Star-nosed mole embryo. Courtesy of Kenneth C. Catania, Vanderbilt University, Tennessee, USA.

Not all sensory surfaces are created equal. Our sense of touch, for example, is exquisitely good on our hands and face, and less so on our abdomen. How is this so? It has long been recognized that more cortical brain space is devoted to certain sensory surfaces, and the increased brain space probably underlies the heightened perceptual abilities associated with the corresponding areas. It has been estimated, for example, that there is about a hundred times as much cortical tissue devoted to similarly sized regions of the fingers as compared with the abdomen. The same principle of cortical magnification holds true for cortical areas that represent behaviourally critical regions in other species, such as the facial whiskers in rats and the platypus's bill. But how do nerve fibres from certain sensory surfaces capture more cortical space to begin with? By studying a very unusual creature, the star-nosed mole, Kenneth Catania of Vanderbilt University offers some interesting clues as to how this cortical magnification might arise during development.

Star-nosed moles have eleven appendages surrounding each nostril. Each appendage or 'ray' contains thousands of small mechanoreceptor organs, and the moles use their rays to explore the environment and find prey. One ray in particular acts as a tactile 'fovea' (analogous to the central region of the retina), and it is used for detailed exploration of objects of interest. Catania has previously shown that this foveal ray is over-represented in the adult cortex, even though it is not different in any obvious way from the other ten rays. Now, by studying the development of mole embryos, Catania shows that the ray destined to become the fovea has a distinct developmental history: its mechanoreceptor organs mature early and its associated nerve terminals are active in the cortex before the rest. In short, it gets a head start in development, which might give it an early advantage in the competition for cortical space. The results bear striking similarities to observations made in the visual system, and suggest that the final wiring pattern of the brain might be determined in part by the timing of events during development. Star-nosed moles might provide a good system to investigate the underlying mechanisms in more detail.