Eye Conditions

by Dr Andrew Blaikie for VI Scotland

Medical information on Binocular Vision

For whom is this information intended?

The information contained in this document is intended for use primarily by parents, other members of the family and older children with visual impairment. The information will also be of use to interested health professionals, carers and teachers.

The purpose of each information document

The purpose of the information is to explain:

• The way the eyes and brain normally work to make 'vision'
• The reason why vision may become impaired by a specific condition
• The cause of the condition
• The effects of the condition on the child's vision
• How the condition is diagnosed
• What can be done to help

This document is written with the minimum use of medical terms and jargon. It is impossible to avoid all medical terms but where we have used them we have attempted to explain them as clearly as we can. Although the information is intended to describe most aspects of the condition each child is different and there will always be exceptions to the rule. As far as we can determine these pages are true and accurate and have been written in good faith. Inevitably there will be some mistakes. We apologise for this.

This document is not a substitute for a consultation with a Health Professional and should not to be used as a means of diagnosing a condition.

We hope the information will help you

After reading the information we hope you will:

• Have a better understanding of the condition
• Know what tests and treatments are normally available
• Know when to seek professional advice
• Be able to discuss the condition in a more informed way
• Make the most of consultations with carers, teachers and health professionals
• Be reassured and more able to cope

Authors

These pages represent the consensus of opinion of many different people who include Parents of Visually Impaired Children, Visually Impaired Children themselves, Community Paediatricians, Ophthalmologists, Educationalists and Psychologists.

The main author and person responsible for their content is Dr Andrew Blaikie who is an Ophthalmology Research Fellow with Visual Impairment Scotland and a member of the Royal College of Ophthalmologists. We are very keen that parents and children feedback to VI Scotland about these information documents. If you have any questions or comments regarding the information please contact:

Visual Impairment Scotland
Scottish Sensory Centre
The University of Edinburgh
Holyrood Road
Edinburgh EH8 8AQ

Telephone Number : 0131 651 6078
Fax Number : 0131 651 6502
Textphone : 0131 651 6067

Email : viscotland@ed.ac.uk
Website : www.viscotland.org.uk

Medical information on Binocular Vision

What we see is made in the brain from signals given to it by the eyes.

What we see is in fact made in the brain. The brain makes sight from signals given to it by the eyes.

What is the normal structure of the eye?

The eye is made of three parts.

• A light focussing bit at the front (cornea and lens).
• A light sensitive film at the back of the eye (retina).
• A large collection of communication wires to the brain (optic nerve).

A curved window called the cornea first focuses the light. The light then passes through a hole called the pupil. A circle of muscle called the iris surrounds the pupil. The iris is the coloured part of the eye. The light is then focused onto the back of the eye by a lens. Tiny light sensitive patches (photoreceptors) cover the back of the eye. These photoreceptors collect information about the visual world. The covering of photoreceptors at the back of the eye forms a thin film known as the retina. Each photoreceptor sends its signals down very fine wires to the brain. The wires joining each eye to the brain are called the optic nerves. The information then travels to many different special ‘vision’ parts of the brain. All parts of the brain and eye need to be present and working for us to see normally.

What is Binocular Vision?

Usually the brain gets images from both (bi) eyes (ocular) at the same time. The brain combines the two images into one, to make vision. The images that the brain gets from the eyes are however slightly different from each other. The brain uses these small differences to work out how far away an object is. This is called depth perception. It can also help to work out how quickly an object is moving towards or away from a person. This is a type of movement perception.

What causes loss of binocular vision?

There are lots of reasons why binocular vision might become reduced or lost altogether. Reasons include:

• Reduced vision in one eye
• Loss of coordination of movement between the two eyes (squint)
• Problems with the brain comparing images from both eyes

Reduced vision in one eye

The brain needs clear images from each eye to compare any slight differences. The differences allow the brain to work out depth and speed of movement. If the sharpness of vision from one eye becomes poor the brain will be less able to do this. Binocular vision will become poorer. If the image becomes very blurred binocular vision may become lost altogether.

Loss of co-ordination of movement between the two eyes

The brain needs images of the same visual scene to compare any slight differences. The differences allow the brain to work out depth and speed of movement. If the eyes do not point in the same direction then the visual scenes will be too different. The brain will be unable to work out depth and speed of movement. Binocular vision will be lost.

When the eyes point in different directions it is called squint or strabismus. There are many different causes of squint. One of them is itself loss of binocular vision.

Problems with the brain comparing images from both eyes

There is a special part of the brain that compares the slight differences in the images coming from both eyes. If this bit of the brain does not develop properly or becomes damaged binocular vision may become lost. There are many different causes of poor development or damage to this part of the brain. Most of the time no cause can be identified.

How is the diagnosis made?

Poor or no binocular vision may be noted during a vision-screening programme. Parents may also notice by the way their child acts that they have difficulty judging distances or speed of approaching objects. There are many different types of tests for binocular vision. The diagnosis can usually be confirmed during an examination by an ophthalmologist or orthoptist.

What kind of visual difficulties is a child with no binocular vision likely to have?

One of the main benefits of binocular is to be able to judge depth and speed of objects well. Children with poor or no binocular vision can have difficulty with these tasks. This may lead to problems with:

• Pouring juice into a cup
• Catching or hitting fast moving balls
• Judging how quickly a car is approaching while crossing roads
• Walking up or down stairs

At first a bit more care must be taken when doing these things. Most children however quickly learn new ways to judge depth and do not end up having too many difficulties. There are however certain jobs where good binocular vision is important and necessary. These include flying planes and performing surgical operations. Most jobs however do not require binocular vision.

If you close one eye you can see what loss of binocular vision is like. At arms length try pouring water into a glass or bringing two pencil ends on top of each. It is more difficult than when both eyes are open. This is because your skill in judging depth is poorer. With a bit of practice these tasks become easier. This is because there are many other clues that help in judging depth. These include the size of objects and the way shadows form on them.

If you would like more information on this subject please contact us.