To determine whether a similar distance-discrimination function also applies to learned behavior, we tested animals on the two-alternative forced-choice task after training. Here, the 4 min measurement period was preceded by a training session during which a 1 min presentation of the innately less aversive odor was followed by a 1 min presentation of the innately more aversive odor with electric shock

(Claridge-Chang et al., 2009). Trained decision bias was no longer bounded by a logistic function of distance BTK activity inhibition between ePN signals; instead, it remained virtually constant at 73.5% ± 1.6% (mean ± SEM), even for the two odors separated by the shortest distance among all 5,995 possible pairs in the panel (Figure 4; Table S4). Given that innate and learned behavior are thought to be controlled by separate brain regions (the LH and MB, respectively) (Heimbeck et al., 2001), differences in innate and learned discrimination may arise because the LH and MB use different odor-coding formats, the MB supporting finer discrimination than the LH. If untrained

flies disregarded information encoded in the MB and made use of LH signals exclusively, then they would display only coarse discrimination. To test this conjecture, we expressed lexAop-shits1 under mb247-LexA control AZD6244 in Kenyon cells (KCs), the principal intrinsic neurons of the MBs. Switching off the efferent synapses of KCs during testing occluded the effects of learning: the decision bias of trained flies now followed

the same distance-discrimination function as that of untrained flies ( Figure 4B). Both parental control strains showed wild-type (WT) performance at the elevated temperature ( Figure S4). Thus, preventing the retrieval of memory in trained animals re-exposed their innate behavioral state. In contrast, blocking KC output in untrained flies had no discernible behavioral consequence; the distance-discrimination functions of untrained animals with intact and blocked MB output overlapped precisely ( Figure 4A). We conclude that flies use two parallel MYO10 odor representations in a state-dependent manner: they rely on the LH alone in the untrained state and engage the MB only after training. Failures of untrained flies to discriminate behaviorally between odors that are separated by small ePN distances, despite strong and opposing preferences to each odor alone, must reflect the coarse grain of odor representation in the LH and a lack of incentive to draw on the fine discrimination system of the MB. The enhancer trap line Mz699-GAL4 ( Lai et al., 2008 and Okada et al., 2009) labels 39.3 ± 0.5 GABA-positive PNs (mean ± SD, n = 4 hemispheres) located in a cluster at the ventral face of the antennal lobes ( Figures 5A and 5D; Movies S1 and S2).