2. Binocular Single Vision Part 1

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Binocular Single Vision (BSV). [image] A cartoon brain and human brain Description automatically generated.

Introduction Development of BSV Mechanism and terminologies in BSV Grades of BSV Advantages of BSV Anomalies of BSV Multiple Choice Questions References.

Introduction. Binocular vision is the coordination and integration of what is received from the two eyes separately into a single binocular percept. Proper functioning of binocular vision without symptoms depends on a number of factors, which can be considered under three broad headings: (1) The anatomy of the visual apparatus (2) The motor system that coordinates movement of the eyes (3) The sensory system through which the brain receives and integrates the two monocular signals. Anomalies in any of these can cause difficulties in binocular vision, or even make it impossible.

The anatomical, motor and sensory systems must be adequate for normal BV to be present..

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, and which enable the eyes to function in a coordinated manner are: 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: Shape of the orbit. Presence of adjacent ligaments, muscles and connective tissues. The extra-ocular muscles have an important role to play as t provide motor correspondence because of the innervations of extra-ocular muscles..

B) Physiological factors:- The development of binocular vision (BV) 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: Postural Reflexes. Fixation Reflexes. The Refixation reflex.

Postural Reflexes:- Postural reflexes are inborn and must be present if binocular vision is to develop. Static Reflexes: Compensate for changes in position of the head relative to the body. b. Stato-kinetic Reflexes: Compensate for changes in head position relative to space..

Fixation Reflexes: Fixation Reflexes form the mechanism from which the binocular vision develops. The Fixation reflex: The Fixation reflex achieves foveal fixation in each eye. The Refixation reflex: Allows foveal refixation from target to target and maintenance of foveal fixation on a moving target.

Refixation Reflexes:- Most Neonates are capable of locating and briefly fixing a moving target and their eyes can move in a coarsely conjugate fashion. The First Reflex: It is the conjugate fixation reflex where eyes learn to move binocularly together during versions. Dis jugate Refixation Reflex: It allows the binocular vision to be maintained through the range of vergence movements that follow changes of fixation distance. The Corrective Fusion Reflex: It allows binocular vision to be maintained under the conditions of stress such as over coming prisms in clinical testing situations. Kinetic Reflexes: It maintains binocular vision through controlled accommodation and convergence..

Binocular Vision development in infant At 2 to 3 weeks: The infant turns his head to fixate an object. At 4 to 5 weeks: The infant can sustain monocular fixation of large near objects. At 5 to 6 weeks: The conjugate fixation reflex has developed, and the two eyes will conjugately fix an object and follow it over a considerable range for at least a few seconds. At first 1 to 3 months: The infant can superimpose images. At 3months: The infants develop binocular vision. At 4 months: Saccades develop..

From 3 to 6 months: The infants develop the ability to perceive stereopsis. At 6 months of age: The infants develop the ability to perceive depth up to 60 arcseconds. Conjugate movements of binocular vision become accurate, and convergence is well developed. From 6 to 8 months: Fusional movement can be detected by placing a small prism over either eye. During Infancy: The Development of ability to fuse images at the horopter and within the Panum's fusional area as well as development of vergence function of the eye is influenced by dramatic changes in eyeball size and orbit position..

Normal routing of visual pathways with overlapping visual fields. Binocularly driven neurons in the visual cortex. Normal retinal (retinocortical) correspondence (NRC) resulting in ‘cyclopean’ viewing. Accurate neuromuscular development and coordination, so that the visual axes are directed at, and maintain fixation on, the object of regard. Approximately equal image clarity and size for both eyes. BSV is based on NRC, which requires first an understanding of uniocular visual direction and projection..

Visual Direction refers to the spatial orientation of objects as perceived by the observer, based on their retinal projection. Principal Visual Direction: This is the primary visual direction associated with the fovea, which is the part of the retina responsible for sharp central vision. It represents the direction of gaze or the line of sight and is associated with the sensation of direct viewing. When we look straight at an object, it falls on the fovea, and this direction is considered the principal visual direction. Secondary Visual Directions: These are the visual directions corresponding to extrafoveal points (points other than the fovea) on the retina. They are referenced relative to the principal visual direction. Secondary visual directions are associated with indirect or eccentric viewing. When an object is not directly in the line of sight but rather in the periphery, it falls on a secondary visual direction..

Projection. [image] Temporal part Of retina Nasasl f retinae Temporal part Of retina Optic Optic chiasma Optic tract - Image perceived by w visual centre in the ccipital lobe Binocular vision.

Single Eye Projection Example with Two Objects: Red Object: Stimulates the right fovea (F). Brain interprets the red object as coming from the straight-ahead position Black Object: Stimulates a temporal retinal element (T) of the right eye, which receives light from the nasal visual field. Brain interprets the black object as originating from the nasal field (to the right side). Retinal Element Correspondence: Nasal retinal elements project into the temporal visual field. Temporal retinal elements project into the nasal visual field. Upper retinal elements project into the lower visual field. Lower retinal elements project into the upper visual field..

Binocular Projection (Both Eyes Open) Red Object: Stimulates both foveae (corresponding retinal points) in the right and left eyes. Brain interprets the red object as being in the same straight-ahead position for both eyes. Black Object: Stimulates temporal retinal elements in the right eye and nasal retinal elements in the left eye. Right Eye: Projects the black object into the nasal field (to the right). Left Eye: Projects the black object into the temporal field (to the left). Since the stimulated retinal elements are corresponding points, the brain interprets the black object as being in the same position in space (to the left side), resulting in a single image without double vision..

Corresponding points are areas on each retina that share the same subjective visual direction (for example, the foveae share the primary visual direction). Points on the nasal retina of one eye have corresponding points on the temporal retina of the other eye and vice versa. For example, an object producing images on the right nasal retina and the left temporal retina will be projected into the right side of visual space. This is the basis of normal retinal correspondence. This retinotopic organization is reflected back along the visual pathways, each eye maintaining separate images until the visual pathways converge onto binocularly responsive neurons in the primary visual cortex..

The horopter is an imaginary plane in external space, relative to both the observer’s eyes for a given fixation target, all points on which stimulate corresponding retinal elements and are therefore seen singly and in the same plane .This plane passes through the intersection of the visual axes and therefore includes the point of fixation in BSV..

Panum's area is a region around the horopter that allows for single binocular vision. Objects located within this zone are perceived as single images by the brain, even though the images may not fall perfectly on corresponding retinal points. If objects fall outside this area, either closer or farther from the observer, they are seen as double, which is referred to as physiological diplopia. Panum's Fusional Area is a conceptual area around each retinal point where two slightly disparate images, one on each retina, can still be fused into a single image by the brain. It explains why the visual system can tolerate a certain amount of disparity between the images seen by each eye without resulting in double vision Panum's fusional area is essential for stereopsis, which is the ability to perceive depth based on binocular disparity. If the images from each eye fall outside Panum's fusional area, the brain cannot fuse them, leading to diplopia. This area therefore defines the limits within which the brain can fuse slightly different images from each eye to produce a single, coherent view of the world with depth perception.

Grades of Binocular Single Vision. There are 3 Grades of Binocular Single Vision as given by Worth's Classification : Simultaneous Perception Fusion Stereoscopic Vision or Stereopsis..

Simultaneous Perception is the first grade of binocular single vision It refers to the ability to perceive two distinct images simultaneously, with one image projected onto each retina This visual function allows the brain to fuse two dissimilar stimuli presented to each eye into a single coherent perception Example: If one eye sees a picture of a cage and the other eye sees a picture of a lion, simultaneous perception allows the brain to merge these images so that the lion appears to be inside the cage when both eyes are open..

2. Fusion. The Second Grade of Binocular Single vision is Fusion. Fusion is the ability of the two eyes to produce a one complete picture from two similar pictures, each of which is incomplete in one small detail. Fusion is of two types namely Sensory and Motor Fusion. Sensory Fusion is the ability to perceive two similar images, one formed on each retina and perceive them as one. Motor Fusion is the ability to maintain sensory fusion through a range of various coordinated horizontal and vertical eye movements. Fusion can be demonstrated by presenting a picture of a rabbit with a missing tail and a bunch of flowers to each eye, which is seen as a complete Rabbit with both eyes Open..

The Third Grade of BSV Stereopsis is the ability to perceive depth and 3D images by superimposition of 2 images of the same object viewed by both the eyes, from slightly different angles. The 2nd Grade of BSV i.e., Fusion: Essential for the development of stereopsis; proper fusion of images from both eyes is necessary for accurate depth perception. The Average standard level of Stereoscopic vision for human eye is 30-40 seconds of arc. Stereopsis can be measured in terms of seconds of an arc..

Advantages of BSV. Enhanced depth perception (stereopsis) Improved visual accuracy and fine motor skills Wider field of view Reduced visual discomfort Enhanced performance in depth-dependent activities (e.g., sports, navigation) Better object recognition and 3D interpretation Reduced eye strain Improved binocular integration and coordination.

What are binocular anomalies?. Strabismus Concomitant Esotropia, exotropia, vertical deviations Incomitant Paralytic, Mechanical, myogenic, neurogenic Sensory adaptation to squint Diplopia, Anomalous Retinal Correspondence (ARC), Eccentric fixation, Suppression, Amblyopia Non- strabismus Accommodation anomalies Accom. Weakness, infacility, excess, spasm, paralysis Vergence anomalies Convergence / Divergence insufficiency, Convergence / Divergence Excess Visual Information Processing (Learning Disabilities).

Sensory adaptations to strabismus. The ocular sensory system in children has the ability to adapt to anomalous states (confusion and diplopia) by two mechanisms: suppression and abnormal retinal correspondence (ARC). These occur because of the plasticity of the developing visual system in children under the age of 6–8 years. Occasional adults who develop sudden-onset strabismus are able to ignore the second image after a time and therefore do not complain of diplopia..

[image]. Greek, Diplos = Double, ops = vision It means double vision; in other words, an object appears double. It is the simultaneous appreciation of two images of the same object. It results from images of the same object falling on non-corresponding retinal points. The separation of the images may be horizontal, vertical or torsional or a combination of these . In Diplopia, one image is distinct and the other is indistinct. The image seen by the squinting eye (false image) is usually less distinct than that seen by the fixing eye (true image). Diplopia occurs only over that part of the field of fixation towards which the affected extra-ocular muscles move the eye..

Diplopia Physiological (binocular) Pathological Uniocular diplopia Binocular diplopia Horizontal Vertical Crossed (Heteronymous) Uncrossed (Homonymous).

Physiological Diplopia It is also called Introspective Diplopia. It is binocular. It exist in the presence of Binocular single vision. It is a normal phenomenon, in which objects not with in the area of fixation are seen as double. In this case when distant object is fixed the near object-situated in the line of vision appears double and vice versa.

This is used in vision therapy to make sure both eyes are turned on, and to practice knowing what that feels like!.

Pathological Diplopia It can be uniocular or binocular. Uniocular Diplopia It is a condition in which there is uniocular perception of double image. An object appears double even when only one eye remains open. It occurs when two images of same object forms on the two different parts of the retina. More commonly there is uniocular polyopia (multiple vision) due to multiple images. It can be caused by : Lens opacity Subluxated clear lens (pupillary area is partially phakic and partly aphakic) Large peripheral iridectomy, iridodialysis, Polycoria (multiple pupil). Retinal detachment due to dialysis when the retina inverts itself. Corneal opacity. In rare case it occurs due to oedema in the macular region..

A close-up of hands Description automatically generated.

Horizontal Diplopia. Horizontal diplopia is a type of double vision where the images are displaced side by side, horizontally, rather than stacked vertically or rotated. It typically results from a misalignment of the eyes in the horizontal plane. Patients perceive two images of the same object placed side by side. The distance between the images may increase when looking in the direction of the deviation and may decrease when looking in the opposite direction. Patients may tilt their head or turn it to one side to minimize the double vision..

Types Horizontal diplopia can be crossed or uncrossed. Crossed Horizontal Diplopia It is associated with divergent squint. In divergent squint the fixation object is viewed by the fovea of the non-squinting and the image formed by the squinting eye lies to the temporal side of the retina result of the squint. The brain is unaware of the divergent deviation of the squinting eye and assumes that the fovea of this eye is directed to the fixation object. The image of the fixating object is projected in a direction opposite to squint. Thus, a divergent squint is associated with a heteronymous (crossed diplopia).

Uncrossed Horizontal Diplopia It is binocular and is associated with convergent squint. The images of the fixation object is received on the nasal area of the deviating eye and is projected temporally. The diplopia is uncrossed (homonymous) with the false image being on the same side of the deviation..

Vertical diplopia is a type of double vision where the images are vertically displaced, typically associated with vertical misalignment of the eyes (vertical squint or strabismus). Patients perceive two images of the same object, one positioned above the other. The degree of separation between images varies with gaze direction and the severity of the deviation.

Torsional Diplopia. Torsional diplopia is double vision where the images appear rotated or tilted relative to each other, unlike horizontal or vertical diplopia. Occurs when one or both eyes rotate around the visual axis, typically due to dysfunction in the oblique muscles (superior and inferior). Patients see double images that are rotated or tilted with respect to each other. The degree of tilt can vary based on the direction of gaze and head position..

Combined Diplopia. Combined diplopia is double vision that occurs in multiple planes—horizontal, vertical, and/or torsional—simultaneously, resulting from misalignment of the eyes in more than one direction. Patients see multiple displaced images that may be horizontally, vertically, and/or torsionally misaligned..

Causes of Binocular Diplopia Following are the causes of binocular diplopia: Paralysis or paresis of the extra-ocular muscles. Displacement of the eyeball, by a space-occupying lesion in the orbit; by fracture of orbital wall or by pressure of fingers. Mechanical restriction of the movements of the globe E.g. Pterygium,symblepheron, thyroid ophthalmopathy etc. Deviation of rays of light in one eye, as in decentered spectacles. Disparity of image size between two eyes (anisokonia) as in acquired high anisometropia (e.g. aphakia with spectacle correction in one eye, and the other eye is normal).

Confusion It is the result of stimulation of corresponding retinal points in the two eyes by two different objects..

In a convergent squint when the fixation object is viewed by the fovea of the non-squinting eye another object in a peripheral part of the visual field is viewed by the fovea of the squinting eye. The brain however is unaware of the convergent deviation of the squinting eye and assumes that the fovea of this eye is directed to the central fixation object and considers that the object lies in a straight-ahead position so that there is a superimposition of the images of the two different objects appreciated by the fovea. In case of a right convergent squint the two axis will be misaligned so that they will not intersect at the point of fixation. The left fovea will still be stimulated by the blue object, but the right fovea will now be stimulated by a different object (red circle). The superimposition of these two different objects and their projection into the same straight ahead position results in confusion..

Suppression. It is a condition in which the image of an object formed upon the retina is not perceived. Here the image is mentally ignored or neglected either partially or completely. It is an adaptation, which occurs mainly in children. It is a temporary phenomenon. Use of this sensorial adaptation ensures that incapacitating diplopia is overcome.; This phenomenon occurs only during binocular vision (with both eyes open) However, when the fixating eye is covered, the squinting eye fixes (i.e. suppression disappears). The patient may experience either diplopia or confusion..

Classification The suppression can be physiological or pathological..

Physiological Suppression Physiological suppression is a normal visual process that helps maintain clear and single vision when using both eyes together. It is an adaptive mechanism that prevents confusion and diplopia in binocular vision. It occurs in the presence of binocular single vision and arises from: Avoidance of physiological diplopia. Neglect of peripheral field Retinal rivalry.

Pathological Suppression Pathological suppression is a visual adaptation mechanism where the brain actively inhibits the image from one eye to avoid visual confusion or double vision (diplopia). Types of Suppression: Central Suppression vs. Peripheral Suppression Central Suppression: Inhibits the image from the fovea (central vision) of the deviating eye to avoid confusion. Peripheral Suppression: Inhibits the image from the peripheral retina of the deviating eye to eliminate diplopia. Monocular vs. Alternating Suppression: Monocular Suppression: The dominant eye's image always prevails, leading to constant suppression of the image from the deviating or more ametropic eye. This type often leads to amblyopia (lazy eye). Alternating Suppression: Suppression switches between eyes, reducing the risk of amblyopia since neither eye is consistently suppressed..

Facultative vs. Obligatory Suppression: Facultative Suppression: Occurs only when the eyes are misaligned. Common in conditions like intermittent exotropia and Duane syndrome. In Facultative suppression visual acuity is not affected under monocular conditions. Facultative suppression occurs only under binocular conditions. Obligatory Suppression: Present at all times, regardless of whether the eyes are straight or deviated. Leads to continuous reliance on one eye, severely impacting binocular vision and depth perception..

Abnormal (anomalous) Retinal Correspondence (ARC).