Chemosensory processing in the fruit fly, Drosophila melanogaster: generalization of a feeding response reveals overlapping odour representations.

Insects are capable of detecting, and discriminating between, a very large number of odours. The biological relevance of many of those odours, particularly those related to food, must first be learned. Given that the number of sensory receptors and antennal lobe (AL) glomeruli is limited relative to the number of odours that must be detectable, this ability implies that the olfactory system makes use of a combinatorial coding scheme whereby each sensory cell or AL projection neuron can participate in coding for several different odours. An important step in understanding this coding scheme is to behaviourally quantify the degree to which sets of odours are discriminable. Here we evaluate odour discriminability in the fruit fly, Drosophila melanogaster, by first conditioning individual flies to not respond to any of several odorants using a nonassociative conditioning protocol (habituation). We show that flies habituate unconditioned leg movement responses to both mechanosensory and olfactory stimulation over 25 unreinforced trials. Habituation is retained for at least 2 h and is subject to dishabituation. Finally, we test the degree to which the conditioned response generalizes to other odorants based on molecular features of the odorants (e.g. carbon chain length and the presence of a target functional group). These tests reveal predictable generalization gradients across these molecular features. These data substantiate the claim that these features are relevant coding dimensions in the fruit fly olfactory system, as has been shown for other insect and vertebrate species.