Cornelia Bargmann joins forces with a tiny worm to study how the brain's wiring influences behavior.

If you've had this unpleasant experience, you won't forget it: You ate something bad and got sick as a dog. Even years later, the thought of eating that particular food again makes your stomach turn.

This response, called conditioned taste aversion, is one of the strongest forms of learning in mammals, says Cornelia I. Bargmann, an HHMI investigator at The Rockefeller University. "All it takes is a single experience to form very long-lasting memories." And the process is not unique to mammals. Fish, snails, and the cuttlefish (a relative of octopus and squid) show a similar response. In the November 10, 2005, issue of Nature, Bargmann's team reported that Caenorhabditis elegans, a nematode with only 302 neurons, does too.

If a worm with so few neurons can learn such a sophisticated behavior, just how many neurons does it take to establish a memory? Theoretically, only two: a sensory neuron to detect a stimulus and a motor neuron connected to a muscle that will carry out a behavior. But in actuality, neural circuits are never quite this simple and invariably involve more—though not necessarily a great many—cells.

The laboratory of Eric R. Kandel, an HHMI investigator at Columbia University who won the 2000 Nobel Prize in physiology or medicine for his work on memory, has studied the sea slug Aplysia. This animal rapidly learns to associate a noxious stimulus, like an electrical shock, with an innocuous cue, such as a light touch on its siphon. It will subsequently withdraw its gill in a protective behavior in response to just the light touch.

Photo: Matthew Septimus

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