Selective retrieval of abstract semantic knowledge in left prefrontal cortex.

Research into the representation and processing of conceptual knowledge has typically associated perceptual facts with sensory brain regions and executive retrieval mechanisms with the left prefrontal cortex. However, this dichotomy between knowledge content and retrieval processes leaves unanswered how the brain supports concepts less reliant on direct sensory experiences. We used neuroimaging methods to investigate whether an increased abstractness in semantic decisions, in contrast to increased response difficulty, is associated with increased left prefrontal activation. Participants were presented with concrete animal names and asked to verify sensory and abstract properties that corresponded only to the animal category. Candidate semantic regions were localized in left inferior, frontopolar, and dorsolateral prefrontal cortex in contrast to a pseudoword control. Activity in each of these prefrontal regions was associated with significantly increased activity for abstract relative to sensory semantic decisions, regardless of increased response difficulty and even when controlling for the response times of participants. These results suggest that more abstract, or verbally-mediated, semantic knowledge of concrete items, in contrast to more sensory-based properties, is specifically supported by the left prefrontal cortex. Semantic retrieval mechanisms may rely on abstract representations, likely coded through a verbal format, to mediate task demands when perceptual information is insufficient.