BINOCULAR VISION

CONTENTS• Introduction• Advantages• Development• Theories of binocular vision• Grades of binocular vision• Normal retinal correspondance• Binocular fusion• Stereopsis• Depth perception• Tests for BSV• Disturbances in development of BSV

Introduction

Binocular vision:

Highly co ordinated organization of series of sensory and motor

processes that culminate in perception of singleness and Stereoscopic depth enabling the eye to attain single vision from 2 retinal images.

Prerequisites: • Central fixation with normal visual acquity• Accurate occulomotor control bifoveal fixation• Normal inter retinal correspondance of visual directions• Sensory mechanisms to provide haplopia• Neural mechanisms to extract selective depth

signals

Advantages• Larger field of view

(horizontal field of view is increased from 160o to 2000 .central 120o overlap)

• Masking of image distortion caused due to optical/pathological defects of 1 eye by another.

• Retention of vision on loss of 1 eye

• Binocular summation and steriopsis

improved function

• Binocular field is larger than either of the fields

alone.

Evolution• Animals preyed upon:

gain from 3600 field of view, with eyes pointing in opposite directions

• Predators:

gain from eyes facing in same direction Stereopsis.

hence predators including humans evolved by developing organisation such as

- Frontally placed eyes (for overlap of visual field)

- binocular coordination of eye movements objects stimulate

corresponding points. (muscles yoked together)

- semidecussation of optic tract (interaction of inputs

from different parts of retina)

- eye hand coordination

Development EYE AT BIRTH AND NORMAL POSTNATAL DEVELOPMENT:

• orbits:

change shape and size throughout childhood,till teenage.

axis they form with one another decreases from 500to 450 in later life.

• Interpupillary distance:

birth: 45mm (avg) adulthood:58 – 66 mm

• Convergence required to fixate an object at 33 cm:

birth: 13 prism diopters later: 19 prism diopters

• Growth of globe: birth: 17.5 cms adulthood: 24mm

• Ciliary muscle: birth: meridonial fibre fully developed

circular portion during 1st one / two years of life.

• Extraocular muscles: smaller but fully functioning at birth

• Postural reflxes: fully functional at birth,innate and unconditioned.

• Visual acuity: 6 months: 6/30 (6/6 or 6/12)

2-3 years: adult levels

• Fixation: monocular at birth.

-rudimentary in the beginning

- 2-3 wks: begins to fixate

- 4-5 wks: sustains monocular fixation

- 6 wks: fixation alternates betweenn 2 eyes

- 3months: fixation is bifoveal & conscious, rather than reflexive

-3-5 months: smooth and gliding pursuit movements, compared to

saccadic ones till then.

• Disjugate vergence movements: develop after conjugate

movements.

convergence is demonstrable at 3 months, stable at 6 months.

• Fusional movements: abt 3-6 mnths,

established by 1 year.

Maturation of binocular functions:

at birth: both eyes act as independent sense organs• Bifoveal fixation:

Fovea develop by 3 month providing stimulus to associate both eyes.child learns that image is most detailed when fixed on fovea.

• Relative space perception:

-objects to right of fixation send images to retinal areas in 2 eyes with common visual direction

- crossfiring of various sensory phenomenon: eg touch with vision• If eye is not allowed to associate child never acquires binocular

vision/ fusion

vision remains monocular and alternating

• Stereopsis: normal eyes & neuromuscular mechanism

stereopsis develops

all or none phenomenon cannot be trained

Theories of binocular vision THEORY OF CORRESPONDANCE & DISPARITY: • Simultaneous stimulation of corresponding points by 1 object

single visual impression with no depth quality

• Simultaneous stimulation with 2 objects that differ in character

Binocular rivalry

• Disparate elements stimulated by 1 object diplopia

• Horizontal disparity within PANUM’S area binocular single vision.

• Other theories:

-motor theory

- theory of isomorphism

- alternation theory of binocular vision

Grades of binocular vision

• Simultaneous perception/ 1st grade:

signals transmitted from 2 eyes to the visual cortex are percieved at the same time

• Fusion / 2nd grade:

ability to produce composite picture from 2 similar picture, each lacking some small details.

• Stereopsis / 3rd grade:

impression of depth/appreciation of 3 dimensions

by superimposition of 2 images of same object

Visual directions• Occulocntric direction/local sign: specific visual direction of a

retinal image location

• Primary visual direction:

occulocentric direction of an object fixated along the 1st line of vision, imaged at fovea

• Secondary visual direction: (secondary line of sight)

occulocentric direction of other retinal image locations relative to primary visual direction.

• Egocentric localization:

point midway between two eyes egocenter / cyclopean eye

• Egocentric direction:

direction of object in space relative to body

occulocentric direction + informaion about eye orientation in head

and head position relative to trunk

Normal retinal correspondance

• Corresponding points: specific pairs of retinal image locations in 2 eyes perceived to have identical visual direction.

eg: Fovea

• Normal retinal correspondance:

when image is formed at corresponding retinal points,they produce perception of identical visual directions when viewing with 1 eye / both eyes simultaneously, permitting single visual impression.

Veith-muller Horopter• For infinite fixation distance,a circle passing through

-the fixation point ,

-object locations imaged on other corresponding points and

-the entrance pupils of the two eyes.

• Every point on the horopter forms an angle with the entrance pupils, equal to that formed by the fixation point.

• At finite distance, the surface is reduced to 2 lines passing through the intersection of lines of sight when eyes fixate at symmetric convergence

- LONGITUDINAL HOROPTER

- VERTICAL HOROPTER

Only points imaged on these two meridians can be

formed on corresponding points without vertical

disparity.

Physiologic diplopia• Diplopia of non fixated objects.

• all object that are nearer or farther away than the horopter (beyond normal fusional range) , are imaged on

non-corresponding points.

2 egocentric directions

Diplopia

• We do not have constant diplopia during daily vision due to

- exclusiveness of attention directed to fixating object.

- low visual acuity of peripheral retina.

Crossed diplopia • For object closer than horopter • Point is imaged on temporal retina• each eye percieves the point on the opposite side.

Uncrossed diplopia• For Points located beyond horopter• Point is imaged on nasal retina.

Binocular fusion Sensory fusion: • normally occurs: when images of an object fall on corresponding

retinal points they are fused into a single mental impression.• Also occurs when stimulus is similar, and not identical

eg: size, shape, form,color, luminosity,contrast.

• ANATOMIC BASIS: course of nerve fibres.

by crossing at chiasma,nasal fibres of one eye are brought near temporal fibres of the other (counterparts)

they terminate in the same cortical cell.

Motor fusion:

ability to align eyes to maintain sensory fusion.

stimulus: retinal disparity

Panum’s fusional space• space in front and behind the horopter where objects located can

be fused into a single image without producing diplopia.• Smallest at the fixation point.

Retinal Panum’s area corresponds to depth of panum’s fusional space.

Fixation disparity:

primary line of sight of 1 eye misses fixation point slightly,being

either underconverged or overconverged

as disparity is less than size panum’s area no diplopia occurs

Theories of binocular fusion

• Synergy hypothesis of Panum• Local sign hypothesis• Eye movement hypothesis of Helmholtz• Supression hypothesis of du Tour and Verhoeff

• PHYSIOLOGIC HYPOTHESIS:

neurophysiologically 4 classes of neurons have been defined by Hubel and Wiesel

1. Binocular corresponding (BC)

2. Binocular disparate (BD)

3. Monocular right

4. Monocular left

Physiologic hypothesis

Stereopsis• Steriopsis is the visual appreciation of 3 dimensions during

binocular vision, occuring through fusion of signals from disparate elements.

• Temporal disparities gives impression of nearness of an object.• Nasal disparity gives impression of remoteness of an objecct.• Vertical disparities do not produce stereoscopic effect.

• Requirements: - bifoveal fixation

-good visual acuity

- fusion

• FINE STEREOPSIS: highly specific pattern matching process operating over narrow range of spatial disparities( probably not more than 0.5 degrees)

• COARSE STEREOPSIS: less specific process that can operate on dissimilar visual images(form, luminance, contrast)

separated by several degrees in spatial position(7 to 10 degrees)

• Stereoscopic acuity:

-minimal disparity beyond which no stereoscopic effect is produced.

- stereoacuity is best at fovea, decreases towards periphery

-normally :depth difference between two images with relative disparity of 10 arc sec can be discriminated

-beyond 600 m, no true stereopsis,

monocular clues take over.

Depth perception• Distances of objects from each other or the observer

• Factors:

1. Stereopsis

2. Nonstereoscopic clues retinal disparity

3. Monocular clues ( nonstereoscopic clues to spatial orientation)

- parallactic movements

-linear perspective

-overlay of contours

-size

-distance from horizon

-influence of highlights, shadows, light

-aerial perspective

4. Influence of accomodation and convergence.

Overlay of contours highlights and shadows

Binocular vision tests1. Test for simultaneous macular perception

2. Test for fusion

3. Test for stereopsis

- vectograph test

- titmus stereo test

- random dot stereogram test (RDT)

- TNO random dot test

- Lang test

- frisby test

- sterescopic contours induced optokinetic nystagmus test

- television random dot stereotest

4. Simple motor task tests

- two pencil test

Titmus stereo test Random dot test(RDT)

TNO Test 2 pencil test

Disturbances in development of BSV• Time of onset of defect in visual system is most important• 18 months to 2 yrs: prognosis for eventual bifoveal single vision is

poor• At later age: normal function maybe regained with adequate

treatment.

• Anomalies in development of BSV:

- Diplopia & confusion

- Supression

- Amblyopia

- Abnormal retinal correspondance.

Diplopia & confusion• Objects in the plane of fixation are percieved as double.

• Misalignment of visual axis in deviated eye

retinal image of fixed target is seen in different egocentric directions

(2 visual directions)

DIPLOPIA and Visual confusion

Intolerable visual environment

ARC Supression

Amblyopia

Horror fusionis: intractable diplopia due to absence of

central supression

• Supression:

Active cortical inhibition of vision in one eye.

can be : - selective

- facultative

- constant (monocular strabismus)

• Amblyopia:

-partial loss of sight in one or both eyes in the absence of

ophthalmoscopic or any other marked signs.

-maybe due to – anisometropic

- strabismic

- stimulus deprivation(amblyopia exanopsia)

-amblyopia sets in early if strabismus begins in

early years of life.

Abnormal retinal correspondance• Active cortical adjustment in the directional values of 2 eyes• The fovea of one eye and a peripheral retinal element of another

eye acquire common visual direction. • Types :

Harmonious ARC:

Unharmonious ARC:

• Advantages:

- gives patient a form of BSV

- stabilizes angle of deviation

- better visual adjustment, as there maybe depth perception

• Disadvantages:

- difficult to establish normal retinal correspondance

Summary • BSV: singlevision from 2 retinal images with depth perception.

• 3 grades of BSV: simultaneous perception, fusion, stereopsis

• Object locations on horopter give same visual directions• Object locations within panum’s fusional area, can be fused to give

single image

• Stereopsis occurs due to spatial disparity

temporal disparity: nearness of object

nasal disparity: remoteness of object

• Disturbances in BSV: diplopia & confusion

supression

amblyopia

ARC

References • Adler’s physiology of eye• Squint and orthoptics – A K Khurana

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