In an eighth-floor laboratory overlooking the East River, Cornelia I. Bargmann watches two colleagues manipulate a microscopic roundworm. They have trapped it in a tiny groove on a clear plastic chip, with just its nose sticking into a channel. Pheromones — signaling chemicals produced by other worms — are being pumped through the channel, and the researchers have genetically engineered two neurons in the worm’s head to glow bright green if a neuron responds.

These ingenious techniques for exploring a tiny animal’s behavior are the fruit of many years’ work by Dr. Bargmann’s and other labs. Despite the roundworm’s lowliness on the scale of intellectual achievement, the study of its nervous system offers one of the most promising approaches for understanding the human brain, since it uses much the same working parts but is around a million times less complex.

Caenorhabditis elegans, as the roundworm is properly known, is a tiny, transparent animal just a millimeter long. In nature, it feeds on the bacteria that thrive in rotting plants and animals. It is a favorite laboratory organism for several reasons, including the comparative simplicity of its brain, which has just 302 neurons and 8,000 synapses, or neuron-to-neuron connections. These connections are pretty much the same from one individual to another, meaning that in all worms the brain is wired up in essentially the same way. Such a system should be considerably easier to understand than the human brain, a structure with billions of neurons, 100,000 miles of biological wiring and 100 trillion synapses.

The biologist Sydney Brenner chose the roundworm as an experimental animal in 1974 with this goal in mind. He figured that once someone provided him with the wiring diagram of how 302 neurons were connected, he could then compute the worm’s behavior.