Learning-related neuronal responses in prefrontal cortex studied with functional neuroimaging

We assessed time-dependent neuronal activity accompanying learning using fu nctional magnetic resonance imaging (fMRI). An artificial grammar learning paradigm enabled us to dissociate activations associated with individual it em learning from those involved in learning the underlying grammar system. We show that a localized region of right prefrontal cortex (PFC) is prefere ntially sensitive to individual item learning during the early stages of th e experiment, while the left PFC region is sensitive to grammar learning wh ich occurred across the entire course of the experiment. In addition to dis sociating these two types of learning, we were able to characterize the eff ect of rule acquisition on neuronal responses associated with explicit lear ning of individual items. This effect was expressed as modulation of the ti me-dependent right PFC activations such that the early increase in activati on associated with item learning was attenuated as the experiment progresse d. In a further analysis we used structural equation modelling to explore t ime-dependent changes in inter-regional connectivity as a function of both item and grammar rule learning. Although there were no significant effects of item learning on the measured path strengths, rule learning was associat ed with a decrease in right fronto parietal connectivity and an increase in connectivity between left and right PFC. Further fronto-parietal path stre ngths were observed to change, with an increase in left fronto-parietal and a decrease in right fronto-parietal connectivity. path strength from right PFC to left parietal cortex, We interpret our findings in terms of a left frontal system mediating the semantic analysis of study items and directly influencing a right fronto parietal system associated with episodic memory retrieval.