This lesson is a little longer than the typical lessons. It is because we are explaining the very complex area of disorders of visual attention, the causes of a number of cerebral visual impairments.
The term visual hemi inattention comes from a combination of words of Greek & Latin origin meaning that attention is absent across half of the visual field.
Hemi inattention is part of a spectrum where visual attention is reduced or absent across half of the visual field. The most severe level is called visual neglect.
As explained in Level 1, particularly lesson 1f, our non-conscious brain feeds our conscious brain.
The frontal lobes of your brain can be thought of as your conscious control centre. Every moment of your waking life, your frontal lobes are using all the information they receive from other parts of the brain, to help make you aware of what you perceive. The pictures they are given may be poor quality, perhaps due to limited visual acuity, but they can only work with what they are given. The frontal lobes can't improve the picture quality, but they can use memory to support vision. A white wall-light switch against a surrounding white background may not be visible, but it can be created from memory, and imagined.
The total amount of information the frontal brain can handle is limited. So even when the visual brain gives the frontal lobes perfect pictures, it doesn't necessarily follow that what you will see is a perfect picture. If your brain is very busy, maybe you are worried about something or preoccupied, then this can limit (frontal) visual attention.
We all experience this. Having a conversation with someone on a mobile phone, or looking at a message when crossing the road, to your peril, can make you miss an approaching car. This is not due to a cerebral visual impairment, but is because the frontal brain has just exceeded its capacity. Something has to give. Visual attention gets reduced, meaning that less is seen.
In other words, extra demands on mental processing, like crowds, noise, thinking, feeling sad, or doing other tasks, and the depressant effects of alcohol and some medications - can decrease visual attention in all of us, (including people with CVI).
Without us knowing, the posterior parietal lobes are constantly mapping our surroundings in relation to our bodies (lesson 1c). When both the parietal lobes are not working well, this causes a type of CVI that means that fewer things are seen. This is called simultanagnostic vision, which was introduced in lesson 3g and will be explained further in lesson 7c. Simultanagnostic vision happens because there are fewer nerve cells to map the scene in both the posterior parietal lobes, probably with fewer nerve fibres to send this information to the frontal lobes. A future level called Dorsal (2) will explain this in more detail.
When the frontal lobes have too much to do, then even fewer things may be seen. The frontal lobes cannot see more items than the posterior parietal lobes can process and send to them.
They cannot improve on what they are given.
So, when the frontal lobes have too much to do, in addition to the challenges of reduced visual attention, for example when somewhere crowded, noisy etc...
...then even fewer things may be seen.
So, what is the difference between frontal reduced visual attention and posterior parietal reduced visual attention?
We need to remember our production line. The brain is full of numerous networks of processes, but there are also orders. Consider this diagram (below) from lesson 6b:
The posterior parietal (dorsal stream) visual challenges come before the frontal - they feed the frontal lobes.
We have explained that reduced visual attention from the frontal lobes can be temporary, for example when leaving a noisy crowded place, for somewhere quiet and peaceful, visual attention should improve. This is not a brain processing atypically, this is normal.
Reduced visual attention due to lack of capacity by the posterior parietal lobes is always due to the brain processing atypically. There can be variants of severity, affected by the environment and how the person is feeling, because people with CVI are still affected by the same frontal challenges everyone is affected by, but the underlying condition is always present. For some with this type of CVI, the impact of frontal challenges is much more severe than a typically sighted person might experience, for example walking in a very busy street might make a typically sighted person clumsy, but a person with CVI may even become unable to see anything. We will explain this in more detail in the lessons in the level Dorsal (2).
Hemi inattention is reduced visual attention due to the posterior parietal lobe on one side not working typically reducing visual attention on the opposite side.
So, in the best circumstances, for example feeling happy on a quiet open beach (image below), a person with left hemi inattention may see a bird flying on their left side, but five minutes later, in a car park, may walk straight into the path of a car on their left side.
But that doesn't make sense, does it?
In the car park, cars (usually) move slowly, more slowly than the bird, and a car is much bigger than a bird - so how is it that the bird is seen but not the car? Surely the car is the more visible of the two?
The answer is because in the car park the brain has to map many more things. This will always be the case for someone with posterior parietal reduced visual attention on one side (hemi inattention).
Another person (who does not have hemi-inattention) may be talking on their mobile phone and may also run out in front of a car, again unseen, because they are for example engrossed in their conversation - this is frontal reduced visual attention and is typically temporary.
Would the bird have been seen by the person with left hemi inattention in the car park? Almost certainly not.
We are aware that we are introducing many new concepts as these lessons develop and will be explaining all areas in more detail in future lessons. If you, or someone you know is affected by hemi inattention or neglect, we recommend you come back and re-read this lesson after learning more about the complex networks of brain processes we describe in the following lessons.
Visual Attention was introduced in lesson 3g. There are different cerebral visual impairments that are due to reduced visual attention, including (from future lessons) lower visual field impairment (lesson 6d) and simultanagnostic vision (lesson 7c).
We have used the example of a production line many times when explaining the visual brain, and 'final product' is what reaches the frontal lobes, for a person to make decisions, choices and actions. We are simplifying what is a very complicated process, but trying to explain that hemi inattention is different from hemianopia, and that whilst part of the same process, visual hemi inattention is also different from impairment of frontal visual attention.
In this lesson we are looking at the cerebral visual impairment where visual attention is reduced to one side or the other, called hemi inattention, which, at its most severe level, is called visual neglect.
So, what happens when your brain creates a picture of what you are looking at, but the visual attention to one side is less?
If you had reduced visual attention to the right of your visual field, so right visual hemi inattention, then when looking at the image above, the child on the left is likely to be easier to spot than the child on the right.
Your brain has successfully created the whole picture, so there is no issue with visual acuity or contrast or colour, and visual field testing may be normal (as visual field testing usually requires only two lights at a time to be seen, one in the centre and one to the side). Think of the above example of the person on the beach who could see the bird on the left where they had hemi inattention. Vision testing is in a perfect environment, a quite dark uncluttered space - quite the opposite to most of life!
The picture created in your occipital lobes is sent to the posterior parietal lobes where everything is mapped, as explained in lesson 3j on the visual pathways. With visual hemi inattention, the visual information is less accessible because the visual map on one side holds less information to give to the frontal lobes, so your attention on that side is reduced.
Someone with hemi inattention to one side, for example the right side, often needs to turn their body to the right side to become more aware of what is present on their right, because the map plotted in the posterior parietal lobes is with respect to the body (see body related, below).
With hemi inattention, the picture is all there, so the absence of visual awareness (meaning things are not seen) is not because only half the picture has been created, it is because the picture is not so well mapped.
So, simply turning your head to the right will not help you see the child on the right if you are not aware they are there due to hemi inattention. To move your visual attention you need to move your body. Try this...
If you have a mobile phone with a camera, or a camera with a screen at the back (view finder screen), set the phone to camera so the screen is showing a live duplication of what you are looking at.
Now (this might seem a little odd!), hold the camera with both hands in front of your tummy, so you can still see the screen, with both of your arms pressed into your sides - so that your arms and hands can't move, like in the image below...
So, to move the picture on your camera or camera phone, you have to physically move your body. That movement, to move the picture on your phone is the body related, also called body-centric movement needed to move your visual attention.
One difficulty we have heard a number of times is people leaving half of their dinner uneaten because they don't have visual attention to that side of their plate - so as far as they are concerned, the other half of their dinner might not even exist - even though the present half plate might seem like a very obvious clue!
It's fascinating to watch someone with left inattention, searching to their left with their head and eyes, yet not seeing the thing unless the body it rotated sufficiently to the left!
Take your body centred camera and stand by a plate of food, and position the camera so that only the left half of the plate is visible in the camera screen...
Keeping your camera in the position we have explained, with your arms immobile - how do you get the rest of the dinner into the view finder? The same question is, how do you shift visual attention so that the rest of the dinner is where there is visual attention? Moving your head to the right isn't going to do anything is it? That would help with hemianopia, but not hemi inattention.
You have to slightly shift your body around, a body turn, approximately half a right-angle, and...
There's the rest of dinner! And now you know what body related, sometimes called body-centric is, and body related movement, as opposed to head and eye related movement.
So, back to our children in the field...
To see the child on the right on the side where there is hemi inattention, the person needs to turn their body - just like you did holding your camera with the plate of food, and...
...there they are - like magic, they've appeared! The child on the left hasn't disappeared, you already knew they were there, and now you are aware of the second child. Just like the salad on the dinner plate doesn't disappear because you have moved your visual attention to be aware of the chicken.
There is a range of visual attention deficit on one side, starting with mild, where something might not be so easily seen on one side, to the most severe, where nothing is seen on that side. The most severe is called visual neglect.
So the conditions ranging in severity between hemi-inattention to visual neglect can be seen as a spectrum.
Whilst a visual field deficit to one side might create similar difficulties, there are two very important differences to understand:
It is possible to have both hemianopia and hemi inattention affecting the same side. Here, both a head and body turn can be helpful. This will be explained in more detail in a future lesson on CVI combinations.
Where the right parietal lobe is affected, this is typically more severe, often entailing complete loss of visual attention on the left side with additional partial visual attention loss on the right side too.
Where the left parietal lobe is affected the attention loss is typically less severe and limited to the right side.
Hemi inattention can be subtle, for example when drawing a picture, one side may be slightly less accurate.
There have only been a few recorded cases of competitive visual inattention, but we are including it because we think it may be more common than is currently known, but easily missed. To explain...
With competing visual inattention, if one child on the left popped up (above), they would be seen, but...also, if a child on the right popped up (below), they would be seen.
So, on their own, each child is seen, one the left and the right - but what happens if they both pop up (below)?
Then only one child is seen, because there is now a competition between the two children for visual attention, and the side with the most visual attention wins, so when both children pop up, only the child on one side is seen (below).
If this condition is unknown, it may be very difficult to identify. Where sometimes things are seen on one side, but sometimes not, a visual cause of the difficulties may be disregarded and other reasons sought, potentially clumsiness, a lack of concentration or attention issues.
We know from many accounts that CVIs can affect the other senses, particularly hearing and how sounds are processed alongside visual processing, and how they affect each other (this will be covered in a future lesson, link below for more information).
Go back to our exercise holding the camera for body centric movements. Where someone does not have much control over their movement, for example a more profoundly affected person who uses a wheelchair. They rely upon another person to move for them, for tasks that are in front of them, remember the absent half, and turning them in their chair is just as effective in moving visual attention.
Anything where a full visual field is needed, including reading, using a computer, watching television, cinema etc, is going to be affected by hemi inattention, and things won't always make sense. A bit like if you start to read a book several chapters in, and there are parts of the story that just don't make sense, because it is assumed you have all the information, so you start to guess, or maybe give up on the book. Things won't make sense and it won't be obvious why, so people supporting those affected need to be extremely mindful of all environments, and help everyone understand how to help them understand - a little body rotation, as we describe (so that the midline of the body is to the side of what needs to be seen) is often all it takes!
We have already discussed people only eating half of their dinner, but this does seem difficult for many to understand. Even if you are not aware of half of the dinner on your plate, you have eaten many plates of dinner before and know there are two sides, a left side and a right side, and both have food on them. It doesn't make sense!
Try to imagine sitting in a restaurant and eating your dinner, then when you've finished and cleared your plate, someone asked why you hadn't eaten all of your dinner - but you didn't understand, your plate was full and now it is empty. Then you are told that the rest of your dinner is still behind you - that's why you didn't eat it, not only did you not see it, you had no idea it was there! Is it reasonable for people to expect you to eat something you don't know exists? This is how it has been described to us, and other examples include:
To understand something not being seen is relatively easy, we can close our eyes and things that were seen, are not any more. To understand reduced visual attention is much more difficult, and it is easy to become impatient with someone who constantly makes what seem like simple obvious errors. For example if a teacher crosses the room to the side where there is hemi inattention for a child in the class, they can become irate because the child is ignoring them - and it is very hard to explain that when they existed only a minute ago, by moving, in terms of how that child experienced the world, they are no longer there!
Before moving on to the next lesson, please check you understand:
Next lesson: Level 6d CVIs Visual Field Impairments - Lower Visual Field Impairment
Further reading is not necessary to proceed, but if interested you may enjoy:
What Is CVI? Visual Neglect & Visual Inattention
Case Study 2 Amelia's Great Climb (child with hemi inattention)
Cerebral Auditory Impairment, considering the altered parallel processes of visual and hearing where CVIs are present.
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