Ocular kinematics, vergence, and orbital mechanics

Abstract

Important aspects of ocular kinematics relate to the geometric configuration of extraocular muscles (EOMs). The orbital layer of each rectus EOM inserts on a connective tissue ring called a pulley that deflects the EOM path. Global layer fibers of each EOM pass through the pulley to insert on the sclera. The orbital layer thus controls linear translation of the pulley, regulating the EOM’s pulling direction, while the global layer rotates the eye. The active pulley hypothesis (APH) states that pulleys are actively positioned to regulate ocular kinematics. The coordinated control postulate of the APH proposes that during conjugate visually guided eye movements, rectus pulleys move the same anteroposterior distance as their insertions, but the inferior oblique pulley moves with vertical gaze by half the amount as the inferior rectus insertion. These motions, observable by magnetic resonance imaging (MRI), shift the pulling directions of these EOMs by half the ocular angle, mechanically implementing a ‘linear oculomotor plant’ appearing mathematically commutative to the brain and consistent with Listing’s Law of ocular torsion. In the non-converged state with the head upright and stationary, rectus pulleys move little transverse to the EOM axes. During convergence and during the static torsional vestibulo-ocular reflex, MRI shows that the rectus pulley array rotates around the line of sight. Oblique EOM orbital layers may implement this shift.