Think about what you are looking at, right now, these words and what else you can see around them, maybe a computer screen, or a window or wall with pictures. Now think about what is in the lower area of your visual field, maybe a table or desk you are working on. That visual information has passed through your eyes and is being processed...
...it travels along the optic nerve to a junction point called the optic chiasm, where it splits into two optic tracts. At the end of each optic tract is a relay station called the lateral geniculate body, and here the visual information is relayed into the optic radiations, which are bundles of fibres within each side of the brain. The optic radiations take the visual information to the occipital lobes.
The optic radiations pass through the posterior parietal lobes on their way to the occipital lobes. Here, often due to damage or injury, but not always, the optic radiations can be incomplete or even absent. This means that when the upper optic radiations have been affected, the visual information that reaches the occipital lobes where the picture is created (lessons 1b, 3b and 5b) is already 'reduced'. How much of it is reduced and to what degree varies from person to person, from simply being hazy down below, or absent where the feet would be seen when walking, to there being no vision at all below the midline when looking straight ahead.
So if looking at five children in a field (above), with a mild lower visual field impairment, the child at the bottom of the image, who would be in the lower part of the visual field, is seen slightly less clearly, but still there, maybe like below...
It is very mild, and this person may not have any issues, or even be aware that they have a lower visual field impairment, but occasionally trip up or down kerbs, especially when somewhere busy.
The most severe cases of lower visual field impairments can involve nothing being seen below the midline or eye line (below):
To explain the process a bit more, let us look at a box of chocolates, like the one below.
As the optic radiations carrying the visual information of the lower part of the visual field passes through damaged posterior parietal lobes, that are altered. In this example (image below) some visual information survives, but a significant degree of the clarity is lost, so that the bottom part of the image is now difficult to make sense of.
The occipital lobes will process the visual information they receives, and give it detail, colour and contrast, but they can only work with what they've got. If the bottom part of the visual information is degraded or absent, the occipital lobes can't patch this up or repair it.
For this example, let us assume that the occipital lobes are working well, and so have added the detail, colour and contrast to the picture they receive. Also, where the occipital lobes are working typically, all of the visual information in the visual field they receive, whether altered, degraded or not, will be processed further, along the dorsal stream, back to the posterior parietal lobes.
The optic radiations have already passed through the posterior parietal lobes from front to back on their way to the occipital lobes, and this is where the damage occurred that caused the visual information in the lower visual field to be reduced. Here we have the reason why lower visual field impairments are linked to posterior parietal visual difficulties (also called higher visual processing difficulties or dorsal stream dysfunction - the dorsal stream running from the occipital lobes back towards the front).
Many with a lower visual field impairment may have this condition alone with no other cerebral visual impairments. However many may also have posterior parietal visual processing difficulties. If damage to the posterior parietal lobes causes damage to the optic radiation, then when it comes to their (the posterior parietal lobes) actual job, of mapping the visual scene (lesson 3f) and giving visual attention (lesson 3g), it is certainly worth checking whether these functions are all working typically or not.
Our chocolates have got to the mental mapping stage of their visual journey.
Here, in the posterior parietal lobes, the chocolates are transformed from a picture (like a photo), to being accurately mapped, in three dimensional (3D) space (lesson 3f) so you can reach for them with accuracy. This means that if you see and like the look of the white chocolate, you can reach for it and easily pick it up.
But we have jumped a stage because...
We are aware that we see and recognise the white chocolate, and we know we can reach for it - this is conscious.
Yet we are not aware of how our mental map of what we see, guides our hand to the chocolate - this is not conscious
We have introduced the conscious (frontal) part of the brain in lesson 1f and in relation to vision in lesson 3k. In future lessons we will continue to explain more about how the many different brain processes, some conscious, some non conscious, work together to support you, and how CVI can affect these.
Here, we have three rows of delicious looking chocolates to choose from. The visual information in the lower visual field is not completely without visual attention, but what is there to see?
In these two images (above), with and without a lower visual field impairment, what is in the lower visual field might not be completely absent of visual information and attention, but it may be slightly or significantly reduced, or in some people, completely absent. Lower visual field impairments, like all CVI's are a spectrum, and vary in how much of the visual field they cover, and how severely altered the visual information is.
Note - we are going to assume the chocolates have been recognised (lesson 3i). We will explain more about the recognition process and how it is affected by CVIs in Level 8.
So, the visual information has reached the frontal lobes and you are deciding which chocolate you want.
With typical vision, you have a choice of twenty five chocolates (image above), but with a lower visual field impairment, it is only fifteen. The person with the lower visual field impairment actually has twenty five chocolates, but they may only be aware of fifteen.
So, the person with a lower visual field impairment may seem to have reduced frontal attention (lesson 3k), and it's true, they only have a choice of fifteen not twenty five chocolates, that is fewer, but the cause of this is further back, in this case much further back in the brain... the optic radiation were altered causing less to be mapped, meaning less to add visual attention to, meaning less chocolates to choose from.
This is just one simple example, and easy to show because the chocolates are in nice neat rows, but how might this affect a person's life, where everything low down is not seen at all or not seen so well?
Two eight year old boys, Alex and Drew (not their real names) were out together with their families. Both boys have different combinations of CVI. Both boys have a lower visual field impairment. Alex's lower visual field impairment is severe (with visual neglect as well). Drew's lower visual field impairment is less severe, and he doesn't have visual neglect.
Both boys were attracted to the temporary metal fencing in the image below.
Each metal panel of fencing was secured with a large, protruding concrete 'foot'.
Both boys, as they were feeling their way around the fence tripped over the foot (parents were there to stop them falling and help safely guide them).
Alex's mother explained that Alex would trip over every fence foot, and the same fence foot over and over again, he would never form a memory, the only way to stop him tripping over the fence foot would be to remove it.
Drew's mother explained that whilst Drew would probably trip over the fence foot once, he would then remember it and not trip over it again, and would also often be able to anticipate that there may be more 'fence feet' at other panel junctions and avoid them.
How are Alex and Drew's lower visual field impairments different?
Think of Alex and Drew standing side by side, looking at five children in a field.
Drew's map in the lower visual field is less detailed so there is less to attend to, so less is seen.
Alex's map in his lower visual field has no detail, so there is nothing to map or give visual attention to, therefore Alex not only never sees anything in his lower visual field, he never forms visual memories about his lower visual field - because there is nothing to form a memory with.
So for the fifth child in the picture, in the lower visual field, Drew might see them, but often wouldn't, especially when somewhere busy or noisy. Alex would never see them.
Going back to the fence-foot both boys tripped over...
For Drew, there was not enough visual information to be mapped to give attention to the fence foot to enable him to see and avoid it, however, once he tripped on it once, his reduced visual attention was enough to form a memory and avoid it in future. For Alex, there was no visual attention, nothing is mapped in his lower visual field (called visual neglect, explained in lesson 6c) so he has nothing to form a memory with.
Place both of your hands just above your lips, with your middle fingers touching, so your hands create a horizontal visual barrier, blocking the visual information beneath them (like the image below).
With someone to support you and keep you safe, walk around like this for a bit, and get a sense of how different it is - how do your feet feel? It is most effective if you try doing this somewhere you have not been before, but always take someone with you as a guide, because you may trip over things, bump into things or even fall down.
A lower visual field impairment can be higher than this (up to the horizontal midline), or lower.
A lower visual field impairment can be very mild, and hardly affect most elements of a person's life, but can make little things difficult. In children, it might be sports where a ball is low down, for example hockey or football. It can also make people seem just a little more clumsy in terms of tripping over things than is typical.
Optometrists and ophthalmologists do not assess this peripheral lower part of the visual field with standard tests, so they may not detect this frustrating difficulty.
A lower visual field impairment can be hazardous. For example, at the top of a staircase, your vestibular system ensuring the correct balance as you move, step by step, going down, heavily relies upon the visual information it is fed. Just try walking a few steps (safely, with a guide close for support) with your eyes closed - very quickly your balance is compromised, and that slightly 'woozy' feeling you experience, which can quickly turn to nausea if you don't stop, is a message from your brain that you are not safe and to stop whatever you are doing!
With a lower visual field impairment, as with many CVIs we have a particular challenge here, because the vestibular (or balance) system doesn't have an absence of information - it can have the wrong information to work with!
So, with this example, at the top of these escalators, your visual brain has told your vestibular system that the first step is right in front of you, which it is, but the actual step you are looking at and your brain is making the calculations for, to ensure safe movement is eight massive escalator steps down, although your brain may think it (until it has learned) that it is right in front of you! So to step onto the moving escalator, or down stairs, there may be conflicting calculations going on in your brain, making the actual movement unsafe, which could lead to hazardous falls.
So - SAFETY FIRST!
Ensure low obstacles are removed where necessary, including rugs that can be tripped over. If needed, ensure a guide is present, especially where there are stairs, including roadside kerbs that can easily be tripped down / up.
For mobility, there may be benefits for some in using a cane (or hiking sticks) for guidance. The cane could provide the absent visual information, in a tactile format, from the lower visual field through the hand in the upper visual field, where there may be sufficient attention to safely interpret the physical environment to allow more independent mobility.
For the more severely affected, like Alex who has visual neglect in his lower visual field, with motivational fun exercises, over time, we have come across children who have learnt to search for things lost, usually dropped, in their absent lower visual field.
If you are supporting someone with a lower visual field impairment, adjust your height to be the same as theirs, so typically lower for a child, then put your hands across your face as shown in the practical exercise above, and walk around every environment, at home, work, school, sports, play, holidays - everywhere they spend time, and see what is and is not likely to be seen, and take the necessary remedial action.
We have heard many accounts where people affected by a lower visual field impairment aren't aware of their own feet. One person described it to us as like 'walking on clouds'. This can affect walking, and a person with a lower visual field impairment may sometimes not seem very steady on their feet. This may be because they are not aware of their feet or the position of their feet when walking.
As you move through any environment, note how many obstacles are low down, everywhere. It is very easy to take for granted that others can see and avoid them as well as you may be able to.
Before moving on to the next lesson, please check you understand:
Next lesson Level 7a CVIs Movement and Dorsal - Dyskinetopsia
Further reading is not necessary to proceed, but interested you may find the following enjoyable:
What Is CVI? Lower Visual Field Impairment
Amelia's Great Climb, Case study following a girl with a lower visual field impairment who loved to climb
Katie's Reading Storyboard, how a lower visual field impairment affects reading
In rare cases, damage to the upper parts of the occipital lobes can also create a lower visual field impairment, in nature, this is like a hemianopia (lesson 6a).
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