The eye(s)

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(Photo by artofdreaming)

Ed Yong has an absolutely brilliant article in National Geographic about the receptical for neuroscience’s favorite sensory system, vision:

But simple eyes should not be seen as just stepping-stones along a path toward greater complexity. Those that exist today are tailored to the needs of their users. A sea star’s eyes—one on the tip of each arm—can’t see color, fine detail, or fast-moving objects; they would send an eagle crashing into a tree. Then again, a sea star isn’t trying to spot and snag a running rabbit. It merely needs to spot coral reefs—huge, immobile chunks of landscape—so it can slowly amble home. Its eyes can do that; it has no need to evolve anything better. To stick an eagle’s eye on a sea star would be an exercise in ludicrous excess…

“Insects and crustaceans have become so successful despite their compound eyes, not because of them,” says Nilsson. “They would have done so much better with camera-type eyes. But evolution didn’t find that. Evolution isn’t clever.”

Eric Warrant, Nilsson’s next-door neighbor at Lund University, takes a more lenient view. “Insect eyes have a much faster temporal resolution,” he says. “Two flies will chase each other at enormous speed and see up to 300 flashes of light a second. We’re lucky to see 50.” A dragonfly’s eye gives it almost complete wraparound vision; our eyes do not. And the elephant hawk moth, which Warrant has studied intensely, has eyes so sensitive that it can still see colors by starlight. “In some ways we’re better, but in many ways, we’re worse,” Warrant says. “There’s no eye that does it all better.” 
Our camera eyes have their own problems. For example, our retinas are bizarrely built back to front. The photoreceptors sit behind a tangled web of neurons, which is like sticking a camera’s wires in front of its lens. The bundled nerve fibers also need to pass through a hole in the photoreceptor layer to reach the brain. That’s why we have a blind spot. There’s no benefit to these flaws; they’re just quirks of our evolutionary history.

It does make you wonder what it is like to be a bat, so to speak. Consider the scallop:

The mantle of the bay scallop (Argopecten irradians) is festooned with up to 100 brilliant blue eyes. Each contains a mirrored layer that acts as a focusing lens while doubling the chance of capturing incoming light.

What is it like to have access to 100 eyes? It is not so simple as just imagining that you could ‘see more’. Our eyes sense more than what we just see (so to speak). In human retinas, melanopsin isn’t used to form images but to help entrain circadian rhythms. These neurons send information to a different part of the brain (the suprachiasmatic nucleus) in a way that we can fundamentally feel in a different way. Now imagine those 100 eyes: are they all there for the same thing? Is the feeling of one the same as the feeling of another?

 

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