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5. Development of Binocular Vision
A major question is whether binocularity and stereopsis are present at birth or whether infants must be learn to see binocularly and three-dimensionally. The visual system takes approximately 6 weeks to become sensitive to visual stimulus deprivation, and binocular vision first appears at about 3 months of age. Although it never tapers off completely[52], visual experience has its greatest effects at about 6 months of age, with effects diminishing rapidly until about 6 years of age [6,11,19]. During the critical period of rapid visual change between 6 weeks and 3 months after birth, infants are at a greater risk of developing visual abnormalities than at any other life stage. Therefore, infants are extremely susceptible to severe visual disorders arising from inadequate visual experience during the critical period.
Since Wheatstone (1838), stereopsis has been one of the most popular fields of vision research, and it is routinely measured in clinical practice[40]. Disorders affecting stereopsis include blur, strabismus, and amblyopia, and the clinical measurement of stereopsis is of value as a means of indirect screening. The type and extent of sensory adaptation are important factors in the re-establishment of functional binocular vision for disorders such as strabismus and amblyopia in children[8,24].
Infantile esotropia, a stable, cross-fixational large-angle esotropia with onset before 6 months of age, is the most common form of strabismus. Generally, cycloplegic refraction reveals less than 3D of hyperopia, with no refractive and accommodative component responsible for deviation. Accommodative esotropia, in contrast, usually occurs between 6 months and 7 years of age, with an average age of onset of 3 years[17]. The amount of hyperopic refractive error in accommodative esotropia averages +4D; esodeviation is restored to orthophoria by optical correction of the underlying hyperopia[1]. In the non-refractive form, hyperopia averages +2D; esodeviation (not related to uncorrected refractive error) is caused by a high AC/A ratio.
The normal sensory organization of binocular vision can be altered in infantile strabismus by suppression or anomalous retinal correspondence (figure 5). Therefore, most strabismic patients do not experience diplopia and visual confusion[2,62,64]. Single vision is achieved by suppression, which causes elimination of the perception of objects normally visible to the deviating eye during simultaneous binocular viewing [28,34,65]. Anomalous retinal correspondence is an adapted shift in the visual directions of the deviated eye relative to the normal visual directions of the fixating eye [4,21,35,46]. The net result is that the deviating eye acquires a common visual direction to that of the
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fovea of the fixating eye during binocular viewing of a peripheral retinal area [5,14]. According to the recent study, early abnormal binocular visual input contributes to poor outcomes in both infantile and accommodative esotropia[33]. The accepted strabismus treatment is wearing appropriate glasses and eye muscle surgery. These treatments may prevent the development of sensory and motor dysfunctions[60]. However, several factors, including patient age at surgical alignment and duration of misalignment, influence the outcome of treatment. Future studies should establish critical factors for achieving stable binocular vision.
Abnormal development of spatial vision causes amblyopia, decreased visual acuity that cannot be attributed to suppression scotoma, uncorrected refractive error, and visual stimulus deprivation. Clinically, amblyopia is defined as a reduction in visual function caused by abnormal visual experience during development[30,61]. Strabismic amblyopia refers to amblyopia that is associated with the presence of strabismus, typically either esotropia or exotropia. The strabismic eye also shows a pronounced suppression of the central and peripheral visual field[26,53]. In addition, there is a contrast-dependent that is strongly dependent on spatial frequency and a contrast-independent deficit for position of targets[18,38,59]. Therefore, for infantile esotropia with significant fixation preference, occlusion therapy and surgery are associated with normal acuity development and a potential for at least gross stereopsis[10,40]. Anisometropic amblyopia is caused by significant, unequal refractive errors, exceeding +2D, between the eyes. Ametropic amblyopia may have equal refractive errors that are either extremely myopic (more than -6D) or hyperopic (more than +4D). Yet another kind of amblyopia, meridional amblyopia, is caused by astigmatic refractive errors for long periods (more than 2 years)[3]. Moreover, form vision deprivation amblyopia occurs in patients with a constant obstruction in the image formation mechanism of the eye, such as congenital ptosis, congenital or traumatic cataracts and corneal opacities that remain untreated for some time[9,23]. Pediatric cataract treatment is now undergoing rapid development, and visual prognosis for children with cataracts is improving due to earlier surgery, increased frequency of intraocular lens (IOL) implantation, and improved amblyopia therapy. Traditional amblyopia treatment consists of full-time occlusion of the sound eye, using an adhesive patch. However, recent trends include prescribing fewer hours and using atropine as an alternative or adjunct to patching or even as a first-line treatment[36].
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Conclusion
Binocular vision requires a high level of coordination between motor and sensory processes—binocular vision and stereopsis will be compromised if any component in this system fails. Visual inputs from both eyes are combined in the primary visual cortex (V1), where cells are tuned for binocular vision. The observation of the cells tuning in V1, together with psychophysical evidence that stereopsis occurs in visual processing, suggests that V1 was the neural correlate of stereoscopic depth perception; however, more recent work has indicated that this occurs in higher visual areas (in particular, MT area). In the future, we would like to review the neural integration of depth perception and binocular vision. The present review provides the basic information on normal and abnormal binocular vision that forms the foundation for the clinical disorder of binocular vision. We look forward to new ideas and research on binocular vision[12].
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