Distinct spatiotemporal activity patterns reflecting similarity-based and category-based semantic processing in anterior temporal lobe and inferior frontal cortex revealed via magnetoencephalography

Semantic processing of similarity and category are closely related yet distinct. In a prior functional magnetic resonance imaging study, the neural correlates of similarity and category processing were examined using prime–target word pairs in which similarity and category were independently manipulated. Specifically, they included "marine mammal", which belongs to the mammal category,but is more similar tofish. In some trials, two animal names from the categories of fish, terrestrial mammals, marine mammals, and birds were presented sequentially to investigate the priming effect of the first name on the processing of the second. In other trials, the second word was an inanimate object. Participants classified the second word as living or nonliving in all trials. Neural activity in the left anterior temporal lobe reflected similarity processing, whereas activityin the left inferior frontal cortex (IFC) corresponded to task-relevant category processing, consistent with models positing distinct systems for semantic representation and semanticcontrol. The present study employed magnetoencephalography to measure neural activity during thesame task, achieving enhanced temporal resolution. Neural activity reflecting similarity processing was observed in the left IFC approximately 525 msafter target word onset. This finding suggests that category processing in the left IFC occurred after the living/nonliving decision, which had a reaction time near 600 ms. The present results thus reveal a dynamic temporal sequence in the retrieval of task-relevant semantic features.