The passive electrosensory system is one of the most primitive sensory systems found in vertebrates. When prehistoric creatures crawled onto land,they lost the ability to detect the weak electric fields of objects around them. But the sense is still used today by a smattering of fish species, such as the paddlefish. This ancient fish lives in murky waters and uses the tens of thousands of electroreceptors scattered along its rostrum, a long paddle-shaped snout, to track down tasty water fleas. Michael Hofmann, Boris Chagnaud and Lon Wilkens took a closer look at the paddlefish brain to find out how these extraordinary fish detect their minute prey(p. 4213).
The team examined the responses of neurons in the paddlefish brain to search for evidence of spatial information processing pathways that are commonly found in other sensory systems. They recorded from the paddlefish's lateral line nerve fibres, which run from the electroreceptors in the skin to a region of the hindbrain called the dorsal octavolateral nucleus (DON), the first processing station of electrosensory information in the paddlefish brain. To see how paddlefish process the information from their electroreceptors, the team compared these recordings to those from neurons in the DON.
Given that some sort of mapping is common in most sensory systems, the team suspected that the paddlefish electrosensory system incorporates a topographic map of the fish's body surface. In other words, they expected to find a topographic relationship between the electroreceptive regions in a fish's skin and the position of the corresponding neurons in its DON that respond when these skin regions are stimulated. To investigate whether paddlefish have a topographic map, the team simulated a water flea by moving silver wires surrounded by an electric field along a paddlefish's body. Using an electrode to record the response of the fish's DON neurons, they moved the `water flea'to various skin locations and observed when they got the best response from the DON neurons. To their surprise, they didn't find any correspondence between the stimulated skin locations and the locations of the DON neurons that responded. They conclude that, unlike most other sensory systems, the paddlefish electrosensory system lacks a topographic map. Instead, there must be some other mechanism of organization that allows the fish to pinpoint its dinner.
Then the team discovered something rather unexpected. As they moved the`water flea' along the paddlefish's body, they noticed that the DON neurons respond to changes in the electric field's signal strength over time. `The skin's electroreceptors are a 2D array, and the touch sense normally operates in the 2D plane. But electroreceptors can also detect objects at a distance,thus adding a third dimension', Hofmann explains. The team proposes that paddlefish compute this third dimension by analysing the time domain of a passing signal; that is, the DON neurons process how quickly the `water flea'passes by and use this information to calculate the prey item's distance. They conclude that a paddlefish's electrosensory system processes information about the time course of a passing prey item's electric field to enable the fish to close the appropriate gap between itself and its dinner.