Dissociation of letter and picture naming resulting from callosal disconnection

Background: Detailed mapping of the corpus callosum for functional fractionation in humans remains incomplete. Objective: To examine separable interhemispheric transfer of visual information by callosal fibers, especially in the splenium. Methods: We examined callosal disconnection signs in a 14-year-old boy with a lesion confined to the posterior part of the splenium and reviewed reported cases with callosal lesions. Results and Conclusion: The patient presented with left hemialexia as the only manifestation of callosal disconnection syndrome. The only difficulty demonstrated was in reading aloud or copying letters, which were presented tachistoscopically to the left visual field, with his right hand. He could copy letters presented to his left visual field with his left hand, however. Therefore, left hemialexia was not due to hemiamblyopia or hemineglect. There was no anomia for pictures and colors in the left visual field. MRI revealed that the lesion was limited to the ventroposterior end of the splenium. Review of 40 reported patients with callosal lesions suggests that the anterior to middle part of the splenium is involved in transferring picture information from the language-nondominant hemisphere to the language-dominant hemisphere and that the ventroposterior part is involved in transferring letter information.