Associate Professor Gordon Sanderson is Associate Professor in the Department of Ophthalmology at the University of Otago, Dunedin. He explains how the eye works, focusing on the receptors located in the retina at the back of the eye. There are two types of receptor – rods and cones – that allow us to see in black and white (rods) as well as in colour (cones).
ASSOCIATE PROFESSOR GORDON SANDERSON
You’ve got a retina which is obviously the receptor – that’s the bit that receives all the images and takes the light bits and the dark bits and sorts them all out. But to get an image on that retina, you need a lens – you need a focusing system to make it look clear, and that’s essentially what the eye is doing.
In the front of it it’s got a cornea, which is part of the lens system, and then inside of it, it’s got a lens, which is the other component, and between them they focus a very clear image on the retina, which is at the back.
And from there, the images go down the optic nerve and ultimately to the back of your head, and that’s the bit that interprets the images for you.
The eye has actually only got two receptors – you’ve got rods and you’ve got cones. We’ll dismiss the rods straight away, because all they do is provide you with your peripheral vision. They will detect movement, and they’re actually black and white, so not able to detect colour. In the centre, you’ve got the cones, and the cones are the most sensitive receptors. They’re the ones that pick up the fine detail but they also pick up colour.
Now there are actually three types of cones. You’ve got the same as you have with the video monitor – red, green and blue – but they working in reverse, so they’re receiving red, they’re receiving green, they’re receiving blue, and they convert various components of red and green and blue and make the shades of colour that we’re all familiar with. Part of that processing is actually done on the retina, but part of it takes place behind the retina and even more takes place in what’s called the mid-brain, and then ultimately the visual cortex kind of receives the image and assembles it and turns it into stereoscopic imagery or visual memory or motion or form or whatever it is that we’re actually concerned about. But the whole thing is a massively complex composite of all these stimuli all derived from three receptors.
The cones have got their own personal nerve fibres. By the time you get out to the peripheral retina, there might be 20 or 40 000 rods per nerve fibre, so that’s why my peripheral vision is a lot worse that my central vision. People tend to think that, because you’ve got a composite image that your retina or your brain is receiving a photographic image. It isn’t. It’s actually only receiving a very clear part in the centre. It’s because you moved around and built up this sort of photographic memory if you like of what the room looks like that you think everything is clear.