The incentive circuit: memory dynamics in the mushroom body of Drosophila melanogaster

Insects adapt their response to stimuli, such as odours, according to their pairing with positive or negative reinforcements, such as sugar or shock. Recent electrophysiological and imaging findings in Drosophila melanogaster allow detailed examination of the neural mechanisms supporting acquisition, forgetting, and assimilation of memories. Drawing on this data, we identify a series of microcircuits within the mushroom bodies that reveal, for each motivational state, three different roles of dopaminergic and mushroom body output neurons in the memory dynamics. These microcircuits share components and form a unified system for rapid memory acquisition, transfer from short-term to long-term, and exploration/exploitation trade-off. We show that combined with a novel biologically plausible learning rule, a computational model of the full circuit reproduces the observed changes in the activity of each of these neurons in conditioning paradigms and can be used for flexible behavioural control.