The barrel area is a unique specialization of the cerebral cortex, shared by many species of rodents and some marsupials, in which the somatotopic map of the body surface receives direct morphological expression. Here, the homogeneous sheet of layer IV granule cells seen in most mammals is fractured into large archipelagos, each representing one of the larger subdivisions of the contra- lateral half-body. Within these larger domains are smaller aggregates of granule cells that contain the concentrated terminations of thalamocortical fibers bear- ing messages emanating from constellations of...