Asymmetries of cerebral neuroanatomy

The mammalian cerebral cortex is asymmetrical. One hemisphere does not contain cortical areas or architectonic patterns, histological features, ultrastructural characteristics, or connectivities of the neurons that are not present in the other: homologous areas on the two sides may differ only in size. Asymmetry has directionality: two-thirds of human brains have plana temporale that are larger on the left. Conversely, roughly the same number of non-human brains show asymmetry in one direction as in the other. Asymmetry has magnitude: some brains show a large asymmetry, others show no asymmetry in a given area. Symmetrical areas are larger than their asymmetrical counterparts, which reflects fewer neurons in the latter. Indirect evidence points to variable asymmetry in the germinal zones in the production of symmetrical or asymmetrical cortical areas. These areas differ in their patterns of callosal connections. Fewer connections are seen in the asymmetrical cases, paralleling the smaller number of neurons. The symmetrical cases contain connections that are more widely distributed. These findings of different numbers of neurons and different proportions of callosal connections suggest that symmetrical and asymmetrical cortical areas may have different functional properties.