What is CVI?


Functional Vision

One cannot learn from what cannot be seen

Functional Vision is about understanding what the person can see, to learn:

  • how big things have to be to be seen clearly
  • the levels of colour contrast that are needed to see things clearly
  • whether movement, or no movement help the person see things clearly
  • the other factors that affect the person's ability to see things clearly

Note: Following on from this section is an explanation of some of the measurements you may have, including functional visual acuity and contrast sensitivity. This section is called The Technical Stuff.

What does seeing clearly mean?

Seeing clearly means seeing comfortably, and when necessary, quickly.

When you might read a book, you read it in a text size that is comfortable, and doesn't require you to strain and take a long time. If the text was half or a quarter the size in the book, you may still be able to read it, but it becomes more of a strain and takes longer, and with that some of the enjoyment is lost.

If a person's vision means that things need to be bigger to be seen clearly, you should not aim for the absolute minimum size they have shown they can see, you should aim for a size that would be comfortable for them to see, which will be bigger.

If a person has CVI it is likely that, for a range of reasons, what they can see clearly will vary. In understanding their functional vision, you will need to note detailed observations, as you did with the visual fields in the previous section. It is highly likely that the notes from observing the person's responses in various parts of their visual field, will be highly relevant to understanding their functional vision.

Observe the person and consider, once again

Who, When, Where, What, How and Why?

This time in relation to these three areas:

  • 1) Seeing things This relates to both moving and non-moving objects. This will be demonstrated by looking and following with the eyes.
  • 2) Reaching for things This relates to both moving and non-moving objects. This will be demonstrated by an attempt to reach out with the arms and hands. Note - if there is a body turn but no reaching with the arms, this may be the person trying to see the object more clearly in a part of their visual field where they do not have sufficient attention (see previous section). Here you will see how the observations of visual fields and functional vision start to come together to form a unique representation of the person's visual world.
  • 3) Understanding / Knowing what is seen This relates to both moving and non-moving objects. This can be demonstrated in a number of ways, and many will be highly personal to the individual and their environment. It may be picking a book to be read, of picking up food and putting it on their mouth, or excitement as a favourite toy is produced.

Consider all of the above in:

  • a) Normal (daylight) lighting
  • b) Darkened conditions
  • c) Optimal environments (for example a tent)


There are very few generalisations when it comes to CVI, but keeping the environment free from clutter is one of them. The visual brain is already challenged, and having to sift through clutter to see will make seeing clearly much more challenging, and in some cases not possible. It can also feel uncomfortable and this makes seeing more difficult.

How easy would it be for the person to find the letter A in both of these classrooms (above)?

An environment completely free of visual clutter has only one colour. A cheap and practical option could be using a tent in the home.

This tent is a UPV Beach Tent, inexpensive and available from on-line retailers, with an airbed, and matching blanket and pillowsThis tent is a UPV Beach Tent, inexpensive and available from on-line retailers, with an airbed, and matching blanket and pillows

The tent and accessories all being a single colour is more important that the actual colour.

Optimal Lighting

A small percentage of people with CVI will have a stronger visual response in low lighting conditions. Some people with CVI may find bright lighting very distracting. Through your observations note the optimal lighting conditions that help the person see objects most clearly and comfortably.

As your observations develop the highly personal and individual nature of the person's visual world should be emerging. These observations need to be part of the long term journey with the person. As they develop new skills or face new challenges these observations will provide the information that everyone can use to ensure their environment is always at its best for development opportunities.

The person is very unlikely to know how their visual world is altered compared to others, even highly intelligent articulate individuals.

The best environment is where a trusted dedicated person, with time and compassion, can tune-in to the person's visual world, and help explain the challenges. From this, strategies that are relevant to that person, and their developmental level, can be created.

From these observations you will:

  • reinforce, and possibly alter, your understanding of their visual field
  • make sure that the functional visual field is used to best advantage
  • understand the person's optimal visual environment
  • understand the factors that make the person's ability to see clearly more challenging
  • understand the best lighting conditions
  • understand that the range of their functional vision varies, from at best, and to at worst.

Why can functional vision appear to vary so much?

For people with CVI, the functional visual acuity can seem to vary depending on a number of issues. An understanding of the person's visual acuity will lead you to further assess what factors seem to reduce the acuity level, and what conditions seem to enhance it. This information will help you understand more about the other cerebral visual impairments that may be affecting the individual, for example the ability to recognise, simultanagnosia or optic ataxia, fatigue, discomfort and mood.

The information will also help you to identify key factors that either help, or make things worse, for example feeling secure in a familiar place making vision work better, or feeling tired or hungry, noise, or a cluttered environment, making it work less well.

Why is it so helpful to find out the functional vision at home?

  • professional testing is only occasional, often annual, sometimes less
  • professional testing can only measure the visual acuity during the short period of the session, and usually only finds out the smallest line thickness and gap thickness that can be seen nearby and in the distance (at 6 meters) with each eye, not what is useful in everyday life
  • professional testing may take place on a 'good day' or a 'bad day', so the results may not reflect usual everyday vision
  • professional testing usually seeks the dimensions of the smallest item that can be responded to (this is not a measure of the smallest dimensions that can easily be seen and recognised with both eyes open even when tired (which is called the functional visual acuity))
  • if the person's visual acuity is reduced between professional assessments, no one will know to make things bigger, or how big
  • if the person's visual acuity improves between professional assessments, an important learning opportunity to use a bit more detail may be missed
  • you can find out what is easily and always seen and use this information to make sure that everything the person needs to learn from is either made visible or presented in another way if this is not possible
  • with this knowledge you can work out what different factors (e.g. noise, clutter, stress) make the person's functional visual acuity worse and develop the best backgrounds for everyday living and learning

Peekaboo App

The (free) Peekaboo App is an accurate way of measuring visual acuity. Take a look at this very short film:

Video Link: https://www.youtu.be/---Ow3lgj2w

If you are in the UK and have access to an iPad, this is a great free app you can download to test visual acuity, which is especially helpful for very young and more complex people (although would work on anyone). It is not available on phones (because the screen has to be big enough for the test) or computers.

With very young or more complex children who are unable to point or reach out, you can learn to observe by watching their eye movement (as the film demonstrates). This in its own right is a hugely beneficial thing to learn, even without the test for acuity. Learning to confidently understand what the person with CVI is looking at, and equally importantly is not looking at, is the first step in learning to understand their visual world.

Because Peekaboo is a free app, and also a fun app, it can be used over and over. You will become more confident, but also learn more about the person's vision. If they are straining to see one of the targets, or take longer to see it, they may be showing you that it is possible but difficult. You are looking to understand 'comfortable' vision, just like you wouldn't comfortably read a book with the tiniest text you could just make out (it would be a strain). You will learn a measured acuity, which can be shared with the professionals supporting the person. Key is to know quite simply how big something has to be to be seen.

Connor's mother who has tested the app writes:

"It literally took two turns for Connor to 'get' the 'game' as to him it is a game not a test. I thought it would be too complicated for him to understand but it has clearly been designed with very complex children like Connor, who have previously proved difficult to test, in mind. Connor does not have the dexterity to point and touch the target, although he tried to reach. What was interesting was how obvious his eye movements were. This is something I have tried to observe in the past, but because he thrashes his head from left to right, particularly when he is seemingly 'looking' at something, this has always been really hard. I think with Connor it will be useful with repeated use rather than just one use. As he becomes more familiar with the game, he will become more confident and faster, and this is when we will really learn what he does see, what he doesn't see and what he strains to see."

Once you are confident about what the person can see, the next step is to make things big enough, and then to understand when they don't see. Where something is big enough to be seen but not seen points towards other causes, for example simultanagnosia.

The ideal relationship for everyone, particularly the person with CVI, involves the family working together closely, and with the harmonious support of professionals.

Everyone's aim should be to optimise the potential for the individual by making sure that the person can perceive, understand and learn.

As your understanding of the person's visual world develops you need to tell everyone how big everything has to be!

Not the minimum size things have to be, but a comfortable size that can be seen easily, without strain.

You are taking your understanding of the person's visual world, and using this to ensure that everyone who works with that person also understands their functional vision, that is:

  • how big things have to be to be clearly and comfortable seen
  • if certain levels of contrast are needed for the object to be clearly and comfortably seen
  • the optimum environment for the object to be clearly and comfortable seen
  • if movement is, or is not required for the object to be clearly and comfortably seen.


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At CVI Scotland we are devoted to helping people understand cerebral visual impairments, and together working towards developing the understanding of this complex condition.