A Narrative Review: Learning and Decision-making in Tiny Brains; How Insects Learn to Avoid Predator Odors and Approach Food Odors

One key function of brains, essential for survival across the animal kingdom, is the ability to learn which stimuli in the environment predict rewards and punishments and respond accordingly. However, the circuit mechanisms underlying this function are not well understood in larger vertebrate brains. Insects, such as Drosophila melangaster, provide tractable model systems in which detailed circuit mechanisms can be understood. Here, we review the recent papers describing the circuit basis of appetitive and aversive learning and decision-making in Drosophila and we discuss the open questions in the field. A summary of the articles described the state-of-the art understanding of the role of the different neuron types in the Drosophila learning circuit, called the mushroom body (MB) : the Kenyon Cells (KC) that encode conditioned stimuli (such as odors), the mushroom body output neurons that encode the learnt information, and the dopamine neurons (DANs) that provide the teaching signals that drive learning and pair an odour with a learnt value. This shows that the mushroom body is the prime circuit for differentiating scents and learning whether they predict positive or negative experiences. However, despite recent progress in the field, many open questions remain, including how the learnt information encoded by the population of mushroom body output neurons is processed downstream of the mushroom body and used to select actions.