The three lessons in this level (Level 9) have been written from many first-hand accounts shared by people who have a type of CVI where their posterior parietal lobes have been affected. This is therefore more subjective than our other lessons, but it is the subject of ongoing research. As the accounts have been so consistent, we wanted to share them, because the understanding they give us lets us make simple adjustments to help many people, especially those unable to communicate their challenges, as is the case for young children and those with limited communication skills.
A three dimensional invisible but vision based map filling the space around your body, allowing you to reach for things and move among things with accuracy, which is greatest at the centre of the map.
Through lessons 1c (Posterior Parietal Lobes), 3f (The 3rd Dimension) and 7b (Optic Ataxia) we have developed our explanation of this complex brain process. In this lesson it is assumed these previous lessons have been understood. Please take a few minutes to re-read them, and review further if needed.
We have devoted a lot of time to trying to explain this mapping process - but what exactly is it?
To begin with, we are going to try to help you understand your own map.
Finding Your Map
You can't see your map, you aren't even consciously aware of it, so understanding what it is, is difficult, but we are going to try to show you.
Look around you. Without the map, your visual world would be like a giant photograph of what you are looking at. Right now:
You know this because of your map!
Take an empty cup with a handle, and place it in front of you. Hold it by the handle, then close your eyes and bring it up to your mouth, as if to drink from it (please only use an empty cup for this). How close were you? Maybe you hit your chin?
Now do it with your eyes open. Your guidance of reach was much more accurate. With your eyes open, your brain places the cup at the correct distance, and helps you visually guide it as you bring it towards your mouth.
That's the map.
It is mainly fed by vision, and added to by touch and the soundscape, so when you close your eyes, it's much less 'there'. It is also less there where you have visual field loss (see Level 6), but now we have travelled further along our production line of visual processes and have reached the posterior parietal lobes where this more complicated type of vision is added.
The map can only map what it is fed!
Your map is non-conscious and invisible, and understanding non-conscious invisible things is tricky - because you can't see them and you don't know they are there!
In the video part of this lesson Gordon Dutton explained that the map can be thought of as using units called voxels.
The idea of voxels as an analogy can be helpful for some...so what is a voxel?
A voxel is a three dimensional pixel, or a pixel with volume.
So what's a pixel?
So looking back at our visual acuity lesson (lesson 5b), with typical visual acuity (6/6 or 20/20 or 0.0 logMAR), you should be able to clearly see things that are 0.1mm across and bigger. Let's call that 0.1mm a bit, the smallest bit of anything you can see clearly. There are 100 bits in a centimetre, or 10,000 in a square centimetre. So if you are reading this on a 20cm by 20cm screen, that's 4 million bits. So if your screen was 20cm by 20cm and 4 mega (million) pixels, it would be perfect for you. Fewer pixels on your screen and the image would look less life-like.
A voxel, is a pixel with volume (the vo is for volume).
But these are all terms relating to technology (voxels are used to create the 3D effect in 3D cinema) - how does this relate to the map your mind creates?
Imagine you are wearing an invisible suit that covers you from the top of your head to your toes. That invisible suit extends out in front of you and is made up of the invisible little blocks we are calling voxels. In the middle the voxels are smallest, tiny! So tiny you can thread a needle. But very quickly as you move to the side they get bigger. The straw in the bottle exercise (below) demonstrates this.
Your voxel suit moves with you, wherever you go, but only covers what you can see, and roughly what you remember for a few seconds - so what is behind you only gets a remembered voxel map for a few seconds, so you can step back if need be.
When walking, the voxels in front of you down by your feet help you walk safely, so as you approach a step it is correctly placed (by the map) so you know when to step up (or down) and thus not trip. That's your map, down supporting your walking, as it did higher up to support your reaching for a cup in the exercise above.
To move your map - which in a sense is a full body suit - you have to move your body. The map is not like a head-band or pair of glasses that moves when you move your head and eyes. Although it is visually fed, the map moves when you - and that is your body - moves. It is body centred (or body centric).
Remember our production line - here is where everything starts to come together.
Across your visual field, there is greatest detail in the centre, and that detail is sent to the posterior parietal lobes, so the map has smaller voxels to match the smaller pixels - so not only can you see the needle, but you can thread it. Areas more to the side, or the top and bottom areas of your visual field are less clear because they are made up of bigger voxels.
So, the map can only work with what it gets - reduced visual acuity means bigger pixels so bigger voxels.
But, a person can have typical visual acuity and no visual field deficit, and still have issues with the map. We think this is central to the difficulties many affected by CVI face.
Why is the map important to understand?
Because if the depth map is not processing accurately, then there is a knock-on effect to an enormous network of processes, and it is how these networked processes perform differently, often because of the deficient depth map, that creates the complex profile of CVI for many, including:
All of these can go back to that map. We need you to understand the altered map of people with CVI, but to do that, you first have to understand the map, which we appreciate is difficult as you can't see it and you aren't aware of it.
Place a narrow necked bottle on a table and sit on a chair in front of it, so the bottle is central to your position, like in the image above if you were sitting in the chair. Remember this position, looking straight ahead at the bottle, we will call this your original position.
Now, using a straw or something similarly long and thin, like a pen or pencil, keeping your position still and looking straight ahead, put the straw into the neck of the bottle.
Now move the bottle to the side, and keeping your original position (so looking and facing ahead, where the bottle originally was, not where it is now, so don't turn your body or head and eyes), and try to put the straw in again. It is much more difficult - this is because the voxels at the side are much bigger, meaning your accuracy is reduced.
Moving the bottle even further to the side, again from your original position without turning, try to get the straw into the neck of the bottle - it is almost impossible.
Sitting at the table, just think about your suit of voxels, all the tiny invisible building bricks, getting bigger to the side, and smaller in the centre. That is your depth map, wherever you go, it goes.
Before moving on to the next lesson, where we explain the difficulties due to the map being less accurate, take a couple of days to really think about your own depth map, and think about some of the following, considering all the voxels your mind has accurately placed, so you can:
The list does not have an end, and it is all possible because of your depth map. It is incredible, if it is working well!
Your depth map keeps you safe in endless ways, particularly helping you avoid:
The map in itself is not a cerebral visual impairment, it is a brain process that places everything you can see, in the right place in space in relation to your body. Many of the challenges of CVI can only be explained with an understanding of this process, both how it works typically, and how it works when atypical. In this lesson, we are explaining the typical map.
Imagine you're the pilot in the cockpit of an aeroplane flying through clouds. Your control panels that help you guide your plane include a radar to seek out hazards. This radar doesn't 'look out' from the cockpit. Instead it creates a picture, which maps the position of the plane in relation to the locations of everything around it.
Now allow yourself to move from the cockpit back into your own mind and body. When your eyes and head turn to left and right with respect to your body, the pictures they collect and assemble are mapped in relation to your body, just like the radar does for the plane.
Let us imagine we are going to do some sewing. First, we need to locate and reach for, then pick up our needle and thread, so we move our 'mobile search unit' that is our head and eyes, to find, then map and locate the needle and thread. The location of the needle and thread is mapped in relation to our body, so we can now move our arms, hands and fingers to the correct position to pick them up.
Think about taking a walk over very uneven ground, maybe a rocky terrain or through woodland. Our head stays steady as we negotiate the uneven surfaces, as it is stabilised by our balance system, which works with our vision, to rapidly search and map the ground ahead, again with respect to our body. This way we can move our legs and feet to match the nature of the ground in front of our foot, so we don't fall down steps or trip over obstacles.
Just like the radar on our aeroplane maps the position of the whole plane, our head and eyes provide the 'radar' to create this map within our mind so we can safely walk on the uneven ground.
Try (with another person for safety) getting a large piece of card, position it at neck height, then slightly tilt it up so you can no longer see the ground ahead when looking straight ahead. Walking becomes hazardous, because you have no map of the ground ahead to let you to relate the position of your body to the lay of the land or obstacles ahead (like many children with cerebral visual impairment).
You may be thinking that with the card you are blocking off what you can both see (with your occipital lobes) and map (in your parietal lobes), which is true, but your conscious mind receives the image from the occipital lobes via the temporal lobes to appreciate the countryside, while your unconscious mind does all the work in the background to perfectly position your feet and ensure your centre of gravity is correctly placed in relation to your body so you don't fall over, without even giving it a thought!
One last thing....
In the video part of this lesson Gordon Dutton explained:
So for quite a lot of people with cerebral visual impairment they have challenges being able to move accurately through three dimensional space...they can be thought of as being clumsy, but they're not, they just have bigger voxels because there are fewer fibres within the posterior parietal lobes of their brain mapping space...and that in turn can lead to them moving inaccurately.
That should make sense to you now, except one important word we have not explained yet, fibres.
The fibres Gordon Dutton is referring to are the individual nerve fibres in the brain that are formed into bundles, to create the pathways in the brain that operate the mind. This includes the pathways creating the invisible non conscious map of where things are around you so that you can move, and reach for things accurately.
Earlier we asked you to imagine it, although it is of course a very real part of the only world you know or have ever experience, the world created by your mind.
We will be returning to the fibres in the following two lessons and explaining a little more about them, what they are, what they do, why they are so important, and significantly for those wanting to understand CVI, what happens when there are fewer.
Checklist: Before moving on, please check that you have understood the following:
Next lesson Level 9b Dorsal (2) The Atypical Map
Further reading is not needed to proceed, but if interested you may enjoy reviewing lesson 6c Hemi Inattention and Neglect, where the body-centric nature of visual attention is explained, with an exercise to show a way to see how the map moves, using a mobile phone.
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