Video Link: https://vimeo.com/392458165

Following on from the previous lesson where we asked you to spend a few days thinking about your own map, we are going to do a few more exercises. This is because this subject cannot be understood just by reading about it. You need to first understand your own map, and then try to get a sense of how it is different for those with a CVI.

We will explain using a simple sliding scale....

Exercise:

For this exercise we are going to imagine we are standing in front of shelves of books, close enough to reach them.

In the image above, imagine the middle row is the same height as your head.

Looking straight ahead, and keeping your body position central and still, let us mark the middle area (with a white circle, below), where your voxels are smallest.

We will call this our original position.

The area in the circle is where you can reach most accurately, so once you have chosen a book, if it is in that circle, you should be able to reach out with your arms, put your fingers on it (and not accidentally on another book) and slide it towards yourself to remove it from the shelf.

Now, we are going to create a voxel map sliding scale, it can go left or right, ours will go right....

The scale is not very long is it? But, keeping your original position (body and head and eye position), as the circle moves to the right, very quickly you will no longer be able to see a book well enough in your peripheral vision to recognise it, let alone look at it (without moving your head and eyes, explained below).

So, standing in your original position, the dashed circle in the image above is your best vision, where you have the smallest voxels. The circle to the right, where you see less, recognise less and reach much less accurately, may be the 'normal' central (original position) vision for the person with CVI, whose peripheral vision provides an even less accurate map.

You need to go out and try this, at home, at a book shop, at a library, anywhere there are books. From your original position, think about your vision to the right, and what happens in relation to the map:

• The voxels become bigger, so...
• Less is mapped, and...
• You can only see what is mapped, so...
• Visual search is reduced (similar to simultanagnostic vision), meaning
• You can't visually look at things pointed out to you, because you can't see them, because they haven't been mapped (similar to apraxia of gaze), and finally...
• You can no longer reach for things visually, with accuracy (similar to optic ataxia).

All, because of bigger voxels. This is why understanding the CVI map in the brain of those affected, is so important, because those with CVI can have fewer, and so bigger voxels. What is central and best vision for them, may be similar to how yours is, slightly to the side, although they will be unaware of this.

We can't emphasise enough how important it is to understand the CVI posterior parietal map.

CVI can = Fewer Bigger Voxels

Please take time to practice these exercises, and get a real sense of the difficulties where voxels are fewer and bigger. Always safely.

Checklist: Before moving on to the next lesson please check you have understood.

• With CVI the map may have fewer, so bigger voxels
• What bigger voxels entail in terms of causing difficulties with accurate movement using visual guidance
• What bigger voxels mean in terms of looking at something or for something

Next lesson 9c Dorsal (2) The Dynamic Map