Human angular vestibulo-ocular reflex axis disconjugacy: relationship to magnetic resonance imaging evidence of globe...

Magnetic resonance imaging (MRI) demonstrates that the lateral rectus pulley shifts 0.5 mm inferiorly relative to the medial rectus in 20 degrees upgaze, but 0.5 mm superiorly in 20 degrees downgaze, whereas the globe translates 0.7 mm nasally in adduction and 0.2 mm nasally in abduction. If pulleys influence ocular kinematics, these effects would predict disconjugate alterations of the yaw vestibulo-ocular reflex (VOR) rotational axes. Binocular eye and head movements were recorded using three-dimensional search coils in 8 humans (age 24 +/- 2 years, mean +/- SE) undergoing directionally randomized, transient, whole-body yaw (2800 degrees /s2 peak) in darkness while fixating straight ahead, as well as +/- 18 degrees vertically. Eye and head rotational velocity axes were expressed as quaternions in Listing coordinates. In the initial 70 ms, the ocular axis varied with vertical gaze by one-quarter the angle of target elevation, but this effect summed significantly with a disconjugate effect of horizontal duction. In central gaze, the mean adducting eye (AD) rotational axis tilted 3.4 +/- 0.8 degrees forward relative to the head axis, while that of the abducting eye (AB) tilted 0.6 +/- 0.8 degrees backward. In downgaze, the AD rotational axis tilted 8.6 +/- 1.0 degrees forward, and AB 5.7 +/- 1.2 degrees forward. In upgaze, the AD rotational axis tilted backward by 0.1 +/- 0.7 degrees, and AB backward 3.4 +/- 0.9 degrees. We suggest that nasal globe translation relative to the fixed trochlea produces binocular extorsion accounting for yaw VOR axis disconjugacy, and thus a horizontal duction dependence in VOR rotational axis summating with classic dependence of VOR axis on vertical gaze. Confirmation of predicted duction-dependent VOR disconjugacy supports the idea that rectus pulleys influence kinematics for all eye movements.