Making ‘Peripheral Vision’ Accessible

I’ve tried to make this blog accessible but it turns out I’ve not shone! I knew I kept forgetting the figure descriptions but I fouled up on something else as well. Here’s a demo, if you are sighted.

WRONG: The American Foundation for the blind have some very useful guidelines on blog accessibility.

WRONG RIGHT?: The American Foundation for the blind have some very useful guidelines on blog accessibility.

See the difference? If you have low vision then a new tab opening can be confusing. If you are reliant on screen reader software you most likely will not know it is there. If the tab remains the same you can just use the browser’s back button.

And now below I’ve added the missing figure descriptions. If you use a screen reader you should hear them. If you’ve not got a screen reader it’s a big insight to try one. I’ll put some ideas about free screen readers in another post, along with what I did for the figure descriptions, it turns out I was doing them wrong before!

I should add that WordPress have thought a lot about accessibility and the overall site has done a very good job in my view.

Peripheral vision is usually on the margins of our consciousness. Come with me on a short exploration of a little of what I’ve recently learned about it.

I, you, we focus on what’s in front of our noses but seldom pause the reflect that we can always see stuff off to the side as well.

When gazing at a point ahead (‘fixation’) you know a lot about things to the left and right. They are not in focus, but you are aware of them and you know there is a person, or a building, or a railway platform edge. Or the next few words on the line you are reading.

This is the visual field, which extends horizontal to about 90 degrees left and right. One of the first people to consider how to measure it was Leonardo Da Vinci – as I was startled to find. His diagram is below.

The drawing shows a circle representing an eye with its lens at top. From the lens radiate lines at equal spacing from far left to far right

From Wikimedia

Leonardo’s drawing considers only one eye (and the extreme angles are a bit of an overestimate!) but if we have two eyes the visual field is more complicated. There’s a region to far left that cannot be seen from the right eye because of the nose in the way, and similarly on the right. My sketch, possibly less artistic than Leonardo’s, shows what I mean.
A semicircle. A sector on the left is labelled left-eye only, the large central section is labelled both eyes, a section on the right is labelled right eye only

We can look at this a different way. The next sketch separates the visual field of each eye.
Ketch shows each eye's vision as a circle, with crescents added on at far left and right to represent the regions only visible from one eye

Notice the new-moon shaped parts one the extreme left and right? They are the ‘temporal crescents’, called that because of their shape. More about these later.

If the visual cortex is damaged the visual field will be affected. A common cause is stroke but a blow to the head or a tumor may also be responsible.

Often the result of such damage is half that the visual field is lost in both eyes, on the same side in each case, left or right. This is homonymous (the same) hemianopsia (half-blindness). The next sketch illustrates this.
Drawing shows entire right-hand side of vision missing in both eyes

Why is the loss half of both eyes, not all of one? Because half the nerves cross over between the eyes and the visual cortex. From both eyes there are two bundles. The bundles representing the right-hand side of the visual field in each eye both go to the back of the brain on the left-hand side. In a similar way both of the bundles representing the left-hand field go to the back right of the brain.

I had a stroke so lost right-hand peripheral vision – except that, and this is is important to me – there has been some recovery so that now I am aware of objects far enough to the right and my visual field now looks like this.
Drawing shows rcrescent of far right peripheral vision has been spared

This sparing of the visual crescent is fairly rare. The part of the brain concerned is just next door but in some cases gets damaged less and can recover. I am very, very luck to have regained this far right vision because it helps me not to collide with objects or people and could easily be a life-saver.

Finally the figures in this short article need descriptions, for anyone with low vision. I need to write them and when I have I’ll post a second version in which they are included.

I don’t know how many blind ophthalmologists there are but some of the textbooks I’ve glanced over will be very hard for them if there are no figure descriptions!


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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3 Responses to Making ‘Peripheral Vision’ Accessible

  1. Interesting. I use WordPress but have difficulty with the pale blu colour and don’t seem to have found the right way to enlarge some of the menues and stats pages. Any suggestions? I have Wet Maculat Degeneration and not much ventral,vision.

  2. Ooh that’s a good question, is WordPress accessible for its bloggers, as opposed to the readers of the blogs?

    I tried (Firefox on Mac) the effect of Command + and Command – to make it all bigger and smaller (Command would be Control in Windows). I can see the menus have been designed so when the page is enlarged the layout is modified but I don’t know how easy it is to use. I find the WordPress admin pages a little confusing anyway, I’m not always sure where to find things.

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