Sunday, August 31, 2008

Study says eyes evolved for X-Ray vision

The advantage of using two eyes to see the world around us has long been associated solely with our capacity to see in 3-D. Now, a new study from a scientist at Rensselaer Polytechnic Institute has uncovered a truly eye-opening advantage to binocular vision: our ability to see through things.

Most animals — fish, insects, reptiles, birds, rabbits, and horses, for example — exist in non-cluttered environments like fields or plains, and they have eyes located on either side of their head. These sideways-facing eyes allow an animal to see in front of and behind itself, an ability also known as panoramic vision.

Humans and other large mammals — primates and large carnivores like tigers, for example — exist in cluttered environments like forests or jungles, and their eyes have evolved to point in the same direction.

While animals with forward-facing eyes lose the ability to see what's behind them, they gain X-ray vision, according to Mark Changizi, assistant professor of cognitive science at Rensselaer, who says eyes facing the same direction have been selected for maximizing our ability to see in leafy environments like forests.

All animals have a binocular region — parts of the world that both eyes can see simultaneously — which allows for X-ray vision and grows as eyes become more forward facing.

Demonstrating our X-ray ability is fairly simple: hold a pen vertically and look at something far beyond it. If you first close one eye, and then the other, you'll see that in each case the pen blocks your view. If you open both eyes, however, you can see through the pen to the world behind it.

To demonstrate how our eyes allow us to see through clutter, hold up all of your fingers in random directions, and note how much of the world you can see beyond them when only one eye is open compared to both. You miss out on a lot with only one eye open, but can see nearly everything behind the clutter with both.

"Our binocular region is a kind of 'spotlight' shining through the clutter, allowing us to visually sweep out a cluttered region to recognize the objects beyond it," says Changizi, who is principal investigator on the project. "As long as the separation between our eyes is wider than the width of the objects causing clutter — as is the case with our fingers, or would be the case with the leaves in the forest — then we can tend to see through it."

To identify which animals have this impressive power, Changizi studied 319 species across 17 mammalian orders and discovered that eye position depends on two variables: the clutter, or lack thereof in an animal's environment, and the animal's body size relative to the objects creating the clutter.

Changizi discovered that animals in non-cluttered environments — which he described as either "non-leafy surroundings, or surroundings where the cluttering objects are bigger in size than the separation between the animal's eyes" (think a tiny mouse trying to see through 6-inch wide leaves in the forest) — tended to have sideways-facing eyes.

"Animals outside of leafy environments do not have to deal with clutter no matter how big or small they are, so there is never any X-ray advantage to forward-facing eyes for them," says Changizi. "Because binocular vision does not help them see any better than monocular vision, they are able to survey a much greater region with sideways-facing eyes."

However, in cluttered environments — which Changizi defined as leafy surroundings where the cluttering objects are smaller than the separation between an animal's eyes — animals tend to have a wide field of binocular vision, and thus forward-facing eyes, in order to see past leaf walls.

"This X-ray vision makes it possible for animals with forward-facing eyes to visually survey a much greater region around themselves than sideways-facing eyes would allow," says Changizi. "Additionally, the larger the animal in a cluttered environment, the more forward facing its eyes will be to allow for the greatest X-ray vision possible, in order to aid in hunting, running from predators, and maneuvering through dense forest or jungle."

Changizi says human eyes have evolved to be forward facing, but that we now live in a non-cluttered environment where we might actually benefit more from sideways-facing eyes.

"In today's world, humans have more in common visually with tiny mice in a forest than with a large animal in the jungle. We aren't faced with a great deal of small clutter, and the things that do clutter our visual field — cars and skyscrapers — are much wider than the separation between our eyes, so we can't use our X-ray power to see through them," Changizi says.

"If we froze ourselves today and woke up a million years from now, it's possible that it might be difficult for us to look the new human population in the eyes, because by then they might be facing sideways."

Changizi's research was completed in collaboration with Shinsuke Shimojo at the California Institute of Technology, and is published online in the Journal of Theoretical Biology.

Source: Rensselaer Polytechnic Institute

Saturday, August 23, 2008

Cooking and Cognition: How Humans Got So Smart

After two tremendous growth spurts — one in size, followed by an even more important one in cognitive ability — the human brain is now a lot like a teenage boy.

It consumes huge amounts of calories, is rather temperamental and, when harnessed just right, exhibits incredible prowess. The brain's roaring metabolism, possibly stimulated by early man's invention of cooking, may be the main factor behind our most critical cognitive leap, new research suggests.

About 2 million years ago, the human brain rapidly increased its mass until it was double the size of other primate brains.

"This happened because we started to eat better food, like eating more meat," said researcher Philipp Khaitovich of the Partner Institute for Computational Biology in Shanghai.
But the increase in size, Khaitovich continued, "did not make humans as smart as they are today."

The early shift

For a long time, we were pretty dumb. Humans did little but make "the same very boring stone tools for almost 2 million years," he said. Then, only about 150,000 years ago, a different type of spurt happened — our big brains suddenly got smart. We started innovating. We tried different materials, such as bone, and invented many new tools, including needles for beadwork. Responding to, presumably, our first abstract thoughts, we started creating art and maybe even religion.

To understand what caused the cognitive spurt, Khaitovich and colleagues examined chemical brain processes known to have changed in the past 200,000 years. Comparing apes and humans, they found the most robust differences were for processes involved in energy metabolism.

The finding suggests that increased access to calories spurred our cognitive advances, said Khaitovich, carefully adding that definitive claims of causation are premature.
The research is detailed in the August 2008 issue of Genome Biology.

The extra calories may not have come from more food, but rather from the emergence of pre-historic "Iron Chefs;" the first hearths also arose about 200,000 years ago.

In most animals, the gut needs a lot of energy to grind out nourishment from food sources. But cooking, by breaking down fibers and making nutrients more readily available, is a way of processing food outside the body.

Eating (mostly) cooked meals would have lessened the energy needs of our digestion systems, Khaitovich explained, thereby freeing up calories for our brains.
Instead of growing even larger (which would have made birth even more problematic), the human brain most likely used the additional calories to grease the wheels of its internal functioning.

Digestion question

Today, humans have relatively small digestive systems and burn 20-25 percent of their calories running their brains. For comparison, other vertebrate brains use as little as 2 percent of the animal's caloric intake.

Does this mean renewing our subscriptions to Bon Appetit will make our brains more efficient? No, but we probably should avoid diving into the raw food movement. Devoted followers end up, said Khaitovich, "with very severe health problems."

Scientists wonder if our cognitive spurt happened too fast. Some of our most common mental health problems, ranging from depression and bipolar disorder to autism and schizophrenia, may be by-products of the metabolic changes that happened in an evolutionary "blink of an eye," Khaitovich said.

While other theories for the brain's cognitive spurt have not been ruled out (one involves the introduction of fish to the human diet), the finding sheds light on what made us, as Khaitovich put it, "so strange compared to other animals."