Animal Vision And Colour Vision
Chris - Hi Ron. You were talking about sheep and cows a moment ago, but how do you know that sheep can recognise pictures of sheep upside down or not?
Ron - Well Helen was explaining some experiments earlier about how pandas can see colour. There are many things you can do: you can look at the structure of the eye for instance, and that will tell you quite a lot about what the animal can see. You can also train it to things. So in Helen's case you were training it to distinguish a red target from different greys. If you put an animal inside a rotating drum and there are stripes on the drum, they will tend to follow the stripes. This is called the opto-motor response. Then you just make the stripes narrower and narrower until the animal no longer follows it, and then you can see how fine detail it can see. So you use various training techniques.
Chris - Well let's home in on your speciality, which is colour vision. There's an age old question, which is that dogs can't see in colour; they have black and white vision. But it's not true is it?
Ron - No. Certainly almost all animals can see colour and there are very few that can't. I think possibly it's very difficult for us to imagine what the world looks like to everybody else and we tend to think that humans see the world just about as well as you can. Now it is true that humans are quite good at colour vision and if you compare our colour vision to that of other mammals such as dogs and cats, it's certainly better, But mammals have fairly poor colour vision. Most mammals have what is known as two colour pigments; two types of cone. They are said to be dichromatic, and they see the world rather like a red-green colour deficient person. Humans have three of these pigments and are said to be trichromatic.
Chris - Is it because we go out in the daytime that we have this intense colour vision?
Ron - It is but our colour vision isn't that good when you look outside of mammals. If you look at fish and you look at birds and you look at frogs, they have much better colour vision. They have more visual pigments; maybe four or five. They can distinguish more colours than humans can.
Helen - I think one of my favourites has to be the mantid shrimp that lives in the sea. Am I right in saying that they've got one of the most complex eyes out there, and they've got eight types of visual pigment and then lots of other types of pigment for things like polarised light and distribution of light and these fantastic stalked compound eyes?
Ron - That's right. The mantid shrimp is like the world champion of colour vision. If you combine the number of visual pigments with the other filters it has in its eye, it can actually distinguish sixteen different types of pigments within its eye, which compared to our three is actually quite amazing.
Chris - It begs the question, why, Ron?
Ron - I think to humans, colour vision really isn't that important. If you said to somebody that they're going to lose their ability to see colour, they would be a bit worried but it wouldn't be completely devastating because our survival doesn't really depend and never has depended on our ability to see colour. In terms of evolution, it was quite useful for distinguishing green, unripe berries from red, ripe berries. But if you compare that to something like reef fish, which are so brightly coloured, colour is obviously a lot more significant in their lives than it is in ours.
Chris - How do we actually see colour though? How does the eye discriminate between colour vision and black and white?
Ron - In order to see colour, you need to compare the output of two different types of cells, which are known as cones. Black and white vision relies on only one type of photoreceptor known as a rod, and because you only have one type, you have nothing to compare, so you can't see colour. You can see colour by comparing the outputs of what we humans call the red cone, the green cone and the blue cone.
Chris - This is very light-hungry isn't it, so it doesn't work at night very well. Is this why we only see black and white at night?
Ron - If it's very dark and we're only using our rods, you can't see colour at all. That's because the ability to see colour and the ability to see low light levels, our so-called absolute sensitivity, are more or less mutually exclusive. So you either have to make the most use of the light or you're able to see colour, but you can't do both at once.