Two scripts, two pathways: dorsal-ventral biases in post-stroke kana-kanji agraphia

Stroke-induced writing disorders offer valuable insights into the neural mechanisms of writing. The Japanese writing system is particularly useful for such investigations, as it includes both kana (phonograms) and kanji (morphograms). Kana is primarily associated with phonological processing, whereas kanji relies on lexical-orthographic pathways. Although previous research suggests there are distinct neural substrates for kana and kanji, most studies have focused on small cohorts with pure agraphia, and large-scale investigations of aphasia remain underexplored. To address this gap, we examined a large stroke cohort to identify anatomical differences underlying phonogram versus morphogram processing. We analysed 315 patients with post stroke aphasia who underwent a comprehensive battery of language assessments, including kana and kanji writing tests, and MRI at multiple stroke centres between 2016 and 2024. Using multivariate support vector regression-based lesion-symptom mapping and structural disconnection analyses based on a continuous permutation-based family-wise error correction, we investigated associations between lesion/disconnectome maps and various language scores, with a particular focus on writing scores. To complement voxel-wise mapping, we prespecified a tract-level multivariable regression to quantify disconnection load across language-related white matter tracts. We evaluated 315 patients (mean age 67.2 ± 13.3, 34.6% female). Lesion-symptom mapping suggested that impairments in kana writing were associated with left frontal regions, whereas lesion clusters associated with kanji writing emerged primarily in the white matter underlying the supramarginal and angular gyri. Disconnectome analyses implicated left dorsal pathways (e.g. arcuate fasciculus, superior longitudinal fasciculus) in kana writing impairments, with left ventral pathways (e.g. inferior fronto-occipital fasciculus, inferior longitudinal fasciculus) implicated in kanji writing impairments. Complementary tract-level regression mirrored this dissociation: kana outcomes showed significant associations with dorsal pathways, whereas kanji outcomes were predominantly associated with ventral pathways. These findings highlight distinct neural pathways for writing phonograms (kana) and morphograms (kanji), providing novel insights into the neural mechanisms of writing disorders. In this large multicentre aphasia cohort, disconnectome mapping and tract-level regression provide convergent evidence for a dorsal-ventral dissociation between pathways associated with phonogram- and morphogram-related writing impairments. Our results contribute to understanding script-specific neural processing and may inform future assessment and rehabilitation strategies for aphasia in languages with complex writing systems.