Abstracts

Rationale: Word recognition is a fundamental component of human language. Functional magnetic resonance imaging (fMRI) has shown that cortical networks involved in language processing are modified by temporal lobe epilepsy (TLE). However, the effect of TLE on the organization of cortical areas involved in visual word recognition and, in particular, semantic processing is unknown. In this study, we therefore examined the efficiency of lexical and semantic processing and associated brain activation using fMRI in subjects with left TLE.Methods: Twenty participants with left TLE (including 10 with hippocampal sclerosis, the HS group, and 10 without hippocampal sclerosis, the NL group) and 12 controls underwent an event-related fMRI analysis of word and nonword processing during a lexical decision task (LDT). Stimuli included concrete words, abstract words, and nonwords. Efficiency of processing was assessed by measuring the response times and accuracy of lexical decisions. Lexical and semantic processing were examined by comparing behavioural and imaging data associated with words and nonwords (lexicality) or with concrete and abstract words (concreteness), respectively.Results: During the LDT, all groups exhibited significant effects of lexicality and concreteness, such that response times were longer to nonwords than words, and to abstract words than concrete words (p < 0.001). Further, both TLE groups showed larger lexicality effects than did the control group in response times, but all groups achieved equivalent accuracy (p < 0.001). While the control group showed greater activation associated with word stimuli than 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, Figure 1). The left TLE groups also exhibited differential activation patterns during the processing of abstract and concrete words compared to controls, and compared to each other (Figure 2). 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. Conclusions: These results suggest that left TLE is associated with altered functional organization of cortical networks involved in lexical and semantic processing but that this may not completely compensate for the inefficiency of the word recognition system in patients with left TLE. In addition, the organization observed varies as a function of hippocampal pathology. Specifically, with memory structures compromised in HS, these patients must recruit more frontal areas in order to judge lexicality for abstract words.