Cortical cartography: what's in a map?

It has been known for over a century that sensory and motor areas in the mammalian cerebral cortex contain maps whose topography mirrors the physical arrangement in the body of the sensory inputs or the motor outputs. For example somatosensory cortex contains a map of the body surface (Figure 1a), motor cortex contains a map of the body musculature, visual cortex contains a topographically ordered map of visual space and auditory cortex contains a map of sound frequency (Figure 1b). Sensory receptors are concentrated in body regions where sensation is more acute, for example in the lips, tips of the fingers, and central foveal region of the eye, and the corresponding regions in the brain maps are enlarged, roughly in proportion to the density of receptors. In addition to being distorted in ways that reflect the functional importance of different body regions, cortical maps are plastic and can adapt to experience, especially if it occurs early in life. For example, the area of cortex devoted to inputs from the fingers appears to be unusually large in violinists who have begun their training before the age of 11 years, whereas amputation of a limb or digit can lead to a shrinkage of the cortical area devoted to the missing sensory inputs. Map organisation also varies not just within species but between them, in ways which reflect the varying importance of different sensory modalities and submodalities to the species concerned.