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Symptoms of strabismus include double vision, crossed eyes, eyes which are not aligned, independent eye movements, and loss of depth perception. If left untreated, the less dominant eye will become even weaker, eventually leading to amblyopia. Though strabismus is the most common cause of amblyopia, it can occur independently. Astigmatism, cataracts, nearsightedness, and farsightedness can all cause amblyopia.
Symptoms include eyes which do not move together, poor vision in one eye, loss of depth perception, and eyes which turn in or out.
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Binocular Vision: Disorders and Treatment. The objects in space are localized by us in two ways —one is relative to one another and is called relative localization and the other is in relation to ourselves and is called egocentric localization. Location of an object point in physical space was separated from its localization in visual space. The objective lines of direction determine which retinal area will be stimulated, their subjective counterpart, the visual direction determines the direction in which the object will be seen in visual space.
It is defined as a retinocerebral apparatus engaged in elaborating a sensation in response to excitation of a unit area of retinal surface. It is the line which connects an object point with its image on the retina. If the visual axis of the two foveas also known as the principle visual axis intersects at the fixation point, it is said that there is binocular fixation. If only one principle line of direction goes through the fixation point, then fixation is monocular.
All object points that simultaneously stimulates the two fovea appears in one and the same subjective visual direction coinciding with the median plane of the head. This is known as the common subjective visual direction of the fovea. It lies in as imaginary plane passing through the root of the nose known as third central imaginary eye or the binoculus or cyclopean eye.
Retinal elements of the two eyes that share a common subjective visual direction are called corresponding retinal points. All other retinal elements are non-corresponding or disparate with respect to a given retinal element in the fellow eye for a particular visual direction. It states that existence of corresponding retinal elements with their common relative subjective visual direction is the essence of binocular vision.
Retinal correspondence is called normal when both the fovea have a common visual direction and the retinal elements nasal to the fovea in one eye corresponds to the retinal elements temporal to the fovea in the other eye. Retinal correspondence is abnormal when the fovea of one eye has a common visual direction with an extrafoveal area in the other eye.
This is generally seen if the angle of squint is small and the extrafoveal point is close to the fovea. It is an attempt to regain the binocular advantage, although anomalous because it is foveo-extrafoveal and not foveo-foveal. This results in the eyes seeing binocularly single inspite of a manifest squint.
In ARC under binocular conditions the fovea and the extafoveal point share the common subjective visual direction. But when the normal eye is closed the extrafoveal element loses any advantage over the fovea of that eye, which retains its primary visual direction. Thus under monocular conditions the central fixation is retained by the fovea, this is the basis of the cover test.
The quality of binocular single vision obtained in ARC varies from patient to patient, in some there is useful gross stereopsis while in the others binocular vision is rudimentary.
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The quality of binocular vision is usually inversely proportional to the angle of deviation. The term Horopter the horizon of vision was introduced in by Aguilonius. It is defined as the locus of all object points that are imaged on corresponding retinal elements at a given fixation distance. Thus a line can be drawn through the object of regard such that all the points on the line are imaged on the corresponding retinal elements and are seen singly. According to this model of horopter if corresponding points have a regular horizontal distance from the retina the horopter would be a circle passing through the center of rotation of the two eyes and the fixation point.
Thus the circle becomes smaller as the point of fixation gets nearer. The empirical horopter curve or the longitudinal horopter is slightly flatter than vieth muller geometric horopter i. The deviation of the perceptual from the geometric horopter is known as the Hering-Hillebrand deviation. This deviation is felt to be due to both neural and optical factors.
What is Binocular Vision?
The nasal hemi-retina at any given eccentricity contains more photoreceptors per unit area than the temporal hemi retina producing a deviation in the horopter mapping in the visual cortex. Thus all the points not lying on the horopter are imaged by disparate retinal elements and are seen as double. This diplopia elicited by object points off the horopter is called Physiological diplopia. However we normally do not always experience this diplopia. It is defined as a narrow band around the horopter within which the objects stimulation disparate retinal elements transmit the impression of single vision.
Fusion is defined as the unification of visual excitations from the corresponding retinal images into a single visual percept. It is the ability to appreciate two similar images, one with each eye and interpret them as one. Single visual image is the hallmark of retinal correspondence. For sensory fusion to occur, the images not only must be located on corresponding retinal areas but also must be sufficiently similar in size, brightness and sharpness to permit sensory. Unequal images are a severe obstacle to fusion. It is the ability to align the eyes in such a manner that sensory fusion can be maintained.
Unlike sensory fusion, motor fusion is the exclusive function of the extrafoveal retinal periphery.
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Fusion, whether sensory or motor, is always a central process i. The simultaneous stimulation of non-corresponding or disparate retinal elements by an object point causes this point to be localized in two different subjective visual directions. An object point seen simultaneously in two directions appears double.
Double vision is the hallmark of retinal disparity. When dissimilar contours are presented to corresponding retinal areas fusion becomes impossible and retinal rivalry may be observed.
Simultaneous excitation of corresponding retinal areas by dissimilar objects does not permit fusion and leads to confusion. In order to remove this confusion, image from one of the eyes is suppressed. This constant foveal suppression of one eye with cessation of rivalry leads to complete sensory dominance of the other eye, which is a major obstacle to binocular vision. Return of retinal rivalry is a requisite for re-establishment of binocular vision. It is a neuro-physiological active inhibitory mechanism in which when corresponding retinal areas are stimulated by dissimilar stimuli or when non-corresponding retinal areas are stimulated by similar stimuli, one or the other is temporarily inhibited or suppressed to prevent confusion or diplopia respectively.
Suppression is foveal in order to tackle confusion and extrafoveal in order to avoid diplopia. The eyes are separated in the horizontal plane of the head and thus each eye has a slightly horizontally disparate view of the world. The sensory fusion of these horizontally disparate unequal retinal images results in a three dimensional percept.
An object confined to the horopter is seen as flat because it projects to corresponding retinal regions, causing zero horizontal disparity.
It is the smallest binocular disparity that can be readily detected i. There are no standardized clinical stereoscopic acuity tests, but generally speaking, a threshold of 15 —30 arc sec.
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Since there is a stereoscopic threshold, it follows that stereopsis cannot work beyond a certain critical distance. This distance has been computed to be between meters. The stereoscopic acuity also varies depending on whether the target is stationary or moving, and whether the eyes are stationary or moving. The steroacuity threshold for static targets is in the range of arc sec. For targets in motion towards and away from the observer the threshold increases to about 40 arc sec.
Stereacuity is maximal about 0. Stereopsis is nil beyond 15 degrees eccentricity. Stereoacuity diminishes in similar exponential fashion when the target is moved in front or behind the horopter along the y-axis. Though stereopsis is essential for depth perception it is not the only mean for spatial orientation. The various monocular clues to spatial orientation can be:.
During the first few years of life certain normal anatomical and physiological conditions are required for the development of binocular vision.
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