Functional anatomy of the extraocular muscles during vergence

Abstract

Magnetic resonance imaging (MRI) now enables precise visualization of the mechanical state of the living human orbit, enabling inferences about the effects of mechanical factors on ocular kinematics. We used 3-dimensional magnetic search coil recordings and MRI to investigate the mechanical state of the orbit during vergence in humans. Horizontal convergence of 23° from a remote to a near target aligned on one eye was geometrically ideal, and was associated with lens thickening and extorsion of the rectus pulley array of the aligned eye with superior oblique muscle relaxation and inferior oblique muscle contraction. There was no rectus muscle cocontraction. Subjective fusion through a 1° vertical prism caused a clockwise (CW) torsion in both eyes, as well as variable vertical and horizontal vergences that seldom corresponded to prism amount or direction. MRI under these conditions did not show consistent torsion of the rectus pulley array, but a complex pattern of changes in rectus extraocular muscle (EOM) crossections, consistent with co-contraction. Binocular fusion during vergence is accomplished by complex, 3D eye rotations seldom achieving binocular retinal correspondence. Vergence eye movements are sometimes associated with changes in rectus EOM pulling directions, and may sometimes be associated with co-contraction. Thus, extraretinal information about eye position would appear necessary to interpret binocular correspondence, and to avoid diplopia.

MRI study of the changes in crystalline lens shape with accommodation and aging in humans

“…However, MRI is a relatively time-consuming technique and is of lower resolution compared to optical and ultrasound techniques. MRI has been used previously for ocular imaging to study overall eye shape, extraocular muscle anatomy, crystalline lens shape, refractive index distribution, and ciliary muscle anatomy (Atchison et al, 2008;Atchison & Smith, 2004;Demer et al, 2008;Jones, Atchison, & Pope, 2007;Strenk, Strenk, & Semmlow, 2000;Strenk, Strenk, Semmlow, & DeMarco, 2004). We have previously described the dependence of refractive index distribution of the crystalline lens on accommodation and age using MRI (Jones et al, 2007;Kasthurirangan et al, 2008).…”

A 1-Diopter Vertical Prism Induces a Decrease of Head Rotation: A Pilot Investigation

“…This prism causes binocular disparity, providing a visual signal to the CNS that could stimulate compensatory eye movements, and thus modifying the extra oculomotor muscle proprioception. The eye with a 2-diopter prism is moving in the direction of the prism and the opposite eye (i.e., without prism) also moves in the opposite direction before stabilizing ( 27 , 41 ). In such case, in addition to proprioceptive signals of extraocular muscles, there are central efferent signals of motor commands sent to the occulomotor system.…”

Functional anatomy of human extraocular muscles during fusional divergence

“…It has recently become possible to observe the functional states of EOMs using magnetic resonance imaging (MRI) to demonstrate posterior shifts in their cross sections (Miller 1989) and increases in their volumes (Clark and Demer 2016a;Demer et al 2008) during contraction. Comparisons of several MRI indexes of EOM function have suggested posterior partial volume (PPV), the volume in four contiguous 2-mm-thick image planes in the midorbit, as the most robust measure since changes in PPV of the horizontal rectus EOMs accounted for 95% of variance in horizontal duction (Clark and Demer 2016a), and also correlate closely in cyclovertical EOMs during vertical duction (Clark and Demer 2016b).…”