Abnormalities of lexical and semantic processing in left temporal lobe epilepsy: an fMRI study

We examined the efficiency of lexical and semantic processing and associated brain activation using functional magnetic resonance imaging (fMRI) in patients with left temporal lobe epilepsy (TLE).

Twenty patients with left TLE (10 with hippocampal sclerosis, the HS group; and 10 with nonlesional MR scans, the NL group) and 12 healthy controls underwent an event-related fMRI analysis during a lexical decision task (LDT). Lexical and semantic processing were examined by comparing behavioral and imaging data associated with words and nonwords (lexicality) or with concrete and abstract words (concreteness).

Although the control group showed greater activation associated with word stimuli than with nonword stimuli in a bilateral language network, both TLE groups showed greater activation for nonword stimuli than word stimuli, including greater activation of inferior frontal language areas (bilaterally in the HS group and left-lateralized in the NL group). The TLE groups also exhibited differential activation patterns during the processing of abstract and concrete words compared to controls, and compared to each other. For abstract words, in particular, the HS group showed activation of frontal areas typically associated with executive functions, whereas the NL group showed activation of more posterior semantic processing regions.

These results suggest that left TLE is associated with altered functional organization of cortical networks involved in lexical and semantic processing. In addition, the organization observed varies as a function of hippocampal pathology.