Olfactory memories are intensity specific in larval Drosophila

Abstract

SUMMARYLearning can rely on stimulus quality, stimulus intensity, or a combination of these. Regarding olfaction, the coding of odour quality is often proposed to be combinatorial along the olfactory pathway, and working hypotheses are available concerning short-term associative memory trace formation of odour quality. However, it is less clear how odour intensity is coded, and whether olfactory memory traces include information about the intensity of the learnt odour. Using odour-sugar associative conditioning in larval Drosophila, we first describe the dose-effect curves of learnability across odour intensities for four different odours (n-amyl acetate, 3-octanol, 1-octen-3-ol and benzaldehyde). We then chose odour intensities such that larvae were trained at an intermediate odour intensity, but were tested for retention with either that trained intermediate odour intensity, or with respectively higher or lower intensities. We observed a specificity of retention for the trained intensity for all four odours used. This adds to the appreciation of the richness in ʻcontentʼ of olfactory short-term memory traces, even in a system as simple as larval Drosophila, and to define the demands on computational models of associative olfactory memory trace formation. We suggest two kinds of circuit architecture that have the potential to accommodate intensity learning, and discuss how they may be implemented in the insect brain. Supplementary material available online at

Synapsin is required to “boost” memory strength for highly salient events

“…Other details as in Figure 2. Regarding the wild-type WT, these results are qualitatively in line with Mishra et al (2013) concerning odor concentration as well as with Neuser et al (2005) and Schipanski et al (2008) Fig. 6A; Table 1).…”

“…This is because during the test with a higher-than-trained odor concentration the mushroom body Kenyon cells would be activated more strongly, leading to at least as strong output as with the trained odor concentration. Contradicting this prediction, memory scores were found to be less when odor concentration was increased between training and test (i.e., memory is specific for the trained odor intensity: Mishra et al 2013;also Yarali et al 2009). In terms of physiology, both the level of activity and the combination of activated mushroom body Kenyon cells varies, albeit slightly, with odor concentration.…”

“…It thus appears that in the syn 97 mutant, different from the wild-type WT ( Fig. 3A,B; Mishra et al 2013), memory strength cannot be properly adjusted to be higher for higher odor concentrations. This made us wonder whether a similar effect would be seen if stronger memories are established on Synapsin and event salience www.learnmem.org Figure 1.…”

“…Regarding our odor-reward learning paradigm, we therefore predicted that Synapsin is particularly critical for forming memories of highly salient events. To put this to the test, we parametrically vary odor as well as sugar salience (both affect memory scores in wild-type larvae: Schipanski et al 2008;Mishra et al 2013) and ask whether Synapsin is selectively involved in forming stronger memories for high concentrations of odor and/or of the sugar reward.…”

The impact of odor–reward memory on chemotaxis in larval Drosophila

“…Such a block, however, could not account for the repulsion observed after unpaired training under conditions of overall lower preference (e.g., Supplemental Fig. S8; Chen et al 2011;Saumweber et al 2011a;Schleyer et al 2011;Mishra et al 2013). In contrast, the summation scenario we propose can account for such odor repulsion after unpaired training, if the level of innate valence were smaller than the negative learned valence.…”