Previous studies have shown that flies are able to identify and remember visual features such as size and colour, but whether they use visual cues to form spatial memories that can guide selective navigation was unclear. The authors therefore developed a spatial learning assay inspired by the Morris water maze and a heat maze test developed for rodents and roaches. Their 'thermal–visual arena' contained a hidden cool tile (at the flies' preferred temperature of 25 °C) surrounded by tiles that were heated to an aversive 36 °C. During training sessions, the position of the cool (target) tile was changed, but its location relative to the surrounding visual panorama — a pattern of evenly spaced bars in three orientations — remained constant, providing visual cues to its location.
The time needed by the flies to locate the cool tile dramatically decreased over the course of ten training sessions, demonstrating that they are capable of spatial learning. They showed no improvement in locating the cool tile in the dark or when the visual panorama was uncoupled from the tile location, indicating that they were using visual cues in the environment to find the target tile. The strongest evidence for the formation of spatial memories is provided by the tests during the so-called probe trial, when the cool tile was removed from the maze after training and the flies spent more time in the quadrant where visual cues indicated it should be. Remarkably, when the authors re-tested the flies at various time intervals, they found that the spatial memories were retained for at least 2 hours after training.
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