Homotopic language reorganization in the right hemisphere after early left hemisphere injury

To determine the areas involved in reorganization of language to the right hemisphere after early left hemisphere injury, we compared fMRI activation patterns during four production and comprehension tasks in post-surgical epilepsy patients with either left (LH) or right hemisphere (RH) speech dominance (determined by Wada testing) and healthy controls. Patient groups were carefully matched for IQ, lesion location and size. RH patients’ activation across all tasks was greatest in right hemisphere areas homotopic to areas activated by LH and control participants. Differences in right vs. left dominant hemisphere activation were limited to homologous areas typically activated by language tasks, supporting the hypothesis that language localization following transfer to the RH is the mirror-image of localization in the absence of transfer. The similarity of these findings to those in patients with larger, peri-sylvian lesions suggests that these areas in both hemispheres may be uniquely predisposed to subserve various language functions.

► Small, structurally remote lesions can result in considerable neocortical language plasticity.
► Right hemisphere language is mediated by right Broca’s and Wernicke’s homologues.
► These areas thus appear to be uniquely predisposed to mediate language.
► This is true across a range across productive and perceptive language tasks.
► Certain tasks are more sensitive to variations in inter-hemispheric transfer.