Let’s discuss the human vision system. Now I am not a biologist, so I am going to give you an engineer’s impression of how the human vision system works.
Firstly we have the eye itself, and looking at an eye perhaps the most obvious feature is the white part, which is called the sclera, and the iris. Now the sclera is fibrous tissue, which is an important part of the structure of the eye and some kinds of animals the sclera actually has bony components in it and in this fossil here you can see a ring of bone, which is actually part of the sclera of that animal’s eye.
If we look at the cross section of the eye on the right here, we can see that the sclera which is part of the membrane that surrounds the eyeball itself. We can see the iris and the pupil which regulates the light going into the eye. We can see the lens, which is a transparent, crystalline protein structure which focuses the light onto the retina on the back of the eye.
The inside of the eye is filled with a fairly transparent fluid, and at the back of the eye we have a surface which is called the retina, a curved surface which contains the photoreceptors, that is, the light sensitive part of the eye. And that is then connected by this big nerve bundle—the optic nerve—to the brain. In fact, we can actually think of the eye as being a part of the brain. It is connected by a significant nerve bundle, and then the eye is essentially a sensory part of the brain located in the front of our heads.
Let’s now look in more detail at the retina of the eye. So if we take a transect, we move from inside the eye to outside of the eye along the direction of that arrow we pass through a number of layers of cells and blood vessels and whatever. Now this is all on a pretty small scale; this total structure is about 500 microns thick.
The light sensitive cells are at the bottom of this structure. So they are not on the surface of the retina, they are actually about 500 microns below the surface of the retina. So the light has to pass through—defuse through—these cells and blood vessels before it gets to the light sensitive cells.
Now you’ve probably heard people talking about rods and cones. The rods cells and the cone cells are the light sensitive elements. They are the photoreceptors in our eye. The rod cells are these spaghetti shaped structures. They are kind of thin. And they are sensitive to low light levels. So we use the rod cells when we are working in a very low light environment. The cone cells are these strange, stumpy shaped cells; they are a little bit conical, kind of squat, not as tall as the rods. And these cells are sensitive to colour. And there are three different types of cones cells: there are some that are sensitive to red light, some sensitive to green light and some sensitive to blue light. But the characteristic they all share is that they are this somewhat conical shape.
Another way we can consider the eye is to look at where the optic fibre joins the retina and that creates an area on the retina which is called the optic disk where there are actually no light sensitive cells. So there is a part of the retina that does not respond to light. And there is another part of the retina where we have an enormous concentration of light sensitive cells and that area is called the fovea. The fovea and the optic nerve are separated by some distance.
So in this graph here what we are going to see is the density of photoreceptor cells, and there is a huge peak in the number of cone cells per square millimetre in this area we call the fovea. So we have exquisite resolution in this one small area of our eye. The rest of the eye, we have much less resolution and unconsciously our eye is continually moving, focusing, on different parts of the scene, directing the fovea to these different parts. So we build up a high resolution of the image by directing the high resolution fovea all over the scene we are looking at.
Now with the rod cells they have a quite different spatial density pattern. There are very few rod cells in the fovea area, but there’s many more rod cells in what we call our peripheral vision; away from the direction that our eye is pointing. This is very useful at night and these cells are also somewhat motion sensitive. They are what give us the ability to see something moving out of the corner of our eye.
So what is really interesting then is this business of the optic nerve entering the retina and creating what we call the blind spot. Now we can actually detect our blind spot using a pattern, something like this. So what you do is close your right eye and stare at the plus sign. And then move your head toward and away from the screen making absolutely sure that you are looking with your left eye at the plus sign. As you move your head in and out, the circle here will at some point fall onto the optic disk on your retina and it will, quite surprisingly, it will just disappear from view. So you can have some fun with a test like this, you can find others online. Have a play and see if you can find your blind spot. It is quite surprising that in everyday life we don’t notice that we have got a blind spot. We go to a bit of trouble, we can actually detect that it is there.
Now if we look at, for instance, our horizontal cross section of the brain. We can see the eyes located at the front of the head and the optic nerve bundles carry the visual information through the brain to the back part of the brain, where we have our visual cortex. And that is where we have all the really exciting visual processing going on. That is where part of our brain is responsible for recognising people and objects and motion and so on. But the human brain is a very complex structure. It weighs something like 1.5 kilograms and is about 1011 neurons in there. So an awful lot of computation and memory capability. Nearly a third of that is devoted to processing visual information.
Earlier we used the argument that the eye has been invented so many times by evolution that it must be a very effective sensor for animals. But there is a cost for the sense of vision, and in the case of human beings one of those costs is that a third of our brain is devoted to processing this visual information. So yes, it is a very effective sensor that as an organism we pay quite a cost in order to run that sensor. Additional costs are associated with the eyes themselves. They are very delicate structures, so we evolved mechanisms to protect the eye. For instance, our eyelids and eyelashes, mechanisms to clean the eyes with tear ducts and so on, and the eyes are also moved within the head by some very high performance muscles. So there is a lot of auxiliary machinery involved in maintaining the sense of vision.
The human eye is quite amazing, let’s look at its various components including the light sensitive rod and cone cells.