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which include motor and sensory fusion. The third degree and highest level of binocular visual function is stereopsis—binocular, three-dimensional depth perception resulting from the neural processing of horizontal binocular disparities (figure 2). However, stereopsis is not the only way to obtain depth information; even after closing one eye, we can still determine the relative positions of objects around us and estimate our spatial relationships with them. The clues that permit the interpretation of depth with one eye alone are called monocular clues. They include pictorial clues, such as the size of the retinal image, linear perspective, texture gradients, aerial perspective, and shading, as well as non-stereoscopic clues, such as accommodation of the crystalline lens, motion parallax, and structure from motion[62].
Figure 2. The classical model of binocular visual function is composed of three hierarchical degrees.
4. Binocular Viewing Conditions on Pupil Near Responses
Here, the effect of binocular clues on near pupil response as our preliminary research is introduced. When changing visual fixation from a distant to a close object, accommodation, convergence, and pupil constriction occur, three feedback responses those constitute the near reflex[42]. We investigated the amplitudes of vergence eye movements associated with pupil near responses for subjects of prepresbyopia and presbyopia under binocular and monocular viewing conditions in dynamics of step change in real target position from far to near (figure 3). The
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findings of these experiments were that the convergence response with pupil miosis was induced in all cases under binocular viewing conditions (figure 4A,C), whereas only presbyopic subjects showed version eye movement without pupil constriction under monocular conditions (figure 4D).
Our findings imply that accommodation, which is high in younger subjects, but becomes progressively restricted with age, is a most important factor in the induction of the pupil near response. However, the results of presbyopia subjects under binocular conditions suggested that binocular visual function such as fusion of the real target, depth perception, and proximity induces pupil constriction in presbyopia resulting from the inability to accommodate[27,31]. When both eyes are oriented toward a target, a fused perception of the target is formed, and through the processing of retinal disparity, depth perception can be achieved. As object distances from the plane of fixation increase, retinal image disparities become large and an object appears to be in two separate directions i.e., viewing a nearby target binocularly yields proximal and disparity clues[20,43].
Consequently, in young subjects, accommodation is active, thus, the pupil near response with convergence by blur-driven is well induced despite the monocular viewing condition. On the other hand, in presbyopic subjects, since the change in real target position was performed in real space and binocular viewing conditions, proximity and disparity clues were all available and were in conjunction with each other[47].
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Figure 3. We measure and record the dynamics of pupil and convergence simultaneously with the step stimuli of a real target in real space.
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Figure 4. Measured data of binocular viewing conditions. The upper trace is from a young subject (A), and the lower, from a subject with presbyopia (B). The young subject’s typical results under monocular (non-dominant eye occluded) visual conditions (C). Typical trace of a subject with presbyopia showed conjugate eye movement without pupil constriction
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Figure 5. Suppression and retinal correspondence in strabismus with esodeviataion. (A) Normal subject; (B) Strabismic patient with normal retinal correspondence and without suppression would have diplopia (B-1) and visual confusion (B-2), a common visual direction for two separate objects. (C) Elimination of diplopia and confusion by suppression of retinal image (C-1) and anomalous retinal correspondence (C-2): adaptation of visual directions of deviating eye.