Switch-like and persistent learning in individual Drosophila larvae

Associative learning allows animals to use past experience to predict
future events. The circuits underlying memory formation support immediate
and sustained changes in function, often in response to a single example.
Larval Drosophila is a genetic model for memory formation that can be
accessed at molecular, synaptic, cellular, and circuit levels, often
simultaneously, but existing behavioral assays for larval learning and
memory do not address individual animals, and it has been dicult to form
long lasting memories, especially those requiring synaptic reorganization.
We demonstrate a new assay for learning and memory capable of tracking the
changing preferences of individual larvae. We use this assay to explore
how activation of a pair of reward neurons changes the response to the
innately aversive gas Carbon Dioxide (CO2). We conrm that when coupled to
odor presentation in appropriate temporal sequence, optogenetic reward
reduces avoidance of CO2. We nd that learning is switch-like: all-or-none
and quantized in two states. Memories can be extinguished by repeated
unrewarded exposure to CO2 but are stabilized against extinction by
repeated training or overnight consolidation. Finally, we demonstrate
long-lasting protein synthesis dependent and independent memory formation.