The word hemianopia comes from the Greek words:
So, how does someone with hemianopia see differently from someone with typical vision?
With hemianopia, like typical sight, the visual information is received through both eyes.
You may have seen the word homonymous with hemianopia. Homonymous is from the Greek word homonymous meaning 'to be the same'. In the context of Homonymous Hemianopia, homonymous refers to the visual field loss being the same from both eyes.
The visual information is processed and sent initially to the back of the brain (the occipital lobes lesson 1b), to be processed to create the picture you see, and from there along different pathways for further processing (lesson 3j).
What if the occipital lobes are not working typically?
Imagine you are walking in beautiful open countryside, somewhere like the image below:
Let us turn this image of what we are looking at into a clock...
The occipital lobes at the back of the brain process the visual information the opposite way around.
The image below shows the occipital lobes in red, as they would be seen if someone was standing behind you looking at the back of your head.
The left occipital lobe creates the picture (using the visual information from both eyes) that you see of the right side of your visual field.
The right occipital lobe creates the picture (using the visual information from both eyes) you see of the left side of your visual field.
So, if something happened to one of the occipital lobes, let us say the right occipital lobe, it will affect the left side of the image of what you are looking at.
Right occipital lobe damage can mean that the left side of what is seen is absent. This is called a left hemianopia, affecting the left visual field, but it is due to damage to the right occipital lobe.
So the brain is only able to process half the image, like this:
But this is not how vision is experienced for everyone with left hemianopia. The image can be experienced as central, and often the absent half is forgotten.
So what is experienced, and how it is different to typical vision may be something like these images below:
The diagram below shows the different combinations of visual field loss due to damage to the occipital lobes. The diagram shows, using a clock, how the quarters of what you are looking at, relate to the opposite quarters in the occipital lobes.
The main cause of hemianopia is injury due to stroke (blockage of the blood supply to the brain), which can occur at any time in life, including in a developing foetus in the womb. Other causes including bleeds in the brain (haemorrhage) and tumours.
These injuries are not always confined to the occipital lobes.
In some cases other parts of the brain can be affected as well.
Some people with hemianopia may also have difficulties interpreting what they see.
While some people who have had a stroke, making one side of the body weak, may not see to the side affected by the stroke.
Hemianopia is relatively straight forward to identify, measure and diagnose using standard visual field tests. Further difficulties with visual processing due to the wider injury may be less easy to pick up. Our future lessons (in Levels 7 and 8) covering cerebral visual impairments from the parietal and temporal lobes may prove helpful for those with brain injury causing hemianopia extending beyond the occipital lobes.
Support if required needs to include allowing for the fact that not only does the person not see on one side, they might not be aware that they are not seeing on that side.
A person with hemianopia, due to not seeing things can:
Hemianopia can affect different people very differently. Where a child is born with hemianopia or has acquired it soon after birth or in early childhood, they may appear to cope remarkably well, even appearing to see just as well in the half of their visual field without vision. One suggestion is that due to the anatomy of the most common causes of hemianopia in young children, the middle temporal area of the brain (lesson 1d), working with reflex vision (lesson 3h), with the optimal plasticity of the young developing brain, are able to sort of fill in the missing gap with a different type of vision that is so effective, that such people can go through their lives unaware they have hemianopia.
Where hemianopia is acquired, most commonly due to stroke in people over the age of fifty, this development of regaining conscious vision in the absent visual field is less common, although there are more and more reported cases. The brain is most 'plastic' in childhood, but the brain never loses its capacity to grow and learn, and there may be great opportunities in the areas of habilitation and rehabilitation due to hemianopia, to develop conscious visual awareness in the absent visual field. It is early days but every exciting!
It is very difficult to appreciate how hard it can be when something a typically sighted person takes for granted, simply doesn't exist for a person with hemianopia, because they can't see it.
Much of the support for hemianopia is around strategies to remind people of the other half of the visual field around them.
Examples of mishaps due to hemianopia we have heard of include:
These might at first sound amusing, but living with hemianopia can be stressful and embarrassing, and those unaffected can do an enormous amount to help, by learning and by understanding.
Before you move onto the next lesson, please check you understand:
Next lesson: Level 6b CVIs Visual Field Impairments - Blindsight
Further reading is not necessary to proceed, but if interested you may enjoy the following:
What Is CVI? Hemianopia
Nicola McDowell's Blog 6 - A World of Halves
Homonymous Hemianopia (New Vision in Absent Visual Field) Paper.
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