Mole nose its own mind

Nerves in the mole's ultra-sensitive nose stake out their space in the brain.

Touch and go: the sensitive snout of the star-nosed mole.

"It's a biological novelty," says Ken Catania of his favourite organ, the incredibly touch-sensitive snout of the star-nosed mole. Now research from his lab shows that nerves from the nose fight for extra space in the brain¹. The findings could help us to understand how our brains allocate space to the senses.

Twenty-two fleshy little fingers protruding from around the star-nosed mole's nostrils probe its swampy underground world. "It probably has the best sense of touch of any mammal," says Catania, of Vanderbilt University in Tennessee. This due to the large numbers of nerve fibres packed into the animal's nose.

In the mole's brain, a large part of the cortex is taken up with processing the flood of information from its touchy nose. One proboscis in particular, a hypersensitive facial phallus called the tactile fovea, hogs a quarter of the nose's brain space. "The question is, how does so much cortex get devoted to these inputs?" asks Catania.

To answer this, he looked at the nose and brain of embryonic moles. Despite being the smallest proboscis in the adult, in the embryo the tactile fovea was larger and more advanced in development than any of its neighbouring nasal fingers. What's more, the fovea was the first part of the nose to make nervous connections with the cortex.

Catania thinks that nerves have to compete for space in the cortex -- and the fovea uses accelerated development to stake out its space. "This early enlargement and maturation may give inputs from the most important regions of the body a competitive advantage in 'capturing' brain areas," he says.

"Competition for cortical territory is a well established principle," says David Price who works on cortical development at the University of Edinburgh. In previous experiments on primates who grow up with one eye closed, the inactive eye loses its territory in the cortex -- and the surrendered space is taken over by the active eye. This showed competition for cortical space is based on nervous activity.

Catania's proposal that competition happens early on during development "is an interesting idea" says Price.

Scientists have long debated how cortical maps are formed -- early on in the embryo, or later, with activity and experience. "It's bounced back and forth between the two ideas," explains Dale Purves, who works on the visual cortex at Duke University Medical Center in North Carolina. Although Catania's findings support the early hypothesis, Purves believes both are probably involved.

A dense network of nerve fibres in the mole's nose makes it super-sensitive to touch

The mole's touch fovea is comparable to our own retinal fovea, the area at the back of the eye where light receptors are most tightly packed and which produces the sharpest vision. Like the mole's fovea, the retinal fovea develops early and takes up a big portion of cortex. "This could be a potential mechanism for capturing its space," says Catania. "And could relate to how brain maps are developing more generally."

The size of our own touch-sensitive areas, such as the fingertips and lips, are also mirrored in a cortical 'touch' map of the body. "We're not far along in understanding our own map," says Purves, and he's sceptical about how much the mole will help.