Displacement experiments provide evidence for path integration in Drosophila

Like many other animals, insects are capable of returning to previously visited locations using path integration, a memory of travelled direction and distance. Recent studies suggest that Drosophila can also use path integration to return to a food reward. However, the existing experimental evidence for path integration in Drosophila has a potential confound: pheromones deposited at the site of reward might enable flies to find previously rewarding locations even without memory. Thus, we designed an experiment to determine if flies can use path integration memory despite potential pheromonal cues by displacing the flies shortly after an optogenetic reward. We found that rewarded flies returned to the location predicted by a memory-based model. Several analyses are consistent with path integration as the mechanism by which flies returned to the reward. We conclude that while pheromones may often be important in fly navigation and must be carefully controlled in future experiments, Drosophila may indeed be capable of performing path integration.

Methods

There were two independent experiments: the pheromone experiment and the displacement experiment.

In the pheromone experiment, the trajectories of groups of flies walking in the circular 20 cm diameter arena were recorded. The comparison is between two conditions: "without emitter" and "with emitter", emitter meaning presence of another group of flies in the smaller circular area of the arena floor (reward zone) prior to the test recording (see the manuscript for details). This dataset contains processed data: pooled histograms of flies' positions and the computed fractions of time spent in the reward zone.

The displacement experiment was performed on individual flies. The fly was walking freely in the 60 cm diameter arena and was tracked using IR sensitive camera. We performed stimulation by the red LED, when the fly was inside the reward zone (a small circular area of the floor). After stimulation the fly was displaced on a flat slider. In the non-rewarded condition the LED was plugged out, but the times when it would be turned on are indicated with the positive value of intensity as in rewarded condition. The dataset contains the preprocessed recorded trajectories (coordinates over time), as well as the files with the processed data, necessary to generate the figures of the associated article.