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Monocular and Binocular Constraints on Eye Orientation
Published in Michael Fetter, Thomas Haslwanter, Hubert Misslisch, Douglas Tweed, Three-Dimensional Kinematics of Eye, Head and Limb Movements, 2020
A.V. van den Berg, P. Bruno, J.Tj.H.N. de Faber
Cyclovergence, or the difference between the torsional orientations of the eyes, depends on the head-centric distance and the elevation of the binocularly viewed target. This constraint on cyclovergence resembles the restriction that holds for the torsional position of a single eye which, according to Listing’s law, depends on the head-centric direction of the target. How do these constraints depend on binocular vision? We investigated this question by comparing cyclovergence for monocular and binocular viewing of nearby targets by normal subjects,by comparing the orientation of the eye of a monocular subject for fixation of distant and nearby targets andby an analysis of the eye orientation in subjects with intermittent exotropia.
Diplopia after Strabismus Surgery
Published in Seminars in Ophthalmology, 2018
Torsional diplopia can be a particularly disabling complication in patients who had normal binocular function prior to strabismus surgery. The new torsional disparity generally has to be quite severe to create symptoms, as most normal individuals have a robust torsional fusion capacity. On average, normal subjects tested with large-field stimuli can generate eight degrees of cyclovergence, and they can further generate an additional eight degrees of sensory cyclofusion. Thus, up to 16 degrees of surgically induced cyclodisparity can generally be overcome with normal cyclofusional adaptations.7 When the amount of induced torsion approaches or exceeds this amount, torsional diplopia results. When torsional disparity is the source of diplopia, it is necessary to perform torsional surgery; however, it is not necessary to perform extreme procedures in an effort to reduce torsional disparity to zero. If the postoperative difference is reduced to within the range of cyclofusional and sensory adaptation, the patient’s torsional diplopia should be eliminated.
Albrecht Nagel on vision with two eyes
Published in Strabismus, 2020
Ocular torsion was an issue of considerable contention in nineteenth century visual science. Speculations that the oblique muscles could rotate the eyes around the z-axis were frequently stated. For example, Bell15 wrote: “By dissection and experiment it can be proved, that the oblique muscles are antagonists to each other, and that they roll the eye in opposite directions, the superior oblique directing the pupil downwards and outwards, and the inferior oblique directing it upwards and inwards”15(p.174). However, evidence supporting it in humans was hard to find and when it was presented it was usually contended.16 The possibility of torsion in opposite directions seemed fanciful and yet this is what Nagel proposed in order to maintain cyclofusion for lines inclined in opposite directions relative to the horizontal. Similar rotations about the vertical resulted in a depth effect with no cyclovergence. As Nagel noted, achieving cyclofusion with single lines required a lot of practice and patience. He returned to the issue later17-19 and showed that the cyclofusion could be produced more readily with arrays of lines. It was largely as a consequence of this demonstration that Hering20,21 became convinced that cyclovergence occurred. The involvement of cyclovergence remained hotly debated until photographic recording of eye movements verified it and Crone and Everhard-Halm22 recommended that large stimuli should be used in order to measure it. Readers can observe this for themselves with Figure 2; cyclofusion can be experienced using a simple stereoscopic display with the aid of red/cyan glasses or they can experience it with free-fusion in the manner of Nagel’s observations. The arrays of lines inclined relative to horizontal in the monocular views will appear horizontal with two eyes.