Anatomical asymmetry as the basis for cerebral dominance

Despite repeated assertions by some critics of the impossibility of linking behaviour to study of the brain, there have been major, although not always continuous advances in this field over the past 100 years, some of them based on simple techniques. One example is the study of cerebral dominance, for which it was believed until quite recently that no anatomical basis had been specified. The paper summarizes the findings of striking anatomical asymmetry of the upper surface of the temporal lobe, in an area known to be involved in speech functions. Asymmetries observed in autopsy brains of adults and fetuses, both by gross and by cytoarchitectonic techniques, are reported. By use of radiological methods asymmetries have been shown in the brains and skills of living persons. Asymmetries also appear to be present in the endocasts of skulls of ancient humans, and in nonhuman primates. The distribution of asymmetries is different in left and right handers. The implications of these findings are discussed.