Fruit flies can learn non-elemental olfactory discriminations

Abstract

Associative learning allows animals to establish links between stimuli based on their concomitance. In the case of Pavlovian conditioning, a single stimulus A (the conditional stimulus, CS) is reinforced unambiguously with an unconditional stimulus (US) eliciting an innate response. This conditioning constitutes an 'elemental' association to elicit a learnt response from A⁺ without US presentation after learning. However, associative learning may involve a 'complex' CS composed of several components. In that case, the compound may predict a different outcome than the components taken separately, leading to ambiguity and requiring the animal to perform so-called non-elemental discrimination. Here, we focus on such a non-elemental task, the negative patterning (NP) problem, and provide the first evidence of NP solving in Drosophila. We show that Drosophila learn to discriminate a simple component (A or B) associated with electric shocks (+) from an odour mixture composed either partly (called 'feature-negative discrimination' A⁺ versus AB^-) or entirely (called 'NP' A⁺B⁺ versus AB^-) of the shock-associated components. Furthermore, we show that conditioning repetition results in a transition from an elemental to a configural representation of the mixture required to solve the NP task, highlighting the cognitive flexibility of Drosophila.

Keywords: Drosophila melanogaster; Pavlovian conditioning; associative learning; feature-negative discrimination; insect; negative patterning.

Figure 1.

(a) Schematic of a typical training cycle. Blue and orange boxes show CS presentation, while red bars show US delivery. (b) Schematic of the conditioning protocols. Clouds represent the CS odorants while lightning bolts indicate the delivery of electric shock during training. A, 3-octanol; B, 4-methylcyclohexanol. (c) Relative PIs computed as the difference between paired and unpaired scores. Performances were compared within the same protocol (i.e. one cycle versus five cycles) but not between protocols. Data are plotted as boxplots. The middle line represents the median, while the upper and lower limits of the box are the 25 and 75% quantiles. The whiskers are the maximum and minimum values of the data that are, respectively, within 1.5 times the interquartile range over the 75th percentile and under the 25th percentile. Raw data are superimposed as jittered dots. ‘n.s.’ stands for ‘non-significant’, *p < 0.05, **p < 0.01 after a t-test (DC and NF) or after a two-way ANOVA (NP).

Figure 2.

(a) Schematic of the tests performed after the three conditioning protocols to determine the nature of the CS representation. A, 3-octanol; B, 4-methylcyclohexanol; C, benzaldehyde. (b) Relative PIs computed as the difference between paired and unpaired scores. Performances were compared within the same protocol (i.e. one cycle versus five cycles) but not between protocols. Data are plotted as boxplots. The middle line represents the median. The upper and lower limits of the box are the 25 and 75% quantiles. The whiskers are the maximum and minimum values of the data that are, respectively, within 1.5 times the interquartile range over the 75th percentile and under the 25th percentile. Raw data are superimposed as jittered dots. ‘n.s.’ stands for ‘non-significant’, *p < 0.05 after a t-test. Grey shading indicates performances that were not significantly different from chance level while white filling indicates a significant difference from chance level (t-test against zero). (Online version in colour.)