Olfactory sensory preconditioning in Drosophila: role of memory forgetting in gating S1/S2 associations

Learning and memory storage is a complex process that has proven challenging to tackle. It is likely that, in real nature, the instructive value of reinforcing experiences is acquired rather than innate. The association between seemingly neutral stimuli increases the gamut of possibilities to create meaningful associations and increases the predictive power of moment-by-moment experiences. Here we report physiological and behavioral evidence of olfactory unimodal sensory preconditioning in fruit flies. We show that the presentation of a pair of odors (S1 and S2) before one of them (S1) is associated with electric shocks elicits a conditional response not only to the trained odor (S1) but to the odor previously paired with it (S2). This occurs even if the S2 odor was never presented in contiguity with the aversive stimulus. In addition, we show that inhibition of the small G protein and known forgetting regulator Rac1 facilitates the association between S1/S2 odors. These results indicate that flies can infer value to non-paired odor based on the previous associative structure between odors, and inhibition of Rac1 lengthens the time of olfactory "sensory buffer," allowing linking of neutral odors presented in sequence.