Brain Advance Access originally published online on November 9, 2005
Brain 2006 129(3):754-766; doi:10.1093/brain/awh679
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Distinct right frontal lobe activation in language processing following left hemisphere injury
1 Centre for Functional Magnetic Resonance Imaging of the Brain, Department of Clinical Neurology, John Radcliffe Hospital, University of Oxford, 2 Department of Clinical Neurology, 3 Russell Cairns Unit, Oxford Department of Clinical Neuropsychology, and 4 Department of Neurological Surgery, Radcliffe Infirmary, Oxford, UK
Correspondence to: N. L. Voets, FMRIB Centre, John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK E-mail: natalie{at}fmrib.ox.ac.uk
Right hemisphere activation during functional imaging studies of language has frequently been reported following left hemisphere injury. Few studies have anatomically characterized the specific right hemisphere structures engaged. We used functional MRI (fMRI) with verbal fluency tasks in 12 right-handed patients with left temporal lobe epilepsy (LTLE) and 12 right-handed healthy controls to localize language-related activity in the right inferior frontal gyrus (RIFG). During the phonemic task, LTLE patients activated a significantly more posterior region of the right anterior insula/frontal operculum than healthy controls (P = 0.02). Activation of the left inferior frontal gyrus (LIFG) did not differ significantly between the two groups. This suggests that, following left hemisphere injury, language-related processing in the right hemisphere differs from that with a functionally normal left hemisphere. The localization of activation in the left and right inferior frontal gyri was determined with respect to the anatomical sub-regions pars opercularis (Pop), pars triangularis (Ptr) and pars orbitalis (Por). In the LIFG, both healthy controls (8 out of 12) and LTLE patients (9 out of 12) engaged primarily Pop during phonemic fluency. Activations in the RIFG, however, were located mostly in the anterior insula/frontal operculum in both healthy controls (8 out of 12) and LTLE patients (8 out of 12), albeit in distinct regions. Mapping the locations of peak voxels in relation to previously obtained cytoarchitectonic maps of Broca's area confirmed lack of homology between activation regions in the left and right IFG. Verbal fluency-related activation in the RIFG was not anatomically homologous to LIFG activation in either patients or controls. To test more directly whether RIFG activation shifts in a potentially adaptive manner after left hemisphere injury, fMRI studies were performed in a patient prior to and following anatomical left hemispherectomy for the treatment of Rasmussen's encephalitis. An increase in activation magnitude and posterior shift in location were found in the RIFG after hemispherectomy for both phonemic and semantic tasks. Together, these results suggest that left temporal lobe injury is associated with potentially adaptive changes in right inferior frontal lobe functions in processing related to expressive language.
Key Words: right prefrontal cortex; language; functional MRI; epilepsy
Abbreviations: BOLD = blood oxygen level dependent; fMRI = functional MRI; LI = laterality index; LIFG = left inferior frontal gyrus; LTLE = left temporal lobe epilepsy; MNI = Montreal Neurological Institute; Pop = pars opercularis; Por = pars orbitalis; Ptr = pars triangularis; RIFG = right inferior frontal region; ROI = region of interest
Received July 12, 2005. Revised September 20, 2005. Accepted October 12, 2005.
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