In all patients, before surgery, vertical fusional vergence was assessed with our eye-tracking haploscope (frame rate ≈15 Hz, accuracy ≤ 0.358, precision ≤ 0.2°),
4,10,11 which we fabricated from an old Bausch and Lomb arc perimeter (Bausch and Lomb, Rochester, NY, USA) presenting a separate image to each eye of the patient. Concentric circle targets without torsional cues, subtending greater than 54 degrees of visual angle, were mounted on the two arms of the arc perimeter and viewed in two 45-degree mirrors at a near viewing distance of 33 cm. Tilting the arc perimeter arms introduced vertical disparity between the two targets. Two webcams (240 × 320 pixel resolution, equipped with close-up lenses
10), connected to a desktop computer, recorded horizontal, vertical, and torsional eye movements, binocularly, through the use of pupil-based and iris-crypt–based video-oculography
10 as well as horizontal, vertical, and torsional in-plane head movements by means of video-based monitoring of black adhesive dots with a white border placed near the patient's inner canthi,
11 all in near infrared light. Data were acquired and analyzed using custom MATLAB software (Mathworks, Inc., Natick, MA, USA), interfacing with commercial eye-tracking software IRIS (Chronos Vision, Berlin, Germany). For a more detailed description of our eye-tracking apparatus and method, please refer to Refs. 10 and 11.
Patients were examined with the targets aligned with no vertical disparity except when the patient complained of diplopia. In that case, the targets were realigned (using a lever to tilt the arc perimeter's arms, thus bringing one target upward and the other one downward), so that the patient could barely fuse them in straight-ahead gaze with the head upright. Eye movements were recorded during alternating binocular and monocular viewing sessions by covering and then uncovering one target or the other (haploscopic cover testing
4) while the patients were instructed to look straight ahead and attempt fusion when both targets were visible. Haploscopic cover testing was performed with head straight and when tilted 45 degrees to the left and right so as to obtain positions that could facilitate fusion and thereby ensure detection of measureable values for vertical vergence and accompanying torsional movements, from which we determined the fusion mechanism used.
More precisely, we analyzed the directions of simultaneous vertical and torsional movements of each eye to identify which pairs of muscles were primarily acting during fusion. For example, if the higher eye is moving downward and simultaneously intorting with vertical fusional vergence, the superior oblique muscle is the primary mover, but if it is moving downward and simultaneously extorting, it is the inferior rectus muscle that is the primary mover for that eye.
We also calculated a cyclovergence to vertical vergence ratio by computing the absolute value of the change in torsional deviation (the difference between the right and left eye torsional position) from before to after fusion divided by the change in vertical deviation from before to after fusion.
Zero reference positions for horizontal, vertical, and torsional tracings were determined at the beginning of each recording with the targets aligned and head upright, by covering each eye (target) in turn, and having the other viewing eye fixate on the center of the uncovered target pattern. An absolute zero torsional position could not be defined, but it was the relative changes in torsion that we were investigating in this study.