Flexible Specificity of Memory in Drosophila Depends on a Comparison Between Choices

Memory guides behavior across widely varying environments and must therefore be both sufficiently specific and general. A memory too specific will be useless in even a slightly different environment, while an overly general memory may lead to suboptimal choices. Animals successfully learn to both distinguish between very similar stimuli and generalize across cues. Rather than forming memories that strike a balance between specificity and generality, Drosophila can flexibly categorize a given stimulus into different groups depending on the options available. We asked how this flexibility manifests itself in the well-characterized learning and memory pathways of the fruit fly. We show that flexible categorization in neuronal activity as well as behavior depends on the order and identity of the perceived stimuli. Our results identify the neural correlates of flexible stimulus-categorization in the fruit fly. Editor's evaluation Memory recall is more precise when discrimination is required. This work in Drosophila shows that two related odors trigger near identical Kenyon cell responses when tested in isolation, but trigger different responses to the second odor if these are experienced in sequence within a small temporal window. The authors argue that this template comparison requires some activity downstream of Kenyon cells, that is recruited by MBONs. Overall, the experiments, building on a clever method to build "miminal memories" via optogenetically restricting the formation of memory traces in selective output compartments of the Kenyon cell (KC) axon terminals, provide very nice physiological evidence for a neural mechanism that underlies a contextual basis for the precision of memory recall.