An olfactory pattern generator for functional neural circuit analysis in <i>Drosophila</i> larva

For physical stimuli, it is straightforward to engineer patterns to stimulate the sensory circuits and probe the functional properties. However, this framework is generally not applicable to olfaction, since olfactory stimuli lack a well-defined space. The typical olfactory stimuli, fixed odor panels of pure chemicals, natural scents or mixtures, lack the capability to systematic sample the olfactory sensory periphery. Here, we present a stimuli method that project activity pattern directly onto the olfactory receptor neurons. This method is based on the flexible and precise mixing of an optimized set of primary odors using microfluidics. The primary odors were derived for the Drosophila larva to separately target each free variable in the olfactory code, the activity of an individual olfactory receptor neuron type. Our device allows us to program and deliver any mixture of primary odors from all combinatorial possibilities during an experiment, allowing selection of any receptor neuron activity combination on demand. We combine this stimulus method with imaging the olfactory representations in the Drosophila larva from receptor neurons to interneurons of the antennal lobe and Kenyon Cells of the mushroom body.