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Departments of Ophthalmology Surgery (Division of Otolarlyngology) and Neurology, Bioengineering and Neuroscience Interdepartmental Programs, and the Jules Stein Eye Institute, University of California, Los Angeles, California 90095-7002, USA
Address for correspondence: Joseph L. Demer, M.D., Ph.D., Jules Stein Eye Institute, 100 Stein Plaza, UCLA, Los Angeles, CA 90095-7002. Voice: (310) 825-5931; fax (310) 206-7826. jld{at}ucla.edu
Magnetic resonance imaging (MRI) demonstrates that the lateral rectus pulley shifts 0.5 mm inferiorly relative to the medial rectus in 20° upgaze, but 0.5 mm superiorly in 20° 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°/s2 peak) in darkness while fixating straight ahead, as well as ± 18° 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° forward relative to the head axis, while that of the abducting eye (AB) tilted 0.6 ± 0.8° backward. In downgaze, the AD rotational axis tilted 8.6 ± 1.0° forward, and AB 5.7 ± 1.2° forward. In upgaze, the AD rotational axis tilted backward by 0.1 ± 0.7°, and AB backward 3.4 ± 0.9°. 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.
Key Words: extraocular muscles • eye movement • magnetic resonance imaging • pulley • vestibulo-ocular reflex
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