Stars and stripes from mole's nose to brain.

A star-nosed mole, digging tunnels in the dark, uses 22 fleshy rays that radiate from its nose to feel for edible morsels in the soil. These rays, dense with tactile receptors, enable moles to do "things with that nose that we can't do, that we don't even know," says Jon H. Kaas, a neuroscientist at Vanderbilt University in Nashville. Kaas wondered how such a complex sensory organ would be represented by the arrangement of nerves in the mole's brain.

He and Kenneth C. Catania, a neuroscience graduate student at the University of California, San Diego, examined 15 moles' brains and found that each ray corresponds to a distinct stripe of high neural activity in the cortex. Furthermore, the stripes form the same star pattern as the rays themselves. The investigators reasoned that because each nose ray moves independently, it activates a distinct group of clustered neurons in the cortex.

The finding helps explain principles of brain organization, the scientists report in the Aug. 5 NATURE. It supports the theory that "neurons that fire together stay together, and those that don't fire together separate from each other during development," Kaas says. Thus, neural activity patterns generated by distinct sensory surfaces, such as each nose ray, create a corresponding brain map of neurons. What's more, the mapping process begins in utero, stimulated by the physical activity of the developing fetus.

Scientists have found similar correlations between sensory organs and brain neurons in other species, Kaas says. Each individual whisker of a rat or mouse is represented by a barrel-shaped cluster of neurons in the cortex. Nerve clusters in the brains of monkeys and raccoons mirror their fingers. In each, the sensory organ is exquisitely sensitive and critical to survival. Small-eyed raccoons, for example, can barely see; instead, they feel vibrations generated by prey in water.

Such correlations are clearest in the compact brains of small animals, Kaas says. Neurons are distributed more broadly in large mammals, making the nerves' spatial patterns less obvious. However, "we think these brains are constructed the same way," Kaas adds. In humans, the most active sensory organ is the skin of the fingers, lips, and tongue. Thus, says Kaas, a human fetus sucking its thumb may stimulate neural activity that in turn shapes the arrangement of nerves in its developing brain.