There are only three types of eyes that are fundamentally different: simple eyes (like ours), apposition compound eyes (found in most insects) and superposition compound eyes (less common, but found in lobsters, for example). But hidden beyond this relative uniformity, there lies an enormous diversity. Every species is the product of evolution, and different environments place very different demands on the eyes. When you think of vision, you probably think of your own highly mobile, forward-facing eyes. But for most of the animal kingdom, vision is something very different.

A particularly striking example of specialized vision can be found in marine animals that live about 200 to 800 meters beneath the surface (Land, 2000; Land & Nilsson, 2002). A little bit of sunlight still penetrates to these depths. Sunlight obviously comes from above, so these midwater creatures live in an environment where everything below them is pretty much completely dark and everything above them is, during daytime at least, relatively light. This is a peculiar environment to live in and it has given rise to a peculiar type of vision.

Essentially, midwater animals have two pairs of eyes. One pair looks up at the surface, and one pair looks down into the dark deep. This is true for a wide variety of midwater species, regardless of what type of eyes they have, which in itself is a beautiful illustration of convergent evolution.

Some animals do not really have two distinct pairs of eyes, but rather a single pair with specialized optics …

“To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.”

It's not hard to see why this famous quote from Darwin's “The Origin of Species” is a favorite among creationists, particularly when taken out of context. But actually, the story of the evolution of the eye is a wonderful illustration of the power of natural selection. And when told step-by-step it's not absurd at all.

Nowadays having eyes is nothing special, but it wasn't always like that. Although life has been around for billions of years, it was not until the Cambrian explosion, about half a billion years ago, that animals (that we would recognize as such) appeared. Before the Cambrian explosion, life consisted of simple cells and cell colonies, which, it's safe to assume, were blind. After, life was mostly still like that (it still is), but there were also complex animals, such as Trilobites, which were equipped with compound eyes, not unlike those of modern insects.

So what happened in between? There is no fossil evidence, but it's a good bet that the first “eye” was simply a light sensitive patch on the body of some lucky mutant (see (a) in the figure on the right). Of course, such a patch offers very little in terms of resolution and directionality …

The cogsci.nl software family has a new member! Mantra is a program for object tracking and has been designed specifically to be used in psychological experiments. For example, with Mantra you can track the hand of a participant while he/ she is performing some kind of experimental task. There are a number of example experiments available, showing how to use Mantra in combination with E-Prime and Python.

Although there are, of course, many object tracking systems available, the beauty of Mantra is that it doesn't require expensive dedicated hardware. You simply plug in a decent webcam and you are good to go!

You can find downloads and more information here!

You are, of course, familiar with the five classic senses: vision, hearing, smell, taste and touch. But when you take an elevator you can readily feel that this list is incomplete. The queezy sensation that you get in an elevator is largely due to our vestibular system, which provides us with a sense for balance and acceleration. We also have relatively distinct senses for pain, temperature, various aspects of our internal organs (interoception) and the position of our body (proprioception). But even more interesting, I think, are those senses that are completely alien to us, specialized senses that are found only in a few species of animal.

The philosopher Thomas Nagel famously wondered what it's like to be a bat (Nagel, 1974). Although being a bat must be very different from being human in a lot of ways, Nagel referred specifically to the bat's sense of echolocation, which, he felt, is so different from anything that we can experience that we can only guess how a bat perceives the world. And he has a point. Unlike nocturnal birds, which simply have very sensitive eyesight, bats listen to the echoes of their screams (too high for us to hear) to navigate through the dark. The idea behind echolocation is pretty simple: The delay between the scream and the echo tells the bat how far away an obstacle is. If the echo is heard first with the left ear, the object is on the left. And more subtle clues, such as how …

If you rank animals by intelligence, you get a nice, orderly progression (leaving aside the question of what intelligence really is, assuming that I know it when I see it). Invertebrates show little intelligence to speak of, with the exception of arthropods, such as bees, cockroaches and spiders, who exhibit remarkably flexible behavior. Still, remarkable though they may be, they are not as smart as amphibians, fish and reptiles, who are, in turn, not as smart as birds. And at the top of the IQ-chain, of course, we find ourselves, the mammals.

But then there is the octopus, the nerdy cousin of the snail. Octopuses [1] are the only animals I can think of that are fairly intelligent, but are in no way part of the evolutionary axis-of-intelligence formed by the vertebrates. They have a kind of weird, alien intelligence, which is why I think the octopus is the single most fascinating species of animal. (I regret to say that I've never seen one in real life, because all the zoos that I've visited either did not have an octopus or it's aquarium was mysteriously vacant, as cages and aquariums in zoos tend to be.)

Let me give you some numbers, mostly taken from Hochner's delightfully accessible review (2008). An octopus has about 50,000 times as many neurons (brain cells) as a garden snail, which, like the octopus, is a mollusc (molluscs form a rather large phylum, but still). This may not sound terribly impressive, because 50,000 times …