Understanding Vision
Aspects of Vision 1: Field of Vision
I expect most people would have expected me to begin by discussing detail vision, and the question of whether our vision is sharp enough to see small things. That will come (it’s next), but I think an understanding of how our overall vision works is important before we get to that particular aspect of our sight.
The total extent of our vision — from left to right, top to bottom — is known as our visual field.
Most people don’t realise that most of our vision doesn’t see detail at all well. We feel like we are seeing good detail everywhere we can see — it’s clear straight ahead, it’s clear up there, it’s clear over there. But in fact only one small part of our visual field can see fine detail — the very centre part, which corresponds to the macula (so we call it the macular field). And there is a spot of extraordinarily fine detail vision right in the middle of the macular field, corresponding to the fovea (so we call that area the — you guessed it — foveal field).
The reason it seems like our entire vision is clear is that whenever we pay attention to something, we instinctively turn our eyes so that the image of that thing lands on our macula, and so the part we most want to see lands on our fovea. That’s what we mean when we say we are ‘looking at’ something.
Macula? Or Macular?
The macula is a thing — a spot on our retina that has a higher density of photoreceptors, and so allows us to see detail vision.
Macular is a word that means ‘relating to the macula’, just like ‘visual’ means ‘relating to vision.’ So macular degeneration means there is degeneration of a type that relates specifically to the macula. Macular field means the part of your vision that is seen by your macula.
Similarly, the fovea is a thing — a tiny spot right in the middle of the macula — and foveal means ‘relating to the fovea.’
Fast Vision — just the highlights, and right now.
The job of most of our visual field is to keep us alive, and help us get around safely in the world. It’s a very fast system, that provides us with a sketchbook version of the world, covering only the bare essentials:
- Where are things around us?
- Are there things moving?
- Are there things changing?
The emphasis is very much on speed, because that’s what we need for survival. If something leaps out at us, we don’t need to know whether it’s a lion or a tiger — we just need to dodge in time. Speed means keeping the information content low, so most of the general visual field has lots of nerve processing that helps us detect things that are moving or changing, and determining where everything is in relationship to us.
In essence, the general field tells us where everything around us is, but not what everything is.
Slow Vision — filling in the interesting bits.
The macular field has the job of filling in the detail that lets us know what the things are.The job of the broader visual field is to find the things, so the brain knows how to guide the macular field to the points of interest.
Once our brains receive the fast outline of the world around us, our subconscious picks out the bits that are most interesting, and shifts our attention towards them. Our attentional spotlight sends commands to the eye muscles to turn the eyes so that the image of that thing lands on our macula — that is, it tells our eyes to ‘look at it.’
The macula sees the finer detail, the shades, the colours, the texture. In effect, it paints in all the richness and subtleties on top of the sketch outline provided by the fast visual system.
All this detail is a lot of data. So much so that about half of all the nerve fibres travelling down the optic nerve and back to the brain are from the macula, despite the fact that it’s only a very small part of our vision. If we got that level of detail from our entire field of view, our optic nerves would have to be massive, and our brains would have to be vastly larger to keep up with the crazy amounts of visual information coming in.
Attentional Spotlight
Our attention is a high-level brain function. What are we concentrating on? If we’re paying attention to something around us, it’s like we have a torch (flashlight) that we shine on it. It might be an intense focus of attention on one thing (like, “Is that thing a bee, or a wasp?”), or it might be a more diffuse focus on an area. Either way, our visual system is wired to turn our eyes (specifically, the macula) towards whatever our attention is on. If it’s very strong attention, it will point directly at one thing and stay there. If it’s a bit more diffuse, our gaze wanders around a bit, sampling the details of one area and then the next.
If our attention is directed more internally (“I wonder what’s for dinner?”), our attentional spotlight is not on the outside world, but our gaze tends to wander around automatically, with our macular field idly sampling the detail of the world around us. The information’s still being processed, and our attention might be caught by something we ‘notice’ (“Oh! Is that Rob over there?”).
If our attention is very strongly focused internally (“What was my password again?” “Should I be looking for a new job?”), our gaze tends to stop wandering, as if our subconscious is saying that it doesn’t want any interruption. To others, we are just staring into space.
It is possible to put your attention on something and resist the urge to look directly at it, but it takes real effort. An example is when a soccer player is lining up a penalty, and doesn’t want to show the goalkeeper with their gaze where they are going to kick the ball. And in eye care, if you have a visual field test, you will be asked to pay attention to something (a target) and keep your gaze on it while saying when you see other things appear in other parts of your vision without looking at them — it only takes a few minutes, but it’s quite fatiguing to resist looking around, and some people can’t manage it at all.
Gaze Tracking and Attention
Gaze tracking devices can tell exactly where we are looking, and therefore exactly what we are paying attention to. An example is here, in an experiment that monitored how people’s gaze shifted as they looked at a painting by Heironymus Bosch, The Garden of Earthly Delights. It’s full of fine details, and the gaze tracker shows how much time people spent with their maculas pointed towards which areas, letting us see what attracted their attention most.
Source: The Guardian 21st June 2023.
The Fovea — the Macula's Macula
Just as the macula is a spot of detail vision in the middle of the retina, the fovea is a spot right in the middle of the macula. The level of detail detected by the fovea is extremely high.
As noted above, the job of the macula is to paint in the detail of the things. But just like the broader visual field, it’s also got a ‘where’ job. It’s in charge of detecting smaller points of interest, so that it can guide the fovea to them for ultra-high-detail examination. A common example is when we’re reading — while the foveal field looks at a word and sees the letters, the macular field is detecting where all the other words are around it, so the brain knows exactly where it needs to shift gaze to get the fovea seeing the next word.
In summary: the job of most of our visual field is to monitor what's around us, warn us of danger, and guide our macula to the interesting bits to fill in the details. In turn, the macula guides the fovea to fill in the very fine detail.
Attention and the Visual Field
Magicians understand all about the visual field and the way our macula follows our attention. Many magic tricks and illusions rely on drawing our attention with a sudden movement, an explosion, or simply a request to ‘watch carefully.’ As long as our attention is on one spot, they can do things in another part of our visual field without us noticing.
For an interesting demonstration of how attention works, have a look at this entertaining video.
And here’s another interesting video of an illusion that illustrates how our peripheral field doesn’t tend to pick up changes in detail.