Two different subcortical language networks supporting distinct Japanese orthographies: morphograms and phonograms.

Language systems worldwide are based on either morphograms or phonograms, but Japanese is unique in that uses a complicated combination of kanji (morphogram) and kana (phonogram) characters. The white matter networks associated with reading have been investigated previously but remain incompletely understood. In this study, we performed intraoperative language mapping under local anesthesia and postoperative language assessments of 53 consecutive patients who underwent awake craniotomy for surgical resection of cerebral glioma within the dominant temporal or parietal lobe. Six cases showing intraoperative dyslexia elicited by direct electrical stimulation (DES) were examined, and all cases showed transient symptoms of kanji or kana dyslexia during DES. We investigated the intraoperative positive mapping points localized near four white matter bundles: the arcuate fascicle, posterior superior longitudinal fascicle, inferior fronto-occipital longitudinal fascicle, and inferior longitudinal fascicle (ILF). The intraoperative DES distributions for kanji dyslexia were especially associated with the anterior-inferior side of the ILF. On the other hand, the DES points associated with kana dyslexia were localized on the posterior-superior side of the complex composed of these four tracts. These results suggest the presence of specific non-interfering networks that subserve the processes of reading morphograms and phonograms.