Basics of Binocular Vision An approach to understand the complexity

Indra P SharmaOptometrist

Objective

1.To understand the basic concept and terminologies used in binocular vision

2. To understand the physiology of binocular vision.

Contents

1. Why to study binocular vision?2. Historical perspective3. Introduction to vision4. Binocular vision5. Mechanisms and terminologies in BSV6.Grades of Binocular vision7.Clues for monocular depth perception8.Advantages and disadvantages of BSV9.Conclusion10.Glossary11.Reference

Why to study binocular vision?

The basic laws of binocular vision forms the very foundation

on which our current understanding of strabismus and its symptoms and sensory consequences is based.

Knowledge on BSV forms basis to orthoptic treatments and many physiological processes in the eye.

Historical Perspective Most of the concepts and terms used today were introduced since the 19th century.

1. Johannes 2. Hermann 3. Ewald Hering Mu¨ller von Helmholtz

Considered among the fathers of modern visual Physiology.

Vision

Vision: Anatomic, physiologic, biophysical, & biochemical arrangements & mechanisms within the retina give rise to excitations that goes to our brain & ultimately result in vision.

Eye is a biological cameraWe see from the brain ,not from the retina.

Types of Vision

1.Monocular Vision with single eye

2.Bi-ocular no co-ordination between two eyes

3.Binocular Co-ordinated vision from two eyes

Binocular Vision

Defination

Binocular vision may be defined as the state simultaneous vision with two seeing eyes that occurs when an individual fixes his visual attention on an object of regard

The coordinated use of the two eyes to produce a single mental impression.

Pre-requisites for development of BSV

Motor Mechanism:•correct neuromuscular development so that the visual axes are directed at the object•overlap of visual fields

Sensory Mechanism:•approximately equal image clarity and size in the two eyes•corresponding retinal areas so that the eyes are cyclopean•normal visual pathways

Mental Process:•ability of visual cortex to promote binocular single vision

Milestone of BSV Development

2 to 3 weeks – the infant turns his head to fixate an object. 4 to 5 weeks - infant can sustain monocular fixation of large

near objects

First 1 to 3 months -superimpose images. 3 months- binocular fusion. 3 to 6 months – stereopsis. 6 month- 60 arc sec. Development of horopter & vergence is also influenced by

dramatic changes in eyeball size & orbital position during infancy.

1. Theory of correspondence and disparity (most widely accepted theory)

2. Neurophysiological basis

4. Alternation theory of Binocular Vision

5. Projection theory of Binocular Vision

6. Motor theory

7. Theory of isomorphism

Theories of BSV

Older theories

Theory of correspondence and disparity

Salient features are :- Simultaneous stimulation of the corresponding points by one

object transmits single visual impression with no depth quality.

- Simultaneous stimulation by two objects points that difference in character results in binocular rivalry.

- Diplopia occurs when disparate elements are stimulated by one object.

- Binocular single vision with stereopsis results when the horizontal disparity remains within the limits of Panum’s area.

Neurophysiology of binocular vision

Optic chiasm. •First structure that contributes for BSV

•Partial decussation at chiasm•serves to project to same place in visual cortex

Mechanisms and terminologies in BSV

Visual Directions

A visual direction is defined as a line that connects an objectpoint with its image on the retina.

Types Principal

Fovea – area of highest VA -carries principal visual direction.

SecondaryOther retinal elements-secondary VD

Corresponding retinal Points

Retinal elements of two eyes that share a common subjective visual direction

– Example• Fovea of two eyes • Temporal retinal points of a eye –

Nasal retinal points of the fellow eye & vice versa

Contd.....

Rest other retinal points – Non corresponding /disparate

Significance:– Corresponding retinal elements are principal

elements of the two retinas that give rise in binocular vision

– Single vision is the hallmark of retinal correspondence

Horopter

1613 Aguilonius

Horopter is defined as the locus of all object points that are imaged on corresponding retinal elements at a given fixing distance.

Horizon of vision

Contd....

Object points lying on the horopter

- seen single

Object points off the horopter - Seen double

Feature 2 Dimensional plane Shape

Fronto-parallel plane : Longitudinal curve

Theoretical horopter curve

Veith-Muller Circle

If corresponding retinal points have a regular horizontal distance from

the fovea of each eye

Shape of Horopter -Circle passing through the

centre of rotation of two eyes and the fixation point

Empirical Horopter Curve

Hering and Hillebrand - showed Vieth-Mu¨ller circle does not describe the longitudinal horopter.

Empirical horpter :

- Flatter than thoeritical horopter - Distribution of the corresponding retinal elements are not the same in the nasal & temporal parts of retinas - Shape varies from person to person

Hering-Hillebrand Horopter deviation

The discrepancy (difference) between theoretical horopter and the empirical horopter .- Can be attributed to disturbing optical properties of the ocular media.

Panum’s fusional area & space

In reality a retinal point of a eye is not only corresponding to a point but to a retinal area of other eye

Panums area- the retinal area surrounding the corresponding retinal points within which BSV can be maintained.

Panums space: A narrow band around the horopter within which objects gives rise to BSV

So objects located slightly off the horopter can remain single

• Object stimulates slightly non-corresponding points

Size of Panum’s Fusional area

Increases with Retinal Eccentricity-

– At fovea • 6-10’ of arc

– 120 eccentricity • 30-40’ of arc

Physiological Diplopia

The Diplopia elicited by an object point off the Pannum’s fusional area

TypesA.Crossed (Heteronymous) Diplopia

Temporal (crossed) disparity

B.Uncrossed (homonymous) Diplopia

Nasal (uncrossed) Disparity

Fixation Disparity

It is the minute image displacement, rarely exceeding several minutes of arc of angle, occurs within Panum’s space while fusion is maintained.

•Due to presence of pannum’s fusional area

– A physiological variation in placement of retinal image displacement from corresponding retinal points

• Even Allow fusion

• Displacement of retinal images in two eyes– Retinal disparity

Fixation disparity

Grades of Binocular Vision

Grades of Binocular Vision

There are three grades of binocular vision as given by Worth's classification:

Grade I : Simultaneous perception

Grade II : Fusion

Grade III : Stereopsis

1.Simultaneous Macular Perception

Simultaneous perception exists when signals transmitted from the two eyes to the visual cortex are perceived at the same time.

It consists of the ability to see two dissimilar objects simultaneously.

2. Fusion

Defined as the cortical unification of visual objects into a single percept that is made possible by the simultaneous stimulation of corresponding retinal areas.

In simple words,It is the ability of the two eyes to produce a composite picture from two similar pictures each of which is incomplete in one small detail

Components of fusion

Sensory Fusion -the unification of visual excitations from corresponding retinal images into a single visual percept, a single visual image-The ability to unify images falling on corresponding retinal areas.

Motor Fusion - It is a vergence movement that causes similar retinal images to fall and be maintained on corresponding retinal areas.

• Ability to align the eyes in such a manner that sensory fusion can be maintained• Diplopia preventing mechanism

• The normal fusional range is 35/40 PD base out and 16 PD base in on near reading.

• 16PD base out and 8PD base in on distance testing.

Source: Rowe Fiona. Clinical Orthoptics 2nd Ed, Blackwell Publishing,2004 2: 23

3. Stereopsis

It is the ability to fuse images that stimulate horizontally disparate retinal elements within Panum’s fusional area resulting in binocular appreciation of visual object in depth i.e. in 3D

Contd...........

– Retinal disparity (Fixation disparity) is the basis of 3 D perception

– Stereopsis occurs when • Retinal disparity is large enough to simple

fusion but small enough to cause diplopia

Contd....

Stereopsis & depth perception are not synonymous.

Not a form of simple fusion.

• Normal stereoacuity is considered to be 40 sec of arc

The difference

Fusion

Corresponding retinal elements are stimulated

Motor system is required

Fusion can occur without streopsis

Fusion occurs horizontal or vertical corresponding retinal points

Stereopsis

Non corresponding retinal elements are stimulated

Motor system is not required

Without fusion it can not occur

Stereopsis occurs only with horizontal disparity

Monocular clues for depth perception

• Relative size

• Interposition

• Linear perspective

• Aerial perspective

• Distribution of lights & shadows.

• Motion parallax

Relative size

Interposition

Linear perspective

Aerial perspective

Distribution of lights & shadows.

Motion parallax

Advantage and disadvatages of BSV

Advantage of having BSV

• Stereopsis

• Binocular summation.– vision shaper, clearer &

more sensitive

• Larger field of view.

• Spare eye

visual field

Abnormal binocular vision

ConfusionDiplopia

Suppresion Eccentric fixation

Abnormal retinal correspondence(ARC)

Amblyopia

Anamolies of binocularity

Confusion When squinting occurs the two foveas view two different objects that are physically separated in objective space, and send two different images to a single cortical perceptual area. This leads to confusion.

Diplopia When squinting occurs an object in space is perceived by the fovea of one eye and some other extra-foveal point of the other eye, which has a different projection or localization value in space. Thus an object would be localized twice in space causing diplopia.

Conclusion

• Without the basic concepts of BSV it is almost impossible to understand strabismus and treat it.

• The advantage of BSV outweights the disadvantage.

• BSV – Every optometrist must know

Glossary

BSV : binocular single vision EH: Emperical horopter F: Fovea LE: left eye PD: prism dioptre RE: right eye VMC : Veith-Muller Circle

Reference

Books

1. Gunter K. von Noorden: Binocular Vision and Ocular Motility, Theory and Management of strabismus, Sixth ed ,2.

2. Kenneth Wright.W: Pediatric Ophthalmology and Strabismus, ed 95: 11:163.

3. Rowe Fiona. Clinical Orthoptics 2nd Ed, Blackwell Publishing,2004 2: 23

4. Khurana A. K.: Theory and Practise of Squint and Orthoptics; first ed .4:61-89.

Websites

1.webeye.ophth.uiowa.edu/eyeforum/tutorials/BINOCULAR-VISION.pdf

2. www.authorstream.com/Presentation/admounion-1133268-binocular-visi...

3. www.google.com