Selective retrieval of abstract semantic knowledge in left prefrontal cortex

Abstract

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.

Figure 1.

To examine the variability of prefrontal activity specific to semantic retrieval across these conditions, property verifications were contrasted with a pseudoword letter detection task. Activity in the aLIPFC. A, The medial region is more active for abstract property verifications than for sensory-based knowledge. B, In contrast, a more lateral region shows a main effect of abstractness and an interaction with response difficulty. Activity in the frontodorsal regions of the left prefrontal cortex. C, The FPC shows greater activity for abstract semantic retrieval than for the perceptual properties with an interaction based on response difficulty. D, The DLPFC shows increased activation for the abstract property verifications. Error bars represent SEM.

Figure 2.

Segregating trials into latency-based quartiles allowed for a more directed matching of the verification questions. Comparing the 75% slowest sensory trials against the 75% fastest abstract decisions shows main effects of abstractness across the left prefrontal regions of interest. Error bars represent SEM.

Figure 3.

The whole-brain main effect contrast (p < 0.01, uncorrected) between the abstract and perceptual property verifications shows left-lateralized activation (L) in the frontal, temporal, and parietal lobes for verification of abstract properties of animals and in the right parietal lobe (R) for perceptual semantic decisions.