A-C, the environment of virtual reality; D-F, virtual reality test in exotropic patients; G-I, virtual reality test in esotropic patients. (A) In orthotropic status, the targets of both eyes are superimposed without virtual prism effect. (B) In exotropic status, the target of the exotropic eye (left eye in this figure) is moved temporally to simulate a virtual prism base-in (BI) effect. (C) In esotropic status, the target of the esotropic eye (left eye in this figure) is moved nasally to simulate a virtual prism base-out (BO) effect. In B and C, the virtual distance between 2 targets is d (cm), and the distance between the eye and the virtual targets is 6 m. Thus, the virtual prism effect is d/6 (PD). The screens that patients see on the virtual reality device with each eye (D, G), the movement of the eyes (E, H), and the real-time result by the eye- tracking system (F, I) are demonstrated separately. The blackening of one screen simulating occlusion in the cover test is presented with gray background in Figs. E, F, H and I (left eye in these examples). The dotted circle is the position of the eye before occlusion, and the solid circle is in the position after occlusion. (D-F) In exotropic patients, when the screen of the left eye is blackened, the right eye moved nasally to track the target. A leftward movement of both eyes was able to be detected by the eye-tracking system. Next, a virtual prism base-in effect as shown in Fig. B was introduced to move the target of the left eye temporally. The operator then again blackened the screens until both eyes remained still. The final amount of introduced virtual prism base-in effect was then calculated to the angle of the ocular deviation (virtual prism effect d/6 (PD) as shown in Fig. B). (G-I) In esotropic patients, when the screen of the left eye is blackened, the right eye moved temporally to track the target. A rightward movement of both eyes was able to be detected by the eye- tracking system. Next, a virtual prism base-out effect as shown in Fig. C was introduced to move the target of the left eye nasally. The operator then again blackened the screens until both eyes remained still. The final amount of introduced virtual prism base-in effect was then calculated to the angle of the ocular deviation (virtual prism effect d/6 (PD) as shown in Fig. C). BI, base-in; BO, base-out; XT, exotropia; ET, esotropia; PD, prism diopter; m, meters
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