Visual Impairment Scotland Medical Information Document – 10/10/2001

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Visual Impairment Scotland Medical Information Document – 10/10/2001

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
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:

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.

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