Brain organization and the roots of anticipation in Drosophila olfactory conditioning

Defining learning at the molecular and physiological level has been one of the greatest challenges in biology. Recent research suggests that by studying fruit fly (Drosophila melanogaster) brain organization we can now begin to unravel some of these mysteries. The fruit fly brain is organized into executive centers that regulate anatomically separate behavioral systems. The mushroom body is an example of an executive center which is modified by olfactory conditioning. During this simple form of learning, an odor is paired with either food or shock. Either experience alters distinguishable specific circuitry within the mushroom body. Results suggest that after conditioning an odor to food, the mushroom body will activate a feeding system via a subset of its circuitry. After conditioning an odor to shock, the mushroom body will instead activate an avoidance system with other subsets of mushroom body neurons. The results of these experiments demonstrate a mechanism for flies to display anticipation of their environment after olfactory conditioning has occurred. However, these results fail to provide evidence for reinforcement, a consequence of action, as part of this mechanism. Instead, specific subsets of dopaminergic and octopaminergic neurons provide a simple pairing signal, in contrast to a reinforcement signal, which allows for prediction of the environment after experience. This view has implications for models of conditioning.

Research highlights▶ The fly brain is organized into discrete executive and behavioral systems. ▶ Olfactory conditioning takes place in an executive center called the Mushroom body. ▶ Distinguishable learning circuitry regulates distinguishable behavioral systems. ▶ Forward conditioning is of fundamental importance to mechanisms of learning. ▶ Olfactory conditioning occurs without reinforcement.