- Fingers and thumbs keep moving
Peripheral vision is what you can see to the far left and far right. Leonardo da Vinci was an early investigator and drew a famous sketch.
The angle Leonardo shows is over 90 degrees on either side or more than half a circle altogether. This may be a bit optimistic and 90 degrees each way might be more accurate for humans, although I don’t know what you might be able to see. Many animals have wider peripheral vision, even those such as cats which have forward-facing eyes like us, so there may be considerable variability amongst humans.
Leonardo’s drawing also show just one eye and a single eye cannot of course see a panorama of 180 degrees because your nose gets in the way. Even if your nose were removed I don’t think a single eye could see such a wide angle, because the part of the brain that processes vision doesn’t expect the left eye to have far right vision, or the right eye to be able to see on the far left.
Stand up and look at an object directly in front of you. It’s very important to keep your eyes on this and not move them to the left or right. This takes a bit of concentration I’ve found.
Now shake you hands about a bit, breathe in deeply, and then extend your arms at shoulder height, as wide apart as possible, and push them back – like an angel, or like Leonardo’s even more famous drawing, the Vitruvian man scribed in a circle, with outstretched arms.
Here’s where the self-control comes it. Please try to keep looking straight ahead and don’t move your eyes or head. If you have been to the opticians and had a field test – the one way you have to activate the clicker each time you see a small light flash – then you know how tempting it is to move your eyes towards where you are not supposed to be looking.
At the optometrist’s you rest your chin on a platform, which stops the head movement, and I believe the apparatus detects eye flicker and compensates for it. But in our experiment the only control mechanism is your will-power.
Can you see your hands? I can’t in this position. But now wiggle your index fingers. Do you see… something out there? If not bring your hands in just a little and waggle again. Continue and at some point you will be able to detect movement but little more. Our far peripheral vision is good at spotting movement (think sabre-toothed tiger leaping) but can’t see very distinctly otherwise. Keep those eyes fixed ahead and try holding up a finger, and see if you can tell the when it’s raised and stationary rather than moving. If not, bring you hands a little further forward, until you can.
At the position where you can spot a raised finger try a second test, the “how many fingers am I holding up’ one.
Our abilities to distinguish between objects in peripheral vision is rather weak. At this point you may not be able to tell whether it’s one or two fingers that are extended.
This inability to resolve individual objects (‘crowding’) out on the edges of vision is partly to do with the retina having far fewer visual receptor cells in its outer regions, but also because in peripheral vision the input from several receptors is combined before being sent to the brain. Understanding its effects is of practical importance to people who present information in a format that requires a large angle of vision, such as maps of underground rail systems.
Bring your hands yet closer together and you will reach a stage where you can clearly count one or two fingers. However you still won’t be able to resolve finer detail; for example you cannot read normal text in peripheral vision, even when it is quite large and quite near the centre. (You may like to experiment by writing letters on your fingers with a felt tip.)
But your peripheral sight has great importance for reading. Unconsciously you use it to scan the text you are reading, and use information about where the words are on the line and where the line ends to make you eye movements very efficient. Those who lose peripheral vision from illness or accident often have reading problems because of this disruption to reading patterns, and may be become alexic – acquired loss of reading ability.
Finally bring your hands together in to meet in your central vision. You can now focus on them. This small central region is called foveal vision. Only in foveal vision do we have full visual acuity, so we can read down to the 20/20 row on the eye chart, assuming we have our spectacles on.
Foveal vision is very narrow. We often have the illusion, for that is what it is, that we can see the whole world in sharp focus. In fact only a tiny fraction is in focus at any moment. But our eyes flit about continually and allow us to build up a vivid mental picture from what is only fragmentary information. An illusion in fact.
Just how small is foveal vision? About 1 or 2 degrees. To understand what this means you need a thumb. Hold it directly in front of you at arm’s length and focus on it. That’s roughly it. Everything to either side is a bit less sharp.
But don’t think peripheral vision is some sort of second-class citizen. On the contrary. Each part of our visual field is a specialist and they work together. Peripheral vision hints at where we should fix our gaze, and foveal information then delivers the detailed information.