The brain of homo sapiens acquired the ability of reading and writing after the long-standing phylogenetical development. The invention of letters more than 5,000 years ago by our ancestors has realized the communication beyond the limit of time and space, and accordingly all the living creatures finally entered into the histological age of evolution. Thus, the cerebral mechanism of reading and writing might be one of the ultimate goals of the evolution of brain.
Scientific approach to the study of cerebral mechanism of reading and writing was initiated by the clinical observations of the patients who had become unable to read and write due to focal brain damage. Through these classical studies, the angular gyrus theory was proposed in the western world to explain the brain function underlying the ability of reading and writing. Although the angular gyrus theory was successful in explaining the anatomo-clinical features of alexia (disability in reading) and agraphia (disability in writing) in westerners, there had been found cases of alexia and/or agraphia in Japan whose clinical pictures could never been explained by this simple classical theory.
The enigma was solved when a patient with alexia and agraphia only in kanji with preservation of reading and writing in kana was found to have a focal damage in the left posterior inferior temporal lobe, which was thought to be involved in the process of semantic reading as well as visual evocation of letter form in writing. Upon the clinical observations we proposed the dual processing model of cerebral mechanism in reading and writing. Our recent investigations of PET scan activation studies are now confirming the validity of this dual processing model.
