Cerebral asymmetry: Historical introduction.

Cortical localization and hemispheric asymmetry of function are key concepts for analyzing the relationship between the brain, mind, and behavior in clinical neurology, neuropsychology, cognitive neuroscience, and modern philosophy. This chapter presents the history of asymmetries in the human central nervous system within the context of right-left asymmetries of bodily structure, as can be detected in individuals of virtually all living species. It has been known since antiquity that behavioral deficits on one side of the body could be caused by brain lesions on the opposite side. Yet, the idea that the two cerebral hemispheres could subserve different functions emerged only in the 19th century following the work of Franz Gall, Marc Dax, and especially Paul Broca. The concept of functional duality flourished during the golden age of cortical localization. Doubts occurred following the antilocalizationist tendencies in neurology and psychology, a movement mainly led by Karl Lashley. Later, a disciple of Lashley, Roger Sperry, and his coworkers unequivocally demonstrated the functional specialization of the cerebral hemispheres, thanks to their work on split brain. Finally, Norman Geschwind and the many scholars who were inspired by him have clarified, complemented, or changed the way of thinking of clinical neurologists about the great neuropsychologic sectors of aphasia, agnosia, and apraxia, as well as consciousness.