Semantic Adaptation and Competition during Word Comprehension

Word comprehension engages the left ventrolateral prefrontal (IVLPFC) and posterior lateral-temporal cortices (PLTC). The contributions of these brain regions to comprehension remain controversial. We hypothesized that the PLTC activates meanings, whereas the IVLPFC resolves competition between representations. To test this hypothesis, we used functional magnetic resonance imaging (fMRI) to assess the independent effects of adaptation and competition on neural activity. Participants judged the relatedness of word pairs. Some consecutive pairs contained a common ambiguous word. The same or different meanings of this word were primed (e.g., SUMMER-FAN, CEILING-FAN; ADMIRER-FAN, CEILING-FAN). Based on the logic of fMRI adaptation, trials with more semantic overlap should produce more adaptation (less activation) in regions that activate meaning. In contrast, trials with more semantic ambiguity should produce more activation in regions that resolve competition. We observed a double dissociation between activity in the PLTC and IVLPFC. LPLTC activity depended on the amount of semantic overlap, irrespective of the amount of semantic ambiguity. In contrast, IVLPFC activity depended on the amount of semantic ambiguity. Moreover, across participants the size of the competition effect as measured by errors was correlated with the size of the competition effect in the IVLPFC. We conclude that the IVLPFC is an executive mechanism within language processing.