Somatosensory fovea in the star-nosed mole: behavioral use of the star in relation to innervation patterns and cortical representation.

Eleven fleshy appendages, or rays, surround each of the nostrils of the star-nosed mole. Each ray is covered with tactile sensory organs, and each ray is represented in the cortex by a stripe of tissue visible in brain sections processed for cytochrome oxidase. Here we report that the 11th, ventral ray is the behavioral focus of the nose. This ray is preferentially used to explore prey items by touch, in a behavior pattern similar to the use of a fovea in the visual system. After prey is first contacted with any nasal ray, subsequent touches are centered on the 11th ray. Although the 11th ray is small and has relatively few sensory organs on its surface, it has the largest cortical representation, greatest area of cortex per sensory organ, and the highest innervation density per sensory organ. In addition, the average area of cortex per primary afferent is highest for this ray. We refer to the differential magnification of first-order afferents in the cortical representation as "afferent magnification." The patterns of both cortical magnification (cortical area per sensory organ) and afferent magnification (cortical area per afferent) of the rays correlated highly with the distribution of touches across the nose scored from videotaped behavior. A simple model of star-nosed mole behavior predicts the distribution of touches across the rays and also correlates highly with both the actual pattern of behavior and the patterns of cortical magnification observed.