Most animals sniff, listen, and look to learn about the world around them. But weakly electric fish probe their environment using pulses or waves of electricity. Now, scientists have shown that the fish's electric sense is sharper than they'd realized: The fish can judge the shape and orientation of objects using electricity alone.
Electric fish get their picture by generating an electric field and checking to see how it gets distorted. Objects that conduct electricity, such as other fish, warp the field differently than, say, rocks do. Scientists think the extra sense aids these stream-dwelling natives of Africa and South America in their nocturnal foraging. Previous work suggested that the fish can use touch and their electric sense to identify shapes.
Now, a team of biologists and neuroscientists has shown that electricity alone is enough to size things up. In the 4 May issue of Current Biology, the researchers, led by Christian Graff of the Laboratoire de Biologie du Comportement in Grenoble, France, describe how they put the fish's electric sense to the test. The team trained six fish--three each of two species--to recognize virtual objects created by the interaction of the fish's electric field with electrodes in an aquarium maze. Depending on the pattern of connected electrodes, the fish would detect different arrangements of bars or planes oriented horizontally or vertically.
The fish first encountered a virtual object in the trunk of the Y-shaped maze. One branch of the maze contained an electrical object identical to the first one, whereas the other branch had a different arrangement of electrodes. If the fish picked the familiar object, the fish earned a treat. It took time for the fish to cotton on to the idea, but once they figured it out, they almost always got it right. And the fish coped quickly when the researchers made the task harder by changing the target object so that only the horizontal or vertical orientation matched the first object.
"This helps us begin to understand the perceptual world these fish live in, how they see their three-dimensional electrical world," says neuroscientist Curtis Bell of Oregon Health & Science University in Portland. It's an interesting question, he explains, because although the fish has a bigger brain relative to body weight than a human does, it lacks a cerebral cortex--the part of the brain in humans that normally checks out shapes and makes comparisons.