TUNNEL VISION

Electric fish manoeuvre in murky depths where it's too dark to see and sounds are too muted to hear. Their electric organs produce an electric field around their bodies, and electroreceptors in their skin detect distortions to this field caused by objects in their environment, so the fish can deftly navigate around any obstacles. And there are plenty of obstacles to deal with;electric fish hover among plant roots and in narrow caves. Angel Caputi wondered how hiding in refuges influences what these fish `see' in their electrical world (p. xxx).

To find out, Caputi's team put weakly electric banded knifefish into metal and plastic tunnels, simulating natural refuges. They wanted to see how the conductivity of the surroundings affected the fish's `electrical images' of objects. As they had suspected, the tunnels changed the electrical`illumination' of surrounding objects. Caputi explains that banded knifefish have an electroreceptor `hotspot' on their jaws, `like an electrosensory fovea.' When they put fish in plastic tunnels, Caputi's team noticed increased funnelling of the fish's electric field to its `electrosensory fovea'. Caputi translates what the fish face: `in vision, this is like a person holding a candle, generating light just like the fish generates electricity, with a mirror reflecting the candlelight and illuminating objects in front of him.'But the metal tunnel presented a different challenge. The team found that the metal tunnel short-circuited the return of currents generated at the fish's tail, preventing funnelling of these currents to the `electrosensory fovea'near the fish's mouth. For the fish, this is the electrical equivalent of `a person with a black board instead of a mirror, absorbing the light from the candle,' says Caputi.

Surprisingly, banded knifefish take these perceptual problems in their stride. Caputi's team found that the fish's brain stores a moving average of past electrical images and subtracts this from current electrical input,allowing the fish to detect anything novel in its surroundings, whatever the background.