When you look at something, the visual information goes to the back of the eye where there are cells that see colour, called cone cells. The different colours of what you are looking at have wavelengths, this is how the cone cells interpret them and recreate the colours you are looking at. So the colours you see right now, have been created by you! They are in your mind.
All the visual information goes first to the area of the brain called V1, also called the Primary Visual Cortex (see lesson 3b), and for colour, this goes to another nearby area in the occipital lobes called V4.
Imagine an enormous set of colouring pencils
With thousands of shades between white and black.
Some people can seem more shades than others.
This picture has the colour red in the middle, and the shades lighten as it becomes white, and darken as it becomes black. Our ability to tell the different shades apart and see them separately, is our colour contrast sensitivity.
Where a person has reduced colour contrast sensitivity, it is a bit like having a smaller set of coloured pencils, where subtle shades are missed.
This image (above) is a view of the City of Edinburgh.
With slightly reduced colour contrast sensitivity it would look more like this (below)
With more severely reduced colour contrast sensitivity, the image would look like this (below).
If you look at this image (above), the fine detail, that is the acuity, has not changed, but it seems less clear because the range of colours used to tell things apart, to make sense of what you are looking at, are reduced.
Some with CVI are affected by both reduced visual acuity and reduced colour contrast sensitivity.
Some are affected by reduced colour contrast sensitivity alone, and pass an optician's eye test because eye tests are often black letters on a white background, which is the highest level of contrast.
So the reduced colour contrast sensitivity is missed, and the difficulties with vision are unexplained, or even considered not to be visual difficulties at all.
Some have reduced visual acuity but typical colour contrast sensitivity.
Reduced visual acuity and reduced colour contrast sensitivity are two different cerebral visual impairments that present different challenges.
With reduced visual acuity, you need to make things bigger.
With reduced colour contrast sensitivity, you need to ensure there are sufficiently strong enough contrasting colours next to one another, to make sense of what is being looked at.
They are different.
Different CVI's require different approaches, to match the specific needs of the person.
Before you move onto the next lesson, please check:
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