The brain adds depth to the picture it has created of what you are looking at, and this depth should match the depth, that is how far away something is, of what you are looking at. We refer to this as your your depth map.
One simple way of testing the accuracy of your depth map is seeing how accurate your guidance of reach is.
Put a cup in front of you, and reach for it.
If you can reach for it accurately, and not knock it, then it is a sign that your brain has accurately mapped it.
If you reached for the cup with your hand open, like in the image below, and then as you got closer to the cup handle closed it, your map may be not quite so accurate.
The open gap between the thumb and fingers, illustrated on the picture above, is called your 'in-flight gap'. 'In-flight' simply refers to the fact that your hand is moving. If you pick up the cup, and have a wide in-flight gap, it is a sign that your map may not be as accurate. In other areas of your life, you may be considered a little clumsy.
If you put your hand behind the cup, to sort of secure its position before committing to moving it, by scooping it up from behind, this is a sign of an even less accurate map.
If you put your hand over the top of the cup, like in the image below, this is also a sign that the map your brain has created of where the cup is, is not so good, in that your guidance of reach is not so accurate.
These simple exercises are just to give an indication of how accurate your guidance of reach is.
The depth of what you are looking at is mapped in a part of the brain called the posterior parietal lobes, which are at the back (posterior) of the parietal lobes (see lesson 1c).
Where the cause of inaccurate guidance of reach is due to the posterior parietal lobes not working as well as they should, and not creating such an accurate depth map (making reaching things with accuracy more difficult), this is called optic ataxia.
Optic Ataxia - vision without order, or as medically defined, impaired visual guidance of limb and body movement.
Inaccurate visual guidance can have different causes, including:
For all of these reasons, it is likely that the person would reach for the cup similarly, depending on how severely affected their guidance of reach is, and so they may look like they have optic ataxia, when there might be a different cause. Equally, optic ataxia may be missed, and mistaken for other challenges affecting guidance of reach, particularly developmental coordination disorder (DCD, sometimes called dyspraxia).
Where a visual behaviour, here inaccurate guidance of reach, has many completely different causes, one has to establish what is causing the difficulty - one has to ask WHY?
If the inaccurate guidance of reach is due to the brain not creating such a good map of how far away things really are, there may well be other challenges the person has.
If you can't accurately judge how far away things are, it can be equally difficult to judge how far away things are if they are moving towards you, and your brain may inaccurately calculate that there may be a collision, which may make you frightened, and avoid places where there is a lot of movement, like busy supermarkets and airports. We have called this particular phenomenon looming and will be looking at it more closely in the level specifically looking at inaccurate guidance of reach (optic ataxia).
We will look at the challenges inaccurate guidance of reach creates, and many suggested strategies to help in following lessons.
The brain receives the information about how far away things are, to create the depth map, from the eyes, and this is added to the image (as explained in this lesson). The information of depth, to create an accurate depth map, comes from both eyes working together.
The two separate images, created separately by each eye (see visual fields lesson 3d) are successfully combined to create a single image. This requires that both eyes are looking at the same thing. This is called binocular vision, and the information provided by the slightly different pictures created by the two eyes, working together simultaneously, sends information to the brain of how far away things are, so the brain can accurately create your depth map by automatically computing the picture differences.
This is called stereopsis.
Many issues can impair stereopsis, which will affect the accuracy of the depth map the brain is able to create. This in turn will affect the accuracy of the person's guidance of reach.
Causes of reduced or absent stereopsis include:
Before you move onto the next lesson, please check:
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