Retrieving rules for behavior from long-term memory

Human behavior is often dictated by rules or prescribed guides for action. Little is currently known regarding how these rules are stored in long-term memory or retrieved and implemented. Here, we examined the roles of ventrolateral prefrontal cortex (VLPFC) and posterior middle temporal gyrus (postMTG) in rule use. We tested two hypotheses: first, that knowledge about actions associated with abstract visual symbols is stored in postMTG, and second, that VLPFC is involved in the controlled retrieval of rule meanings. Subjects viewed a series of road signs during event-related fMRI data collection. Three types of signs were intermixed: highly familiar signs, novel signs whose meaning was explained to subjects prior to scanning, and novel signs whose meaning was not explained. Subjects were asked to think about the meaning of each sign as it was presented during scanning and then to give its meaning in a post-scan test. Left postMTG was more active when subjects viewed signs whose meaning they knew than signs whose meaning they did not know, consistent with a role in storing rule meanings. This region was not modulated by experience, in that it was equally engaged by newly trained and well-learned signs. In contrast, right VLPFC was more active for newly trained signs than for either well-learned or incorrect ones, consistent with a role in controlled retrieval. Left VLPFC was reliably engaged while subjects attempted to interpret the signs but did not differ according to knowledge or experience. These data implicate postMTG in rule storage and VLPFC in rule retrieval.