Re-evaluation of Learned Information in Drosophila

Depending on prediction accuracy at the time of memory recall, specific mushroom body output neurons drive different combinations of dopaminergic neurons to extinguish or reconsolidate appetitive memory in Drosophila. The changeability of memories upon recall allows animals to constantly reassess the reliability of learned information, but the neural mechanisms governing these updating processes remain largely unknown. Now, Scott Waddell and colleagues have mapped the dopaminergic neurons that interact with specific mushroom body output neurons in the Drosophila brain in order to produce either extinction or reconsolidation of a food-related memory (the association of a specific odour with a sugar reward) by the introduction of a prediction error (new exposure to the odour without reward). Finding similarly segregated feedback connections in mammals should instruct on-going procedures aiming to alleviate harmful memories in stressed humans. Animals constantly assess the reliability of learned information to optimize their behaviour. On retrieval, consolidated long-term memory can be neutralized by extinction if the learned prediction was inaccurate1. Alternatively, retrieved memory can be maintained, following a period of reconsolidation during which it is labile2. Although extinction and reconsolidation provide opportunities to alleviate problematic human memories3,4,5, we lack a detailed mechanistic understanding of memory updating. Here we identify neural operations underpinning the re-evaluation of memory in Drosophila. Reactivation of reward-reinforced olfactory memory can lead to either extinction or reconsolidation, depending on prediction accuracy. Each process recruits activity in specific parts of the mushroom body output network and distinct subsets of reinforcing dopaminergic neurons. Memory extinction requires output neurons with dendrites in the α and α′ lobes of the mushroom body, which drive negatively reinforcing dopaminergic neurons that innervate neighbouring zones. The aversive valence of these new extinction memories neutralizes previously learned odour preference. Memory reconsolidation requires the γ2α′1 mushroom body output neurons. This pathway recruits negatively reinforcing dopaminergic neurons innervating the same compartment and re-engages positively reinforcing dopaminergic neurons to reconsolidate the original reward memory. These data establish that recurrent and hierarchical connectivity between mushroom body output neurons and dopaminergic neurons enables memory re-evaluation driven by reward-prediction error.