Alexia and agraphia in Japanese

Abstract

Dissociated performance in the reading/writing of kanji (morphogram) and kana (phonogram) in alexia and agraphia in Japanese has attracted attention because kanji and kana differently convey semantics, orthography, and phonology. Although the dissociation between kanji and kana is relative, but not absolute, detailed case studies, in addition to recent neuroimaging and neurophysiological studies, have revealed the partially separable mechanisms of the kanji and kana writing systems. Kanji words are processed mainly, not exclusively, via the orthographic–semantic route, and kana words are processed via both orthographic–semantic and phonological routes. Pure alexia with lesions in the left medial occipital lobe and splenium shows alexia for both kanji and kana, but at the chronic stage, alexia for either kanji or kana may remain depending on the compensatory mechanism. The left occipitotemporal area is important for reading/writing. The slightly different location and extent of lesions are associated with distinct symptoms: kanji alexia with the lesion in the mid-fusiform gyrus (visual word form area; VWFA), kana alexia with lesions posterior to VWFA, and kanji alexia with kanji agraphia with lesions lateral to VWFA. The left angular gyrus has traditionally been considered important for writing. However, it is unclear whether the crucial site is the angular gyrus or the cortical and subcortical regions adjacent to it. In neurodegenerative diseases, reading/writing abilities are differentially impaired depending on the specific distribution of neuronal degeneration in each disease. Examining the reading/writing abilities of kanji and kana can facilitate understanding of the writing system and related disorders.