position-dependence of the human vestibuloocular reflex during passive and active yaw head rotations

The effect of vertical eye-in-head position on the compensatory eye rotatio n response to passive and active high acceleration yaw head rotations was e xamined in eight normal human subjects. The stimuli consisted of brief, low amplitude (15-25 degrees), high acceleration (4,000-6,000 degrees/s(2)) ya w head rotations with respect to the trunk (peak velocity was 150-350 degre es/s). Eye and head rotations were recorded in three-dimensional space usin g the magnetic search coil technique. The input-output kinematics of the th ree-dimensional vestibuloocular reflex (VOR) were assessed by finding the d ifference between the inverted eye velocity vector and the head velocity ve ctor (both referenced to a head-fixed coordinate system) as a time series. During passive head impulses, the head and eye velocity axes aligned well w ith each other for the first 47 ms after the onset of the stimulus, regardl ess of vertical eye-in-head position. After the initial 47-ms period, the d egree of alignment of the eye and head velocity axes was modulated by verti cal eye-in-head position. When fixation was on a target 20 degrees up, the eye and head velocity axes remained well aligned with each other. However, when fixation was on targets at 0 and 20 degrees down, the eye velocity axi s tilted forward relative to the head velocity axis. During active head imp ulses, the axis tilt became apparent within 5 ms of the onset of the stimul us. When fixation was on a target at 0 degrees, the velocity axes remained well aligned with each other. When fixation was on a target 20 degrees up, the eye velocity axis tilted backward, when fixation was on a target 200 do wn, the eye velocity axis tilted forward. The findings show that the VOR co mpensates very well for head motion in the early part of the response to un predictable high acceleration stimuli-the eye position-dependence of the VO R does not become apparent until 47 ms after the onset of the stimulus. In contrast, the response to active high acceleration stimuli shows eye positi on-dependence from within 5 ms of the onset of the stimulus. A model using a VOR-Listing's law compromise strategy did not accurately predict the patt erns observed in the data, raising questions about how the eye position-dep endence of the VOR is generated. We suggest, in view of recent findings, th at the phenomenon could arise due to the effects of fibromuscular pulleys o n the functional pulling directions of the rectus muscles.