How to use this site: This page is a chapter in an online book. If you’ve arrived here without reading the previous chapters first, it might not make much sense. I strongly recommend you start at the beginning. Click here to go to the beginning.
Chapter 2
Understanding the Eye
If you want to understand vision, you need to start with understanding the structure of eye itself, in all its marvellous complexity.
What are the various parts of the eye?
When I first started Optometry school, one of my textbooks was Wolff’s Anatomy of the Eye, a great lump of a book about 300 pages thick, full of diagrams showing the structure of the eye.
It didn’t include anything about the way the eye actually worked — just the structure. I couldn’t believe there could be so much to know about one little part of me.
And yet, over time I began to realise that Wolff’s was just an introduction, and there was so much more. Extending that complexity to the rest of our body… wow.
Luckily for you, I’m going to give you a only a brief overview on this page — just enough to understand what I’m talking about when we get to thinking about how eyes work, and don’t work.
Structures of the Eye
This is a cross-section, as if you’ve cut an eyeball into two exact halves and lifted one half off. Even though they’re coloured, four of the structures are completely clear. The blue (vitreous) and the red and green bits (aqueous) are all clear fluid or gel, separated by the lens, and the cornea up the front is also clear. That way the light has a clear path all the way from the front right down to the retina at the back.
This is a really simplified diagram. In reality, it’s so complex that every part in this diagram is something scientists could spend their entire careers studying just that part.
The Cornea
The cornea is the clear window at the front of the eye. It’s got a very precise curve, because it also acts as a lens, focusing the light on to the back of the eye. It’s really important that the cornea is clear, and that the surface is very smooth. Every time you blink, the lids spread a very thin layer of tears over the cornea, which creates that ultra-smooth optical surface. Anything that disrupts that tear layer (such as dry spots, or bits of mucous in the tears) can decrease your quality of vision.
The Iris
The iris is what we see as the colour of the eye, usually somewhere between blue and brown. Think of it as like a circular curtain — it restricts the amount of light coming into the eye (the muscle that closes it up is a ring-shaped sphincter muscle). The gap in the middle is the pupil. We see the pupil as black, because light goes into the eye but generally doesn’t come back out. An exception is when we use a super-bright light such as a camera flash. The bright light goes in and illuminates the inside of the eye, and then the light bounces back out again before the sphincter muscle has time to shrink the pupil down, and so we get those eerie red pupils in photos.
When we optometrists look in your eyes with our bright lights, we use a device that has a lens system that focuses that ‘red eye’ light so we can see the details of what’s inside. But if the pupil shrinks down too small it can make it impossible to see in there, which is why we might need to use drops to temporarily prevent your pupils closing up.
The Lens
The lens is a pretty remarkable structure. You can’t see it, because it sits behind the iris, and it’s clear. In young people, it’s very flexible, and lets them adjust their focus from far-away vision to up-close. Kids can focus on objects that are extremely close, but over the years the lens keeps producing new cells, and it gets more and more crowded in there, which makes the lens get tougher and less flexible.
By our mid-forties, most of us are having trouble focusing on fine detail up close, and that’s why we start needing reading glasses. By age sixty our focusing flexibility is pretty much zero, so it levels off there.
The major change later in life is that the lens starts to lose clarity. It’s kind of like those clear plastic bottles that if you leave them outside in the sun they eventually turn milky-white. When a lens loses clarity, it’s called cataract. It’s a normal ageing change, like grey hair or wrinkly skin. Just as there’s no point where we suddenly switch from smooth skin to having wrinkly skin, there’s no exact point where we suddenly say we have cataracts, it creeps up gradually. And like grey hair, some people get it earlier than others, and some medications and other conditions can bring on cataracts earlier. But sooner or later, we all get cataracts, as long as we live long enough.
When a lens loses clarity, it’s called cataract.
The Vitreous Gel
The vitreous (also known as the vitreous humour, with humour here being an old word for a bodily fluid) is a clear gel that fills the inside of the eye. It doesn’t really do much other than fill space, but really becomes significant if there is a bleed from somewhere in the eye that gets into it. If that happens, then it’s like switching from looking through a glass of water to looking through a glass of mud.
The Retina and Choroid
The retina is a really important one for many people with vision impairment. It’s a very complex structure — technically it’s an extension of the brain. It’s a very thin layer that coats the inside of the eye, and acts like the film (or sensor) of a camera. The job of all the bits I’ve described above is to get a clearly-focused image on to the retina. The retina has a layer of light-sensitive cells called photoreceptors, which pick up the picture and transmit it to nerve fibres in other layers of the retina. Those nerve fibres carry the picture to the optic nerve (see below), but those fibres aren’t just simple transmission lines — they have connections to the other nerve cells around them, and those connections mean the image is already partly processed even before the signal has left the eye.
The whole process of receiving and transmitting the image is very intensive, so it needs a lot of nutrients, which are largely supplied by the layer of blood vessels behind the retina called the choroid. The process of converting light into nerve signal also produces a lot of metabolic waste products and a great deal of heat, both of which are taken away in the blood. The choroid is basically the retina’s personal assistant, so if the choroid malfunctions the retina won’t work properly either.
The retina and the choroid are kind of like a carpet and the carpet underlay. You can’t see the underlay (the choroid), unless there are holes in the carpet (the retina). So if there’s something wrong with the underlay, the overlying carpet won’t feel right — and if there’s something wrong with the underlying choroid, the overlying retina won’t work properly.
Viewing the Retina
What are we seeing here?
Imagine having a basketball, one with a clear window in the surface. Looking through the window you’d see the inside surface of the opposite side of the basketball. It would be pretty dark, of course, so you’d need to shine a torch/flashlight in to see it properly.
This is the view we get when we shine all those bright lights into your eye. We are looking straight through the clear structures of the eye — the cornea, the aqueous, the lens and the clear vitreous — to see this, the inside surface of the back of the eye.
This whole area is retina, and the darker patch right in the middle is the macula. That white circle is the head of the optic nerve, just as if you were looking straight down on a basin and seeing the plug hole. You can see the blood vessels on the surface of the retina, but actually 90% of the blood vessels are in the choroid, running beneath the retina.
There’s actually a whole lot more retina that’s not shown in this scan. This is just the part that’s right at the back of the eye. Here’s one (of a different eye) from a ‘wide-field’ scanner.
The area covered by the first scan is shown with the blue circle. Even though it’s only a small area, it’s the most important area.
Wide field scanners are remarkable pieces of technology, big and expensive. Think back again to that basketball with the window in it. It’s easy to see the inside surface back half of the basketball as long as you have a torch, but if you want to see the inside surface of the front of the basketball you have to be looking extremely sideways through the window. Because of the focusing layers of the eyeball, that becomes extremely difficult. Putting drops in to widen the pupil can help, but even with a large pupil it’s challenging.
The Macula and Fovea
The macula is a small part of the retina, right in the middle. It has a much higher density of photoreceptors and nerves, because it is the part that gives us more detailed vision. Having that higher density makes it more energy-intensive — it requires even more nutrients, and generates even more heat and metabolic waste, so it’s the part of the retina most prone to breaking down. Many vision impairment conditions involve the macula.
The fovea is like the macula-of-the-macula. It’s a very tiny area, right in the middle of the macula, which gives us ultra-high detail. This is the bit we use when we say we are looking straight at something.
Viewing the Macula and Fovea
This is the same scan as above. Remember, it’s not a wide-field image, it’s just the central part of the retina. The area marked out by the black circle is the macula (although there area various definitions that make it a bit larger or a bit smaller than that), and the fovea is shown by the tiny (really tiny!) green dot in the centre.
But… why?
Why is it that we have only a small part of our vision that gives detail vision, and only a tiny part that gives us ultra-high detail vision? I mean, a video camera records high detail across its entire field of view. Why not do that?
Excellent question. See further down for a discussion.
The Optic Nerve
All the retinal nerve fibres travel towards the optic nerve head, through which they travel out of the eye, the same way water in a basin travels to the plughole and down the pipe. The nerve fibre part of the optic nerve is only about 1.5mm (about 1/16th of an inch) across, but that’s a lot — that makes it the largest sensory nerve in our bodies, carrying something like 75% of the total amount of the sensory information we get about the world.
If something destroys the optic nerve, it’s like someone cutting your phone line. The phone goes dead, not because of a problem with the phone, but because the signal can’t get through. Conditions like glaucoma and optic neuritis damage the optic nerve.
The two optic nerves join up behind the eyes (close to the middle of your head), and then the nerve fibres from both eyes reorganise themselves into two new groups. The nerve fibres from both eyes that are carrying vision from your left visual field (that is, whatever is to the left of where your foveas are pointed) carry on back to the right side of your brain, to the part that deals with vision. And the nerves from both eyes that are carrying vision from your right visual field go together to the left side of the vision part of your brain.
So wherever you look, the right side of the brain receives information on whatever is to the left of where you’re looking, and gets nothing at all from whatever’s to the right of that point. The left side of the brain gets everything that’s to the right. That demarcation (which we call the vertical midline) goes right down through the macular field, and even neatly through the centre of the tiny foveal field, neatly dividing your overall visual field into two hemifields.
The nerves-crossing-over point has a special name: the optic chiasm (that word chiasm is Greek for ‘crossing’, which in turn comes from the ancient Greek for ‘to mark with an X’, since the two nerves coming in and the two nerve pathways going out form an X shape). The nerve fibres behind the chiasm are continuations of the same nerve cells in the optic nerve, but that pathway is no longer called the optic nerve — instead, the pathways are known as the optic tracts.
The Brain
In the end, all seeing is really done in the brain. The part that is considered ‘the vision part’ is the Occipital Lobe, which is right at the very back of the head. So damage anywhere between the eyes and the back of the head has the potential to affect vision. But the nerve fibres don’t all go just to the vision part of the brain. Some fibres go off to other parts, which can give some interesting results when the vision part of the brain is damaged but other parts of the brain might still respond to information from the eyes.
Beyond the occipital lobe, the information gets shared with other parts of the brain to build up the overall experience we call ‘seeing.’ Sometimes people have perfectly good eyes, and perfectly good occipital lobes, but still have trouble integrating that vision with the rest of themselves and so experience vision problems.
Why?
Back to the question of why we only get a little bit of high-detail vision in a much larger field of low detail. Why isn’t our eye even as good as a standard video camera?
Basically it’s an ingenious way of getting past a bandwidth (data transfer rate) and processing capacity limitation.
The optics do a great job of getting a full-field clear image to the back of our eyes. And in theory the retina could have vastly more photoreceptors and pick up all that clear image and convert it all into nerve signals.
But then the optic nerve would have to be able to transmit vastly more information back to the brain. It’s already the largest sensory nerve in our body, but to carry foveal-level detail it would have to be carrying… I don’t know, probably at least a thousand times more nerve fibres, maybe more. That would make it wider than the diameter of the eyeball itself.
Even more importantly, the brain would have vastly more information to process, so it would have to be enormous, and entirely devoted to processing vision. We’d be just a brain with eyes — an enormous seeing-machine, with no room left over for thinking and doing. (I mean, when you think about it, that’s what a video camera is — it’s great capturing and storing an image, but that’s all it can do).
So, let’s use an analogy to help us understand how our visual systems manage to get wonderfully clear vision despite those seemingly impossible limitations.
Think of your vision as being like a movie being streamed to your TV. Let’s say your bandwidth (internet speed) is limited. It’s so limited that all you can get is a low-definition image coming through. And there’s no way to upgrade to a faster internet speed.
What our visual system does is ingenious. It’s a ‘cheat’ to make it seem like we have an ultra-high-definition stream.
It takes a very low-definition stream of the entire view, but also a high-definition stream of just a very small area of the view, and also an ultra-high-definition stream of just a tiny area in the middle of that. The optic nerve has the capacity to transmit all of those streams with no problem, because the areas of high- and ultra-high-definition are so small.
Then (and this is the ingenious bit), it’s as if the TV shows you the low definition view, but you have an eye tracker linked to it so that wherever you look on the screen it makes just that part of the screen show the tiny ultra-high-definition stream, with the high-definition area surrounding it. Since you see ultra-high-definition wherever you look, you have the illusion that the entire image is in ultra-high-definition. We can’t tell the difference.
Fast and Slow
Our visual system has another amazing trick to make the most of the information that it gets back to the brain, and this is one that is profoundly helpful in keeping us alive.
It runs two parallel visual streams: one fast, and one slow. They’re not quite the same as the high and low-definition streams discussed above, but there’s definite overlap.
The fast one gets our brain just the bare essentials really fast. It pulls information from all over our visual field, but it concentrates on movement and spatial information, leaving all the other stuff for later. This is the system that detects that a predator is leaping at us from the side, or a tree branch is falling on us. This is why our general retina is exquisitely sensitive to movement — it’s looking for change, because change can mean danger, and that means react quickly or die.
In essence, it’s like it gives us a rough outline of our world, a bit like a preliminary sketch. Just enough so we can react.
The slow system carries all the detail, richness, colour and texture. It’s much more detail, so it arrives later, and it acts to fill in the outline our fast system’s already provided. Again, it draws from receptors for this information over our entire visual field, but most of that information comes from the macula (and especially the fovea).
A curiosity of the fast system is that some of the nerve fibres carry information straight to parts of our brain that process space and instinctive reactions, without ever going through the occipital cortex, because the overwhelming priority is speed. That way we can start dodging the thing before we have any idea what it is.
Those reflex actions can also be to close our eyes tight or turn our faces away to protect them from potential impact, so that means we might have collected minimal information about the thing that came at us. So we might not have any idea what it was — perhaps just that it was something big, and it came at us from the side — because that’s all the fast system gave us before.
