For The Eye Which is an Extension of the Brain

Two seated stick figures seated. Figure at right says "you have homonymous hemiaopsia", one at left thinks "?". One at right continues "The most likely cause is a lesion at the back of the brain". Figure at left thinks "How's she know where?"
This was a conversation between me and the specialist, on the afternoon following the stroke. I didn’t catch the name “homonymous hemianopsia” and had to look that up later. It took me a few days before being certain I’d got it right. I’d never heard the expression before in all my life.

But I understood I’d lost the right-hand vision in both eyes.

Is it in his eyes?

Back at my own doctor’s there had been a suggestion that my vision loss might be due to retinal detachment “And if so it’s something that can be fixed.”

In the waiting room at the eye clinic I chatted with a lady of about my own age who did indeed have a detached retinal, and it was fixed – she was called and within minutes the operation was over and done with, and she said goodbye. I was sorry she had gone, but for a while I felt quite hopeful about my own case -“It’s sure to be that, it won’t take long. And then I can go home!”

The specialist at the eye clinic was very logical though. She pointed out something that my own doctor hadn’t realised, or perhaps felt too sympathetic to mention, I shall never know.

It’s highly unlikely that retinal detachment would simultaneously affect both eyes, and in exactly the same way.

So it must be brain, not eye damage. The area which processes the information my eyes are sending about the right-hand side of my world has been knocked out. And that’s a funny feeling if you think about it. Both eyes are sending stuff that’s not getting through.

That’s where it is!

I understood the basic principle of the eye itself, as a sort of camera, and that it sent signals “to the brain” where those signals were processed in some way. But when the ophthalmologist spoke about the back of the brain I innocently thought “Why not the front??”. Nearer the eyes you see.

So as well as finding out what was actually wrong with me and what the future would be (I was looking forward to it) I wondered about how the eyes communicates with the brain.

From eye to brain

There are hundreds of diagrams of the human visual pathway out there but I’ve tried to draw my own, in a way I can understand and which I hope will make sense to you too. I’ve only used two unfamiliar technical terms and I’ll explain them after the picture (which I know owes something to Hunkin).If a viewer gazed at a test chart with symbols Cross, Triangle above Circe, Square, the images of these on the retinal of the two eyes would be reversed and inverted, Square, Circle, Triangle, Cross. From each eye the optic nerve fibres divide into left and right, and the fibres from the left of the right eye cross those from the right of the left eye. This is called the chasm. The crossing of the fibres mean that the signals from the right-hand side of both eyes can join up. Similarly for the left-hand side of both eyes. Information from the two left-hand sides is directed to the left LGN (term explained in the text). This represents the view of the right-hand side of the world, because the image on the retinal is reversed. Similarly information from the right-hand side of both eyes arrives at the right LGN. From each LGN the visual pathways split again, into one path carrying information about the top of the visual field and another one carrying information about the bottom. Finally the pathways reunite. All information about the right-hand side of the world arrives at the left visual cortex at the back of the brain, and information about the left visual field at the right visual cortex.

The two terms that I had never heard of before reading all this up are the “Chiasm” and the “Lateral Geniculate Nucleus”. First the chiasm. That is the crossing-over of half of the optic nerve from each eye. The name comes from the Greek letter Chi which looks like as X. Another word for this crossing is “decussation”, which is ultimately derived from the Latin for ten: think X in Roman numerals.

The crossing of the visual pathways allows all visual information from the right-hand side to be processed on the same (left) side of the brain that controls movement of the right-hand side of the body, and similarly for the right-hand side of the brain (which controls the left-hand side of the body). Why things are crossed this way is not clear: it must provide some sort of advantage, but exactly what is still far from explained.

As for the lateral geniculate nuclei, although they are highly complex structures, for the purposes of our explanation you can think of them just as relay stations.

Sight and stroke

What has all this got to do with vision loss in stroke survivors? Well the effect of a stroke on the field of vision can have many different forms. I have lost most of the right-hand field in both eyes. If O means vision and X blindness, my pattern is this
OX OX (called “homonymous”)

At first I thought that all blindness caused by stroke would be the same, and it took a lot of reading around before I understood that it could take many forms.

Some people have this pattern of loss
XO OX (called “bi-temporal”)

And quite a few lose, not half (“hemianopia”), but a quarter of their vision (“quadrantanopia”), and it may be upper or lower.

These different configurations of field loss are associated with damage to specific parts of the visual pathway, and in fact the symptoms are like a sort of window into the brain, and observations of hemianopia contributed to the unfolding understanding of the visual system. From knowing what vision I had lost, the specialist who saw me was able to predict a lesion – damage – at the left-hand rear of the brain. And she was right, as the brain scan confirmed. And it’s not hard to work out what part of the visual pathway would have to be damaged to cause bi-temporal loss for example, or quadrantanopia.

Why did I write this?

Because I knew nothing about how stroke could affect vision. My understanding could hardly have been vaguer. Vision loss is an eye problem in most people’s view. But now I found it can be a brain problem too, and looked for answers to questions. And answers are quite hard to get. How many people have hemianopia – in the UK, or any country, or the world? How does it affect them? – difficulty in reading, in navigating about, and in locating objects are typical – but every partially-sighted person is different in their own way. And how can those with visual field loss generally, not just from stroke, be helped and assisted? And much more. All this I want to learn, and am hopeful of doing so, and of contributing. This is part of my pathway.


Two scientists were named in the making of this post but suffered no harm.


XKCD and Tim Hunkin for inspiration.


About partialinsight

One evening I had a stroke. Half my sight vanished overnight. Adapting made me grasp how amazing the visual system and brain are. It also taught me to understand disability completely differently and I'm grateful for the lesson.
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