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BINOCULAR VISION & ITS ANOMALIES Presenter - Dr Neha Moderator – Dr Parul

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DEFINITION – It is the state of simultaneous vision with two seeing eyes (neither of which needs necessarily be normal) that occurs when individual fixes his visual attention on object of regard. Binocular single vision (BSV) is the coordinated use of two eyes in order to produce a single mental impression. Each eye receives a separate retinal image which is integrated by the brain to give a single common perception.

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ADVANTAGES OF Binocular Vision Increased field of vision . Optical defects in one eye are made less obvious by normal image of other eye. Safety factor against partial or complete loss of vision. Ability to perceive stereopsis. Enhanced visual acuity ,contrast sensitivity, visual motor skills. The blind spot of each eye is compensated for by the other.

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PREREQUISITE FOR NORMAL Binocular Vision DEVELOPMENT Good visual acuity in either eye. Proper fixation at two fovea. Normal retinal correspondance. Visual fields of two eyes must overlap. Intact reflexes –postural ,fixation and kinetic. Image by two eyes should be of same colour, size and shape. Normal visual pathway.

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DEVELOPMENT OF BINOCULAR VISION During the first few years of life certain normal anatomical and physiological conditions are required for the development of binocular vision. The factors concerned in the development of Binocular vision A) Anatomical factors: The two eyes are so situated in the orbit that the visual axis is directed in the same direction. This is due to -

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Shape of the orbit Presence of adjacent ligaments, muscles and connective tissues. The extra-ocular muscles have an important role to play as they provide motor correspondence because of the reciprocal innervation of the extra-ocular muscles.

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The aim of the motor correspondence is to- 1) Enlarge the field of view by transforming the field of vision into the field of fixation. 2) Bring back the object of attention on to the fovea and to maintain it. 3) Position the two eyes in such a way that at all the times they are properly aligned.

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B)Physiological factors : The development of binocular vision depends upon certain normal physiological binocular reflexes. The reflexes can either be inborn or acquired as a result of appropriate stimulation. The various binocular reflexes are:

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Fixation reflex Refixation reflex Pupillary reflex

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Fusion Reflex and its Development: Fusional reflexes are conditioned reflexes, acquired and maintained by cerebral activity. They are developed by the individual on the basis of experience. Once formed, with continued reinforcement it becomes an unconditioned reflex. Also known as psychoptical reflexes,

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It consists of all the activities mediated from the retina through the brain to maintain the images received on the two foveas with the ultimate aim of attaining a single binocular vision.

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The development of optomotor reflex is essentially a post natal event, with the approximate time schedule being : 2- 3 weeks – follows light uniocularly 6 weeks to 6 months - follows light binocularly

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Convergence is absent at birth, starts developing at 1 month of age and is well established by 6 months. The development of accommodation lags behind the development of convergence due to the delay in the development of ciliary muscles, it parallels with the convergence by 6 months of age.

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RELATIVE SUBJECTIVE VISUAL DIRECTIONS Each retinal element localizes the stimulus as a visual percept in a specific direction, a visual direction which is relative to the visual direction of fovea called secondary visual direction. The visual direction of fovea is termed as principal visual direction. This relationship is stable and this stability makes an orderly visual field possible.

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RETINOMOTOR VALUES Each retinal element has its own retinomotor value. The appearance of an object in the periphery of the visual field attracts attention and the eye is turned toward the object so that it may be imaged on fovea. This function of retinal element is called retinomotor value.

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RMV of retinal elements increases from the centre to periphery RMV of fovea itself is zero. Once an image is on the fovea there is no incentive for an ocular rotation.

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COMMON RELATIVE SUBJECTIVE VISUAL DIRECTIONS All objects point that stimulate simultaneously the two foveas appear in one and the same subjective visual direction. This direction belongs to both the right and left fovea and is therefore called common subjective visual direction of the foveas. Not only the two foveas have a common visual direction , every retinal point or area has a partner in the fellow retina with which it shares a common relative visual direction

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RETINAL CORRESPONDENCE Retinal elements of the two eyes that share a common subjective visual direction are called Corresponding Retinal Points All other retinal elements are NonCorresponding or disparate with respect to a given retinal element in the fellow eye

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Common subjective visual directions form a sheaf that is the subjective equivalent of two physical eyes and may be thought of as the third central imaginary eye and the binoculus or cyclopean eye .

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GRADES OF BINOCULAR VISION Binocular vision was divided into three grades by CLAUD WORTH – Simultaneous perception Fusion Stereopsis

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SIMULTANEOUS PERCEPTION This is the ability to see simultaneously two images, one formed on each retina. A very basic requirement of BSV. Must be accompanied by the ability to correlate two dissimilar images.

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FUSION This is the ability to see two similar images, one formed on each retina and to blend them as one . Two types of fusion are – Sensory Motor

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1. SENSORY FUSION Unification of visual excitations from corresponding retinal images into a single visual image. Single vision is the hallmark of correspondence i.e stimulus to sensory fusion is the excitation of corresponding retinal elements.

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For sensory fusion to come about the images must be sufficiently similar in size brightness, sharpness. Unequal images may present a severe sensory obstacle to fusion. The simultaneous stimulation of non corresponding or disparate retinal elements by an object point causes this point to be localised in two subjective visual directions, hence Diplopia.

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2. MOTOR FUSION If a small base out prism is placed before one eye the stimulus will be deviated from that fovea to a slightly temporal point A and the individual who lacks motor fusion will immediately experience diplopia, however if motor fusion is present there will be desire to maintain the single image and a corrective adducting movement will be made by the right eye to bring the fovea round to the necessory position.

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RETINAL RIVALRY When dissimilar contours are presented to corresponding retinal areas, fusion becomes impossible 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.

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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.

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Each eye sees a set of oblique lines, when observed in a stereoscope , these lines are not seen as crossing lines but as an everchanging pattern of patches of oblique lines going in one or other direction . Binocular rivalry may also be produced by uniform surfaces of different colour {colour rivalry} and unequal luminances of the two targets.

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HOROPTER It is defined as the locus of all object points that are imaged on corresponding retinal elements at a given fixation distance. Vieth –muller circle is the theoretical or mathematical horopter curve

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PHYSIOLOGICAL DIPLOPIA Objects which do not lie on the horopter will not stimulate corresponding points and as non- corresponding points can not project to the same position in space , these objects are seen in diplopia which is physiological

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PANUM’S AREA OF SINGLE BINOCULAR VISION The region around the horopter in which single vision is present Panum’s theory of fusional areas states that the retinal element in one eye corresponds not only with a single point in the other but with an elliptical area surrounding the exactly corresponding point

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By using panum’s area it becomes possible to fuse the slightly differing images seen by each eye when veiwing a three dimensional object extending into a narrow band of space in front of or behind the horopter. If an object penetrates beyond this band retinal elements outside the limits of panum’s area will be stimulated and physiological diplopia will result

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The horizontal extent of these areas is small at the centre ( 6 to 10 min near the fovea) and increases towards the periphery (around 30 to 40 min at 12 degree from the fovea).

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STEREOPSIS The relative ordering of visual objects in depth, that is in third dimension. Stereopsis develops when horizontally disparate elements are stimulated simultaneously. The fusion of such disparate images results in a single visual impression perceived in depth , provided the fused image lies with in panum’area.

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Vertical disparity produces no stereoscopic effect. Whenever a three dimensional object is viewed the two monocular images are inevitably slightly dissimilar by virtue of the fact that the two eyes are viewing it from slightly different aspects. This dissimilarity results in three dimensional percept.

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STEREOPSIS AND FUSION Although it is true in a general way that sensory fusion is required for stereopsis , it is not absolutely required. Stereopsis is also possible with diplopia. Also presence of sensory fusion does not guarantee the presence of stereopsis. There are patients who readily fuse similar targets but do not have stereopsis,such patients selectively suppress the disparately imaged elements of a stereogram seen by one eye.

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STEREOSCOPIC ACUITY There is a minimal disparity beyond which no stereoscopic effect is produced. This limiting disparity characterises a person’s stereoscopic acuity

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MOTION PARALLAX When one looks at two objects ,one of which is closer than the other and moves either the eyes or the head in a plane parallel to the plane of one of these objects. One notes the apparent movement of the objects . The farther object appears to make a larger excursion than the near object

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LINEAR PERSPECTIVE Object points having a constant size appears to subtend smaller and smaller angles as they receed from the object. Rail road tracks which are parallel, seem to approach each other in the distance.

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SIZE OF KNOWN OBJECT If the size of two objects is known to us then we can judge the relative distance of the objects by their apparent size. If an object that we know to be smaller appears to be larger than the other we judge it to be nearer.

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ANOMALIES OF BSV WHEN DOES IT OCCUR ??? When a manifest deviation occurs. Occurs when NRC is disturbed. The corresponding retinal elements are no longer directed at the same object This results in confusion and diplopia.

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CONFUSION It is the simultaneous perception of two different objects projected on to corresponding retinal elements. Two dissimilar images at the fovea result in retinal rivalry. Confusion is more likely at non foveal retinal areas.

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DIPLOPIA It occurs when an image falls on the fovea of one eye and a non foveal point in other eye. The object is outside the panum’s fusional area. Younger the patient, higher the ability to suppress the non foveal image.

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SUPPRESSION It involves active inhibition in the visual cortex of an image from one eye when both eyes are open. stimuli for suppression include diplopia, confusion and blurred image from one eye resulting from astigmatism/anisometropia. Clinically suppression may be- Central-the image from the fovea of deviating eye is inhibited to avoid confusion.

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Peripheral- image from the peripheral retinsa of deviating eye is inhibited to avoid diplopia. Mono-ocular-when the image from the dominant eye predominates over the image from the deviating eye so that the image from the latter is constantly suppressed. This type of suppression leads to amblyopia.

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Alternating- when suppression switches from one eye to other, amblyopia doesn’t develop. Facultative- occurs only when the eyes are mis-aligned.eg intermittent exotropia-obligatory suppression is present all the times.

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Suppression and ARC are inter related adaptations to strabismus occuring before 6 years. It also represents an obstacle in trying to restore normal sensorimotor function.

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AMBLYOPIA It is unilateral or rarely bilateral decrease of best corrected visual acuity caused by form vision deprivation and abnormal binocular interaction for which there is no pathology of the eye or visual pathway.

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It may be anisometropic,strabismic or due to stimulus deprivation – amblyopia exanopsia (eg in a child with congenital cataract, severe ptosis). Amblyopia follows through stage of suppression. Children whose strabismus begins early in life, have more deep seated amblyopia if strabismus is monocular than those whose squint begins later.

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Most of evidence suggests that site of interference in amblyopia is block in cortex & not retinal activity. Most functions of macula are intact in presence of amblyopia - dark adaptation,color vision. It is reported that only 20% of subjects with amblyopia fixate along central foveal axis of poor eye,when good eye is occluded.

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ABNORMAL RETINAL CORRESPONDENCE It is a condition in which non corresponding retinal elements acquire a common subjective visual direction. The fovea of the fixating eye is paired with a non foveal element of deviated eye. ARC is a positive sensory adaptation to strabismus which allows some anomalous binocular vision in the presence of a heterotropia.

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Binocular responses in ARC are never as good as in normal bifoveal BSV. ARC is most frequently present in small angle esotropia associated with anisometropia. May develop throughout the first decade of life.

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TYPES OF ARC Harmonious ARC: if the angle of anomaly is equal to the angle of strabismus. Unharmonious ARC: if angle of anomaly is less than angle of deviation .

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ADVANTAGES OF ARC : BSV preserved to some extent. Tends to stabilize the angle of deviation. Better visual judgement. DISADVANTAGES : extremely difficult to revert to normal correspondance.

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MANAGEMENT OF ARC In the majority of strabismic cases , ARC is considered beneficial and is therefore encouraged and not treated. The exception is if surgery is to be carried out with a chance of obtaining BSV. In general,the cases considered for treatment will be of mixed correspondance. Predominantly normal correspondance will recover any way and predominantly abnormal has too poor prognosis.

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GENERAL PATTERN FOR TREATMENT OF ARC C0rrection of refractive error. Amblyopia treated by occlusion. Aim for bifoveal stimulation. Encourage fixation with strabismic eye. Prism method to remove the angle of strabismus to encourage ARC. In treatment of strabismus it is better to concentrate on treatment of ARC rather than suppression alone.

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