Functional MRI of phonological and semantic processing in temporal lobe epilepsy

Phonological and semantic aspects of language were examined in patients with unilateral temporal lobe epilepsy (TLE) and healthy controls using functional MRI. We expected to replicate previous findings in healthy individuals showing relatively greater activation in frontal regions for phonological compared with semantic processing, and greater activation in temporal regions for semantic compared with phonological processing. We hypothesized that differences between patients with left TLE and healthy controls would be found in the pattern of left temporal cortical activation associated specifically with semantic processing. Patients with right TLE were included as a seizure control group. All TLE patients previously showed left hemisphere language dominance on intracarotid sodium amytal studies. Greater blood oxygen level dependent activation was found during phonological processing compared with semantic processing in frontal regions for healthy participants but, contrary to expectation, semantic processing did not lead to increased temporal lobe activity relative to phonological processing. Furthermore, no differences between left temporal patients and controls were found specifically in left temporal cortex. Rather, patients with left temporal seizure foci showed significantly greater left dorsolateral prefrontal activity compared with controls, as well as increased signal change in left inferior frontal and right middle temporal gyrus. Surprisingly, patients with right, but not left, TLE showed poorer performance on the linguistic tasks compared with controls, as well as a decrease in right superior temporal activation. The results converge with studies of dyslexic patients showing increased left frontal activity in the presence of left temporal dysfunction and are suggestive of both inter- and intra-hemispheric functional reorganization of language representation in left TLE.