Scientists believe the weakly electric fish electrosensory system is similar to our auditory system, and evolved along parallel lines.

The fish has a cerebellar-like structure, called the electrosensory lateral line lobe (ELL), that receives input from skin sensors. "The fish's skin is covered by several thousands of volt meters, like the photo receptors in our retina," describes Rasnow, pointing to the thin, knifelike fish undulating through the water.

Above, simulation of the electrical field surrounding the brown ghost knife fish.

"But unlike the retina -- or the cochlea, our sound receiver deep inside the inner ear -- they are totally accessible. Which is a huge advantage in terms of understanding how those receptors work."

The weakly electric fish may shed light on how the human brain collects and processes sensory information. And what scientists discover may soon settle one of the raging debates in neuroscience.

While conventional wisdom holds that the cerebellum's domain is fine motor control -- the movement of your fingers, for example, when you play the piano -- Bower and colleagues share the rather heretical notion that the human cerebellum actually controls the acquisition of sensory information. They are convinced fine motor control is a consequence, not a direct function, of the brain's relentless drive to collect sensory data. Thus, the cerebellum controls where the eyes are pointed in order to gather visual data, while it controls finger muscles to obtain tactile information. Simply put, fine motor control may be a "spin off" of the cerebellum's main event -- the processing of new sensations. And with its exceptional electrosensory skills, the pulse fish may able to demonstrate this concept -- and settle the argument -- more clearly than other species.

The electric fish gather sensory information through electrolocation. As they swim and discharge electric currents, the skin receptors acutely sense changes in the fish's environment when the electrical field is disturbed by the presence of rocks, plants or other fish. The skin receptors relay this information to the ELL, which uses it to create an "electric picture" of the outside world. The fact that the cerebellum in these fish is so large adds weight to the theory that it is primarily a sensory organ.

Intuitively, if the cerebellum actually did control fine motor movements, "with such a huge cerebellum you'd expect the fish to have the grace of a synchronized swimmer," argues Chris Assad, Rasnow's cubbyhole mate and all around partner in crime. "Instead, there's nothing exceptional about their movements. Many fish do swim very gracefully, but the species with the largest cerebellums -- pulse Mormyrids from Africa -- swim just like regular fish. But they do have this novel sensory modality. And if you examine cells inside their cerebellum, a lot of them are devoted to interpreting and integrating electrosensory input."

It's not a big evolutionary jump, furthermore, from fish to humans. "It turns out the cerebellum -- the actual circuitry, anatomy and connectivity -- is highly conserved across all vertebrates," says Assad, "so what's going on in the ELL gives us very good clues about what may be going on in the human brain.

-- Linda Marsa