The biological basis of learning and memory is often viewed as the holy grail of neuroscience. It is no surprise then that the mammalian memory center, the hippocampus, has been the focus of intense, ongoing research. However, while tremendous advances have been made in our understanding of the neural circuitry within this structure, the sheer number of neurons and connections makes tracing the relevant inputs and outputs involved in specific memory-related tasks rather challenging. Consequently, many have turned to model organisms that possess several orders of magnitude fewer neurons, including Drosophila melanogaster.In this issue of Cell (Claridge-Chang et al., 2009; Krashes et al.,2009), two research groups probed the neural circuitry beyond the fruit fly’s memory center, the mushroom bodies,by dissecting the functional contribution of discrete neuronal populations to associative learning. Remarkably, although the two articles focused on different aspects of olfactory learning and used different conditioning paradigms, they converged upon a single set of dopaminergic neurons in the fly brain called the protocerebral posterior lateral 1 (PPL1) cluster.
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