Brian Rasnow plunges two stick-like amplifiers into one of several large fish tanks crammed inside his narrow cubbyhole of a lab --an elongated walk-in-closet of a room with high ceilings that provide the only relief from claustrophobia. With his enormous, expressive eyes and lithe build, Rasnow looks like a mischievous elf leaning over the tank. Suddenly, a clicking noise that sounds like a Geiger counter going berserk emanates from the speakers perched on the metal shelves that line one wall. It's the sound of the the electric field generated by the nocturnal fish that live inside the tanks. Like primitive sonor, these weakly electric fish, known as pulse fish, emit electrical impulses to either communicate with other fish or bounce off objects in their paths, enabling them to navigate through the murky waters of their natural habitat.
"This is how easy it is to eavesdrop," says Rasnow, grinning like a delighted schoolboy. "The first time I did this, I hooked up my fish tank at home. I just stuck two kitchen forks in the tank and connected them to my stereo. All we're hearing is the voltage between those two wires." The fish's neurons are firing in synchronous millisecond pulses, creating an electrical field that the fish uses to perceive the world. If there's a distortion in the field, the fish senses something is there. But how does an object distort the field? And how does a fish discriminate between different objects? Between something big or small? Far away or close up?
These are questions posed by Brian Rasnow and his colleague Chris Assad as they work to understand how brains work. Scientists have studied the "strong" cousins of weakly electric fish -- the eels and rays -- for centuries. It's well known that Ben Franklin was fascinated by these fish and their relatively powerful currents in the 1700s. And even Volta modeled the battery after the eels' electrical organs. But only weakly electric fish generate electrical currents for use as a sensory organ.
Not until 1951, though, did scientists possessed equipment sensitive enough to detect the faint electrical impulses emitted by weakly electric species like the pulse or wave fish. In the years since, they've learned plenty. "In just a few decades," notes Brian Rasnow, "electric fish went from a zoological curiosity to become one of the best understood neuroethological systems."
-- Linda Marsa
