What does optic ataxia mean?
Optic ataxia is Greek in origin, bringing together three words:
In modern medical terms, the word ataxia is used to mean an absence or loss of full control over bodily movements. Optic (of the eye) ataxia means that visually guided movements have an absence or loss of full control, often called inaccurate visual guidance of reach.
This lesson picks up from lesson 3f, where the 'depth map' the brain creates is explained, please review this lesson if necessary.
So, what do we mean by map?
Take a mental photograph of what you can see, right now, covering your whole visual field.
Let us imagine it is something like this image (below):
With typical visual acuity and contrast sensitivity, every colour and shade, (and you can see thousands), and every detail, down to things that are as small as the width of a hair, are mapped.
But isn't that what the picture, already covered in lessons 5b (reduced visual acuity) and 5c ( reduced contrast sensitivity) does? How is the map different from the picture?
The first thing to understand is the order. The picture, in terms of acuity and colour came first, the picture feeds the map, not the other way around. If part of the picture is missing, due to visual field loss, it can't be mapped. If part of the picture is less clear, due to reduced visual acuity or contrast sensitivity, or both, the map will be less clear. The map, in the posterior parietal lobes, can only work with the information it receives, from the occipital and middle temporal lobes, along the dorsal stream.
It's quite straight forward, if only half the image is processed, due to hemianopia, or the bottom part is absent due to a lower visual field impairment, or a lack of clarity, these can't magically re-appear further along the process - but - and this is really important - here, in the posterior parietal lobes, where the map is processed, things can also affect vision. This means:
These difficulties are sometimes referred to as dorsal stream dysfunction. Combined, and when most severe, they are a condition called Balint Syndrome.
The map can profoundly affect vision even if there are no occipital or visual field impairments. The map only needs to be mildly affected to have a significant impact on a person's life.
As previously explained (lesson 3f) depth is mapped (depth map). Where the map is less accurate, we can't reach for things accurately, affecting:
For those of us unaffected by optic ataxia, we take reaching for things accurately for granted but actually, we are supported by our visual memories of how big things are, so that we can calculate how far away they are, and with our real-time depth map, visually reach with accuracy. To see how good your depth map is without your visual memories, go for a walk in woodland, and walk off the paths through the trees and branches. Trees and branches come in all shapes and sizes, so as you move through the wood, it will be difficult to rely upon your memories of how far away each one is - so you only have your real-time depth map to guide you. You will quickly find yourself bringing an arm up to help and protect and safely guide you through the low branches.
The depth map is good, but it has a collaborative working relationship with your visual memories, which can create particular difficulties for the next group...
Where a child has had optic ataxia since birth, and it is quite severe, particularly for those children who are profoundly affected, the visual memories they form may be inaccurate or confusing. Think of something all children with CVI come into contact with - another person, let us say their mother, and that mother is 5'6'' (168cm) tall and of medium build.
If the child's depth map is significantly affected, then their mother will be experienced as a tiny little person when seen further away, and as a looming giant when close up. They would not have had the consistent visual experiences to build up a memory of their mother according to her actual height and size. If these visual memories are needed to help guide us, how hard must this be for the complex child, especially if they have very limited language.
Mother is still mother, known, loved, adored, and their experience of mother is the only one they know, so it is their normal, and mother going from tiny to giant is also their normal. It is actually all of our normals, but we quickly learn the difference between the same object / person as bigger when close, and smaller when further away.
Try to appreciate how these visual memories, possibly of tiny things to giant things, could cause confusion and affect learning. When out and about, without reliable visual memories, such children are in essence wading through the dense woodlands with only their depth map to help them - which is significantly impaired!
What might happen? When distances of things cannot be accurately judged, as the child approaches something, or something approaches them if they are walking or being pushed in a chair, their brain may calculate an imminent collision. This alert can be in the form of the fight or flight reflex, which is there to protect them, but is not a nice feeling. This may be why so many we know with CVI become anxious, distressed and at times terrified in busy crowded places.
With acquired optic ataxia, existing (pre-injury) visual memories of how big things are, to aid judging how far away they are, to reach with accuracy, can help enormously. There may still be some difficulties, possibly with handwriting, breaking things, general clumsiness and situations in which things may differ from visual memory, for example a tree may be walked into because it was thought to have been a bigger tree further away.
Developmental Coordination Disorder (DCD) has diagnostic criteria, based around a child's motor skills not developing as would be expected. DCD only applies if the difficulties cannot be attributed to another condition, for example cerebral palsy or CVI. We do know that where CVI is missed, often because there are no visual acuity or visual field impairments picked up using standard sight tests, the clumsiness caused by optic ataxia can be attributed to DCD rather than the undetected cause, CVI.
Clumsiness is an outcome or behaviour, and has many different causes, some connected, some not.
We have explained where the cerebral visual impairment, optic ataxia, applies and how this can be confused with, but is different from DCD. But what about dyspraxia?
There is a lot of confusion around the term dyspraxia, as an internet search of 'dcd or dyspraxia?' will show.
Might people who have been told they have dyspraxia actually have optic ataxia?
In lesson 3f we suggested a simple reaching test, to pick up a cup, and get a sense of how accurate your guidance of reach is.
Another test is to draw a small circle, and ask the person to either mark a dot with a pen, or for younger less able children, mark with their finger, in the middle of the circle. Observe how those not affected by optic ataxia do it:
Someone with optic ataxia may:
Understanding the mapping process and the difficulties it causes when not as effective or accurate, is important. The typical map matches where things are in the real world, in terms of how far away they are, and gives every visual detail the properties of depth and distance in your mind - so what you see, that is the three dimensional world around you as you move through it, is a match with the visual world your brain creates. To move through the real world, without bumping into people and things, and to reach for things with accuracy, these two worlds (world 1 = the reality of the surroundings, and world 2 = the non-conscious mental copy of them), need to match. When the 3D non conscious mental copy is inaccurate, then optic ataxia is the outcome.
The mapping process has its limits though, and the more you give it to map, the less effective this become.
Over this and the next few lessons, take time when out and about to really think about the mapping process and what it is adding to your vision. Think about how, if it hasn't quite put things in the right place in your mind, it would affect how accurately you can reach for them.
For those born with optic ataxia, with a consistency resulting from repeated experiences, they can learn how big and far away familiar things are. This will create the much needed visual memories which can help guide the person.
For the more able, try to use an outstretched little finger as a discrete guide when reaching for things, using tactile guidance of reach to support the impaired visual guidance of reach.
De-clutter - for everyone, reaching for a cup on a cluttered table, it is less easy than on a clear table.
Slow down, particularly for acquired CVI, you need to be more careful as you brain probably still thinks it is judging distance and depth as before, so the kerb you step off onto the road might be steeper, shallower or deeper than you realise. Here, telescopic hiking poles, to feel depth, can make a big difference.
Be especially patient with people affected by optic ataxia when out and about. They may quickly start to feel uncomfortable and not know why. Sometimes letting a person link to your arm can help make them feel more secure.
It is great that you have taken the time to understand where vision might be altered, but with optic ataxia you also need to understand the person's visual history and memories, which can have more impact upon what they are experiencing that their actual visual impairment! The person with optic ataxia, especially if from birth, not only is unable to reach for things as you are, they may not understand them as you do.
What can't be reached for accurately is not limited to tasks and learning, it affects many aspects of living, all day every day - every person walked past, every doorway gone through. Even if a person is not reaching for something, their optic ataxia may still be affecting how they are feeling, usually creating confusion or anxiety - this may be invisible to others, so try to be mindful of it.
Note: Lack of stereopsis (explained in lesson 3f) can also affect visual guidance of reach. Please review this lesson if necessary.
Before moving on please check that you have understood the following:
Next lesson: Level 7c CVIs Movement and Dorsal - Simultanagnostic Vision
Further reading is not required to move on to the next lesson, but if interested you may find the following enjoyable:
What Is CVI? Optic Ataxia
Looming - Looking at why optic ataxia can create frights and anxiety.
John's Writing Storyboard - Image heavy description of writing affected by optic ataxia.
CVI & DCD Paper - Looking at confusion around the two conditions.
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