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.
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.
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.
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.
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!