Abstract

Early mammals presumably had at least two visual, two somatosensory/ motor, and one auditory cortical areas. Neocortical expansion has resulted in the increase in area sizes but more importantly in the proliferation of new cortical areas. Hypotheses to explain the addition of new cortical areas relate to the segregation of initially heterogeneous cortical areas in a trend to increase connectional speci- ficity and processing speed, and to the addition of new sources of morphogenetic activity as the brain expands. Other characteristics of the neocortex are the development of corticospinal axons, which permit direct cortical control over the spinal motoneurons and an increase in manual dexterity, as well as interhemispheric connections, which in placental mammals course via the corpus callosum. The latter may have originated as a consequence of the development of topographic sensory representation in the mammalian cerebral cortex, making it possible to establish a continuity between the two hemirepresentations of the sensory surfaces that are located in separate hemispheres.