08 September 2007

How does the dolphin eye work?


If you are a regular listener to the Dolphin Pod, then you'll be aware of the power and versatility of a dolphin's echolocation system. If you are inclined to be jealous of the sensory systems of other animals, you could do worse than to envy dolphin biosonar. It is pretty cool stuff. At least, you can rest assured that humans outshine dolphins when it comes to vision, right? Not so fast my simian friend - while it is true that humans and other primates do have well-developed visual systems, our dolphin friends are not far behind. This episode will provide you with a detailed look at dolphins' sense of vision by focusing (get it, focusing?) on the structure of the eyeball. It is a head-to-head battle between the human and the dolphin eyeball to see who will earn the title of 'coolest eyeball ever'!

The Human Eye

In order to judge the merits of our two eyeball contestants, we first need to know a little more about eyeballs in general. It is important to note that not all eyeballs are created equal - each species on this planet that is able to receive sensory information through photosensitive cells organizes their hierarchy of tissue in slightly different ways. In fact, the eye, and other organs that function like eyes, has evolved at least 40 different times during the course of evolution. Dolphins and humans share a common mammalian ancestor more than 65 million years ago. Still, they are both built on more or less the same plan. The basic process for humans and dolphins works like this: light enters the eyeball by first passing through the cornea. The cornea is the outer layer of the eye, the shiny surface that you can touch with your finger, but that you shouldn't touch with your finger. The light then travels through the pupil, the pitch black opening in the center of your eye, and the lens - a kind of clear, rugby-ball-shaped tissue lurking just behind the pupil. The lens helps to focus light rays that pass through to the interior of the eye. The interior chamber of the eyeball is filled with a kind of liquid. Lining the back of the eyeball is a collection of cells that are capable of turning light into electrical impulses that travel to your brain. This layer of cells is called the retina. The process of allowing light in through the cornea and then the iris/pupil, and focusing it with the lens onto the retina, is basically how humans and dolphins are able to see. The real work of turning these electrical impulses into images that you actually use to see is done by your brain, and this is where things start to get complicated. Luckily for you, and for me, I will skip over this part for now, though wait for a future podcast that will focus on the brain and cognition.

Okay, now that you are an eyeball anatomy expert, it is time to learn how the dolphin eyeball differs from the human eyeball. First and foremost, there is a major difference in where the eyeball is located on a dolphin's head versus the human head. Humans have two eyes positioned in parallel at the front of our head; this is a typical position for the eyes of a hunting animal that requires depth perception. That is, 3D vision comprised of one visual field occupying about 180 degrees of visual space. Dolphins have their eyes placed laterally - on the sides of their heads. They can move each eye independently, which allows them to have two fields of vision. This is the kind of setup that most high-school teachers would love to have as it would allow them to keep one eye on the blackboard and the other focused on the classroom. This is just what dolphins can do; they can look in two different directions at the same time. Dolphins have 300-degree panoramic vision. This might sound strange, but it isn't that hard for the brain to process two separate visual fields coming from each of the two dolphin eyes at the same time. If one of your eyes should happen to fall out of your head, but you were still able to see with it, then you too would be able to see two visual fields at the same time. It probably would not be a very pleasant experience, but your brain would eventually get used to it. Brains are resilient. For example, in a few famous psychological experiments from the late 19th century, subjects were made to wear strange glasses that made them see the world flipped upside down. At first, this made them quite ill, but after a few hours, or in some cases a few days, the subjects started to see the world right side up again, even when wearing the glasses. Their brain(s) compensated for this weird information and made the world go back to normal. So, dolphins have the ability to see two visual fields at the same time, but they can also move their eyes so that these visual fields overlap, just like human vision. This likely gives them 3D (or bi-scopic) vision. The best position for dolphins to get bi-scopic vision is when they tilt their heads up and look underneath themselves, which is why you may often see dolphins tilting their heads upwards to get a good look at you with both eyes. Or, seeing them swimming upside down as they follow flying fish that are above the water's surface.

Dolphins are able to see under water quite well, thanks to a few handy adaptations for living in an underwater environment. In humans, the cornea helps to refract or bend light as it passes through the eye. It accomplishes this simply by the fact that light traveling in air bends as it enters the cornea, which, like the rest of the eye, is essentially filled with liquid. You will likely have seen what happens to light as it enters liquid if you have ever dangled your feet in the pool. You will notice how the strange bending of light makes your feet distorted, usually larger than they look in air. This is because the light is being bent (refracted) in funny ways as it hits the water, just like what happens when it hits the cornea. But for dolphins, the cornea doesn't bend the light at all because the light is already traveling in a liquid. It is the lens (just behind the pupil) that does all of the bending. A dolphin's lens is not shaped like a rugby-ball as is the human lens, it is much more spherical - like a little ball. This shape allows it to refract light much more dramatically, compensating for the lack of refraction that happens at the cornea. When humans swim under water, we have a difficult time seeing things clearly. This is because our cornea is used to bending light that hits our eyes directly from air, not from water. You might think that when a dolphin sticks its head out of water it might have the same problem in reverse, but this is in fact not the case. Even though their cornea is made for accepting light from water, the lens is able to compensate for this sudden shift from water to air - it moves back and forth in the eye, allowing a dolphin to focus in air almost as well as it can in water. This same kind of back and forth movement of the lens is what helps a dolphin focus under water, as well.

The pupil is that little black hole in your eye. If you have ever watched ER or any other medical emergency show, you will have seen doctors shine a light into a patient's eyes to see if the pupil changes size when exposed to the light (a sign that all is well with the hapless patient's brain). The human eye has an automatic mechanism that changes the size of the pupil, allowing in more light when it's dark and less light when it is bright outside. Dolphins have something much stranger going on with their pupil. Instead of a fancy mechanism that makes the hole larger or smaller, dolphins have a flap above the eye called an operculum. When light levels are high, the flap is pulled down over the eye, resulting in two dark spots on either side of the eye and not just one spot in the middle like in humans. And dolphins, like cats and dogs, have a layer of reflective cells behind their retina, called a tapetem lucidem, which reflects lost light back out past the retinal cells a second time. This makes it easier for dolphins to see in low light levels.

What about color vision? Well, there haven't been any experimental tests actually proving conclusively that they do or do not have color vision. But scientists have examined the kinds of cells found in a dolphin's retina and concluded that they are unlikely to have very good, if any, color vision. For animals that can see color, a special type of cell called a 'cone' cell can be found in the retina. Cone cells are sensitive to light occurring in different color spectrums. Humans have three different types of cone cells, which gives us pretty good color vision. Other animals that have color vision have as few as two types of cone cells. But poor old dolphins only have one type of cone cell, so if they have any color vision at all, it is likely limited to colors in the blue-green spectrum. Given that water absorbs colors like red and yellow easily, but lets blue and green though, it makes sense that dolphins only really need to see things in blue and green - the ocean is a blue/green world.

Well, this has been a close contest; the human and the dolphin eyeball have both proven worthy competitors. Humans certainly have the upper hand when it comes to cone cells, beating the dolphin eyeball hands down when it comes to color vision. But, the extra weird features of the dolphin eye give them a coolness factor that humans lack (like the ability to move each eye independently). In an eyeball competition, it is hard to say who would come out the winner. In fact, I think I am going to call this competition a tie. (boo) Hey! I call 'em as I see 'em.

Further reading:

Mass, A. M. and A. Y. Supin (2002). Vision. Encyclopedia of Marine Mammals. W. F. Perrin, B. Wursig and J. G. M. Thewissen. San Diego, Academic Press: 1280-1293.

Mass, A. M. and S. A.Ya (1995). "Ganglion cell topography of the retina in the bottlenosed dolphin, Tursiops truncatus." Brain Behav Evol. 45(5): 257-265.

van der Pol, Bert A.E., Jan G.F. Worst, and Pek van Andel. "Macro-anatomical Aspects of the Cetacean Eye and Its Imaging System." In Sensory Systems of Aquatic Mammals, edited by R.A. Kastelein, J.A. Thomas, and P.E. Nachtigall, pp. 409-414. Woerden, The Netherlands-. De Spil Publishers, 1995.

Mass, Alla M. and Alexander Ya. Supin. "Retinal Resolution in the Bottlenose Dolphin (Tursiops truncatus)." In Sensory Systems of Aquatic Mammals, edited by R.A. Kastelein, J.A. Thomas, and P.E. Nachtigall, pp. 419-425. Woerden, The Netherlands: De Spil
Publishers, 1995.

Ridgway, S. H. (1990). The Central Nervous System of the Bottlenose Dolphin. The Bottlenose Dolphin. S. Leatherwood and R. R. Reeves. San Diego, Academic Press, Inc: 69-97.

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