Science of Sight, Eye Diseases and Animal Vision

15 October 2006
Presented by Chris Smith, Helen Scales


Taking a look at the science of sight is consultant ophthalmologist Nick Sarkies, who will discuss eye diseases and how we can treat them, and Ron Douglas provides insight into colour vision and how the world appears though the eyes of animals. Sticking with our animal focus, Bob and Chelsea reveal that there may be three times as many poisonous fish as there are snakes, and in Kitchen Science, Derek Thorne stops himself bouncing off the walls long enough to discover how superballs spin.

In this episode

Insect Study a Breath of Fresh Air

A study on beetles has revealed why there are no longer 3 foot-long dragon flies buzzing around in your garden (as there were 300 million years ago) - and it's all down to oxygen. Alexander Kaiser, from Arizona's Midwestern University, together with his colleagues x-rayed four different beetle species of increasing size including the 3mm long Triboleum castaneum beetle and the much larger 3.5 cm Eleodes obscura. The aim was to measure the relative sizes of the tubes, called tracheoles, which they use to draw air into their bodies. These tracheae run down the sides of an insect's body, and the animal moves air in and out by pumping its abdomen. But the x-rays showed that, with enlarging body size, the tracheae became disproportionally big, and take up 20% more space than the increase in the insect's body size would predict. This becomes a problem at the site where the body and legs meet. The opening connecting these two structures can only become so big, limiting the size of the trachea that can pass through and therefore the size of the beetle. The team calculated, on the basis of their measurements, that it shouldn't be possible for a beetle to exceed about 15cm in length. And indeed the largest beetle species currently on Earth is the appropriately-named Titanic longhorn from South America, which grows 15-17cm. But, this is based on present-day oxygen concentrations of about 21%. 300 million years ago oxygen levels were much higher - closer to 35% - and as a result insects didn't need such big trachea and huge insects, including dragonflies with a three and a half foot wingspan were common.

Archer Fish Right on Target

Archerfish, which knock their prey into the water with a well-aimed blast of water, tailor the power of their shot to the size of their meal, German scientists have found. Writing in this months edition of the journal Current Biology, Thomas Schlegel and his colleagues from the Universitat Erlangen-Nurnberg used high-speed photography capable of capturing 5000 frames per second to work out how much force the fish was using to knock prey of its perch. There were two key findings: The larger the prey the more force the fish used. The effect was even seen in fish that had previously grown up in a tank where firing any amount of water resulted in their receiving a food reward. The researchers think that the effect is down to the simple principle that the larger something is, the greater the adhesive force it uses to cling on to a surface. The second finding was that the fish minimise the amount of energy they use to retrieve their lunch. For larger targets they increase the amount of water they fire, rather than the pressure or speed at which they fire it. This means that they pack double the punch for double the energy, rather than double the punch for quadruple the energy, which would be the result if they altered the pressure or volume (because kinetic energy varies with the square of the speed but only linearly with mass). As a result they burn off the least energy possible for each shot they take meaning they make the most out of meal times.

Giant Pandas See in Colour

Even though giant pandas are black and white, doesn't mean that they can only see in black and white - because this week we have news from scientists in America who discovered that giant pandas are not colour blind but can distinguish colours from greys. Angela Kelling, a graduate researcher from the Georgia Institute of Technology, worked with Yang Yang and Lun Lun, a couple of giant pandas that live in Zoo Atlanta. She found that they were able to distinguish between colours as well as various shades of grey. To find this out, she conducted a two year study in which she presented the two pandas with three PVC pipes each hung underneath a sheet of paper - two of the pipes were hung beneath grey paper and the third was under coloured piece of paper - either red, green or blue. If the panda pushed against the pipe underneath the coloured paper it received some food as a reward. But if it pushed one of the pipes underneath the grey paper they got no reward. She repeated the tests over and over with different colour combinations and found the pandas were able to distinguish the coloured paper from the grey paper because once they had learned that the colour paper meant food they pushed the coloured paper more often than the grey paper. We still don't know much about the type of colour vision that pandas have, and from this study its not clear whether the pandas could tell the difference between red green and blue. But now we know that they can tell the difference between grey and other colours and that might help them forage better among the green bamboo that they spend the majority of their day eating.

Extreme Environment Changes Fish Appearance

The Devils Hole pupfish is one of the most endangered species of fish in the world, because in the wild the entire species lives only in a single rocky pool in Death Valley in California. Now scientists have discovered that the Devils Hole pupfish, a little inch long fish, look the way they do partly because of their tiny cramped conditions. Some of the wild pupfish were transferred to captivity to help try and ensure the species survival should the natural population in the Devils Hole die out, but the pupfish bred in refuges have begun to look rather different to their wild brothers and sisters with deeper bodies and smaller heads. This change in the appearance of captive-bred fish raises serious questions about how this rare species can be protected from extinction. Sean Lema and Gabrielle Nevitt from the University of California Davis, set about trying to understand how environment changes could affect the way pupfish look. They worked in the lab with a similar but unthreatened species called Amargosa pupfish - and to mimic the conditions in Devils Hole they warmed the tank water up and restricted the amount of food the fish could eat. And low and behold, the Amargosa pupfish started to look more like the wild Devils Hole pupfish. On the positive side, these studies suggest that fish may be more adaptable to changing conditions than was previously thought - but also it could mean that captive-bred pupfish could die if they were reintroduced into the Devils Hole in Death Valley to try and rebuild the wild population. And this also raises a slightly philosophical question, that if the way an animal looks is so intimately linked to its environment, is it still the same fish if it is not preserved in the same habitat? So does keeping Devils Hole pupfish alive in captivity really mean anything if there are none left in the wild?

A football

Science of spin and bouncy balls

This week Derek is with Professor Hugh Hunt of Cambridge University and student helpers James and Martin from the Norwich School. They're going to be finding out about the science of bouncing balls.

To do this experiment, you will need:

Super bouncy ball A little bit of oil or vaseline Kitchen counter that meets a vertical tiled wall

How to do the experiment:

1 - Take the bouncy ball and roll it gently across the work surface towards the wall. Watch to see how far it bounces back.

2 - Put a little oil or vaseline on the tiles where the ball hits the wall.

3 - Roll the ball across the work surface again and watch how far it bounces back.

What's going on?

The point of this experiment is not only to look at how far the ball bounces back, but to think about how the spin direction changes. If you think of the ball rolling clockwise when it moves towards the wall, then on the way back it must be rolling anticlockwise. This change in spin direction requires energy, but where is the energy coming from?

When rolling the ball against a dry wall, you may have noticed that it started to bounce on the way back. It is this bounce that helps the ball to change its spin direction, and it's the wall that's providing the kick.

Taking a closer look, you will see that as the ball approaches the wall, it tries to continue rolling upwards in the direction it was going (ie: clockwise). However, gravity means that it can't do that for very long, and so it has to fall back down again. This makes the ball start to bounce and roll the other way (to see a graphical simulation of this effect, go to Hugh Hunt's website).

So what happens when there is oil on the wall? In contrast to when the wall was nice and dry, the ball can no longer grip and roll upwards. This prevents the ball from bouncing and can no longer use the wall to change its spin direction.

The result is that the ball starts rolling back to us spinning the wrong way (ie: still in the clockwise direction). The only way it can start to spin anticlockwise is by relying on friction between the ball and the kitchen counter. However by the time it's done so, the ball has lost all of its energy and comes to a complete stop.

This is why the ball doesn't roll back as far when there is oil on the wall. The oil reduces the friction, stops the ball getting a kick up the wall, and leaves the ball still rolling forwards even though it's moving backwards.

So can we see this effect anywhere else? You might expect to see balls bouncing in this way when they roll towards the cushion in a game of snooker. In fact, you won't see this at all because the cushions are specially designed so that the ball won't jump when it bounces back. The cushions are angled so that the point of contact between the ball and the cushion is not exactly half way up the ball (as when it hits a vertical wall), but higher up on the ball's surface.

- Eye Diseases And How To Treat Them

The Naked Scientists spoke to Dr Nick Sarkies, Addenbrooke's Hospital, Cambridge

Eye Diseases And How To Treat Them
with Dr Nick Sarkies, Addenbrooke's Hospital, Cambridge

Chris - So sight must be the sense that we value more than anything.

Nick - Yes I think that people do value their sight enormously.

Chris - But how does your eye actually work?

Nick - Well it converts light into chemical energy and then into electrical energy and that's transmitted by a nerve into the specialised cells in the brain.

Chris - Ok but obviously the environment is full of light. How does it actually end up getting to the right place in the eye? What's going on at the front?

Nick - It has to be focussed. So the front part of the eye is concerned with focussing it onto the retina and the retina is very highly specialised to receive the light and convert it into chemical energy.

Chris - But what about the fact that, I reckon, about half of the population wear glasses. Why is that? Why have we ended up with a problem with our eyes that we need glasses?

Nick - Good question. And it's very disturbing particularly in countries such as China and the rest of Asia. Myopia, which is the inability to focus light accurately on the retina because the eye is too big, seems to be increasing. The question is why?

Chris - But what's actually happening in the eye when someone is short or long sighted?

Nick - The problem is that the retina is not at the right focal length. If someone is short sighted, then the retina is too far back and if they're long sighted, then the retina is too far forward as it were.

Chris - Ok, so now if we go to how the eye can actually go wrong, there must be lots of diseases that can make eyes worse, especially as we get older, apart from just short and long sightedness.

Nick - Yes there are lots of diseases. The three most common, and the commonest worldwide, which is curable, is cataract. That's still the commonest cause of blindness worldwide.

Chris - So what's going on in someone with a cataract?

Nick - A cataract is a lens that's become cloudy. The crystalline lens within the eye is designed to remain transparent. But as it ages, it tends to change its chemical structure so that it's no longer transparent.

Chris - It becomes foggy.

Nick - Exactly.

Chris - So do we know why that happens?

Nick - We know why it happens in some people with rare metabolic conditions. We don't really understand why it happens to everybody eventually.

Chris - Is it a familial thing?

Nick - It can run in families, although it's unusual.

Chris - So obviously it's pretty routine to put it right now.

Nick - It's become probably the most common operation that's done worldwide: removing the crystalline lens and replacing it with an artificial lens.

Chris - How do you actually do it?

Nick - With some high tech equipment in the western world, but it can be done much more simply without high tech equipment. It tends to be done more like that in other countries in the world.

Chris - So go on Nick, tell us about the nuts and bolts of what happens during a cataract operation. There are probably lots of people out there that have been told that they're in the early stages of cataract, because it's not an all or nothing thing. Some people have a bit of cataract change and it's not sufficient that they need it replacing there and then but they will do one day. So what are they going to be facing?

Nick - They're going to be facing an operation where the lens is removed and replaced with an artificial lens. In order to do that, we have to make an incision into the eye, so it's a major operation. That's why it's not carried out unless the cataract is causing individual trouble. Perhaps they can't read very well or recognise their friends over the other side of the street.

Chris - So you make a small incision…

Nick - You make a small incision into the eye and with an ultrasound device, we soften the hard part of the lens. This is called the nucleus. We suck it out and then suck out the surrounding part of the lens, called the soft lens matter. Into the capsule, where the lens was, we insert an artificial lens.

Chris - Correct me if I'm wrong, but we have muscles in the eye that either stretch or compress the lens to make it fatter or thinner so that it focuses light on the retina when it's working properly. So does your implant or artificial lens that you put in have that capability or not?

Nick - Not yet. There are attempts to try and design materials and a lens that will have some flexibility within the eye, but so far they're not very successful.

Chris - So does that mean that people who have this done do end up a bit short sighted or a bit long sighted as a result and can that be corrected for?

Nick - Yes, if they prefer they can be long or short sighted. It depends on preference really. People who have been short sighted all their lives often like to remain short sighted and so we put a lens in that keeps them a little bit short sighted so that they can read unaided.

- Animal Vision And Colour Vision

The Naked Scientists spoke to Professor Ron Douglas, City University, London

Animal Vision And Colour Vision
with Professor Ron Douglas, City University, London

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.

Salmon swimming upstream to mate

- Salmon Farms and Venomous Snakes

The Naked Scientists spoke to Chelsea Wald and Bob Hirshon from AAAS, the science society

Salmon Farms and Venomous Snakes
with Chelsea Wald and Bob Hirshon from AAAS, the science society

Bob - This week on Science Update, we're going to bring you the latest news about how salmon farms may be killing wild salmon. But first, in other fish news, Chelsea tells us that there may be a lot more venomous ones out there than anyone guessed.

Chelsea - When you think about venom, you probably think about snakes. And there are about 450 venomous snakes in the world. But a new study from New York's American Museum of Natural History shows there are at least three times as many venomous fish. That's at least 1,200 venomous fish in the world. Ichthyologists Leo Smith and Ward Wheeler made this estimate by placing the 200 previously known venomous fish on their new evolutionary tree and guessing which other fish species would also have venom. They then confirmed their predictions with dissections. Smith explained that this estimate is very conservative.

Leo - Only when someone had actually published a paper talking about venom in a group did I include them. And, you know, plenty of us have been stung by other ones that are clearly venomous.

Chelsea - Smith says he wouldn't be surprised if the number rises to over six thousand - important news for drug researchers who often use venoms to develop new medicines.

Bob - Thanks, Chelsea. A salmon farm can indirectly kill young wild salmon within a thirty-five mile radius. This according to a two-year study of over fourteen thousand fish, led by Ph. D. student Martin Krkosek at the University of Alberta in Canada. He explains that salmon farms are teeming with sea lice: a parasite that's relatively harmless to adult salmon, but deadly to juveniles.

Martin - These fish are only about an inch long. They weigh about half a gram. They don't have any protective scales, and all it takes is one or two lice to kill them.

Bob - In the wild, juveniles rarely get sea lice, because they grow up far away from adult carriers. But Krkosek found that fish farms along juvenile migration routes can infect and kill up to 95 percent of the young fish that pass by. As a result, he says that farming appears to be depleting the wild salmon population, rather than saving it.

Chelsea - Thanks, Bob. That's it for this week. Next time we'll talk about some prairie dogs who are too busy thinking about you-know-what to look out for predators. Until then, I'm Chelsea Wald.

Bob - And I'm Bob Hirshon, for AAAS, The Science Society. Back to you, Naked Scientists!

- Do cool washes kill bacteria?

Some washing powder manufacturers are starting to recommend that we wash our clothes at 30 degrees centigrade instead of 40 degrees centi...

Do cool washes kill bacteria?

That's a really good point and there are a few things to bear in mind with this. The smells that you get on clothes aren't actually the bacteria on your clothes per se. What happens is that your body create the ideal home for bacteria to live. They live on dead skin and sweat that you squirt out onto the body's surface. They produce trace elements and chemicals and metabolites, which soak up into your clothes like blotting paper, and they're a bit whiffy. So your clothes don't necessarily smell just because they're contaminated. When you wash your clothes you wash out those substances and that's why they have a nice smell. The excellent point you've made is whether these temperatures are hot enough to neutralise any bacteria that may be on the clothing. This could be important if you're working in a hospital, I suppose. The answer is that there are a lot of detergents in washing powder and it's very alkaline as well. It can cause caustic burns if you put it on the skin. It would take some pretty hardy bugs to survive it, but some can. Things like mycobacteria that cause TB can survive in those conditions. You maybe should consider washing at higher temperatures if you think there might be bugs loitering on there.

What's glaucoma?

Glaucoma is another important condition of ageing. It causes a gradual loss of peripheral vision. The reason this happens is that the nerve is being gradually damaged and this usually happens because the pressure in the eye is too high. The treatment that we can give to try and prevent this from happening is treatment to lower the pressure either with drops or sometimes with operations.

- What is astigmatism?

I'm very short sighted, but over the years I've started to suffer from a astigmatism. I'd like to know what it is and what causes it.

What is astigmatism?

The normal cornea, the bit at the front of the eye, is essentially spherical. But if it's not quite spherical, and is shaped like a rugby ball rather than a football, it creates a stigmatism. This means that the eye doesn't see perfectly in all planes. It's usually just the cornea that's shaped like a rugby ball, but it can sometimes be contributed to by the lens. This can run in families and there are certain diseases that cause it too.

- Will blood vessels damaged by retinopathy repair themselves?

I've got diabetic retinopathy in both eyes and I was wondering if there was any evidence that if you get your sugar levels just right, th...

Will blood vessels damaged by retinopathy repair themselves?

I'm not sure whether they can repair themselves but what seems to be the case is that if you keep your blood sugar very well controlled, the chance that you will develop severe diabetic retinopathy is reduced. So it's well worth trying to keep your diabetes extremely well controlled. You can stop it getting worse rather than reversing the damage you already have. Once the blood vessels have started to proliferate in the retina, then we know that they can be lasered with a very high energy laser and they can be destroyed. This prevents them from causing the damage that they would otherwise have caused to the retinal function.

- Why can't I recognise a face upside down?

Why can't I recognise a face that I've known for many years upside down?

Why can't I recognise a face upside down?

I'm afraid I don't have a clue! Although what I can say is that although we can't do it and other animals like sheep and cows can't recognise other sheep and cows when they're upside down, monkeys can actually do it. So it would seem to imply that if you habitually hang upside down then it's something you can do. They've not looked at bat face recognition as far as I know!

- Why is black and white better than red and white?

Why is it that we're perfectly comfortable with black and white movies and photographs when we know that the world is in colour, but the ...

Why is black and white better than red and white?

I can fairly say that I don't really know the answer to that. I suspect that it's just what we're used to. We're used to seeing colour photographs and we're used to seeing black and white photographs. But we're not really used to seeing red TV or green TV. I suspect that it's just what we're familiar with.

- Why do eyes sometimes get watery?

My eyes often become watery for no apparent reason. Why do you think this is?

Why do eyes sometimes get watery?

When you go out in the wind, it's a stimulus to tear production. Tears are quite useful because they keep the cornea, that's the front of the eye, moist. They keep the eye smooth and functional. But if you go out on a windy day, often you produce a few too many tears and they fill up the whole conjunctival sac and so you're looking through water. So I suspect that it's that: different conditions will bring it on.

- Could someone be so short sighted they can see at night?

We all know that when we open our eyes under water we see blurry images. I believe that this is because water has a different index of re...

Could someone be so short sighted they can see at night?

The problem is that in air, light is focussed by both the cornea at the front of the eye and the lens within the eye, and actually most of the focusing is done by the cornea. The reason the cornea can focus is because the refractive index of air and water are different. But as soon as you put your head under water, the refractive index of the water and the liquid within the eye are the same and the cornea no longer focuses the light. The cornea is then lost as a refractive surface. In answer to the question, our eye is said to have a focusing power of 50 diopters. 45 diopters is due to the cornea. So about three quarters of the eye's focusing power is due to the cornea. When you go under the water you lose the cornea as a focusing surface. So you would have to be 45 diopters myopic, which means normally that you would be focussed at just over two centimetres. I've never met anyone who's minus 45 diopters.

- What is macular degeneration?

I have neovascular macular degeneration of the right eye, which means that I haven't any sight left in that eye. I went to see if I could...

What is macular degeneration?

I can't give you an exact chance really. This problem of neovascular degeneration of the retina, which is a devastating cause of visual loss is due, we think, to some sort of metabolic fault in the retina. This allows blood vessels to grow into the retina and then distort the retina. Up until very recently we haven't had very good treatment for it, because lasers can be quite destructive. If you laser a retina, you destroy the retina in the process. But there are some very exciting developments because there are now two new drugs that seem to be helpful in this condition. So it's possible that in the future we may be able to treat people like you with this condition.

- How can we develop a squint overnight?

My husband has recently developed a squint overnight. Why is that?

How can we develop a squint overnight?

It's probably because one of the muscles is not working properly. There are six muscles and it's probably because one or maybe two of them are not working very well. So it could be a nerve being pinched somewhere, but as it happened overnight, it's more likely that a blood vessel blocked. So the supply to the nerve is damaged and the nerve doesn't work very well. Usually it will tend to recover in a few weeks.

Can bulls see red?

Yes, bulls can see red but they probably can't distinguish it from green. They only have one visual pigment in that part of the spectrum, which means that they can distinguish blue from other colours, but they can't distinguish red from green. They see the world like a red-green colour deficient person. So waving a red rag at a bull isn't a problem because it's red, but because it's moving. They're responding to the movement rather than the red, although they can still see the colour. They will just get confused between red and green rags.

- Do seabirds have polarised vision?

Do birds that hunt in water or below water have polarised vision, as it would be a good advantage for them to be able to see through the ...

Do seabirds have polarised vision?

A lot of animals have polarised vision, so that's actually a good suggestion. The other thing that I think is really interesting is birds that dive under the water. Earlier on we were talking about the way that humans become long sighted when they look under water; well birds that dive under water don't. They can actually accommodate up to 60 diopters, meaning that they can really change the shape of their lens. They do this by ramming their lens up against a very solid iris, and they form a great big bulge at the front of their lens.

- Why are the cones in our eyes red, green and blue?

Why are the cones in our eyes red, green and blue? Why not red, yellow and blue? How can we see yellow?

Why are the cones in our eyes red, green and blue?

They're not actually red, green and blue at all, although that's what we tend to call them. They're actually blue, green and yellow. The long wavelength cone absorbs mainly in the yellow and not in the red, so that's how we can see yellow.


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