The movements of flies buzzing around a picnic table may seem random, but in fact flies exhibit decision-making processes when choosing whether to circle overhead or land on the potato salad. Flies' ability to choose how to adroitly navigate their surroundings depends on their capacity for short-term, spatial memory. A group of German researchers led by Roland Strauss of the Johannes Gutenberg University in Mainz, Germany, is teasing apart how insects' brains direct their movements and behaviours. Through this work, the authors hope to gain insight into the basic principles that govern memory and decision-making abilities in humans. “Complicated human brains and the small brains of flies are interacting with the same world, and they have to solve the same types of problem,” says Strauss.

Previous work by Strauss and his colleagues had shown that fruitflies can maintain their direction along a straight path towards a target object even after the target disappears. But these experiments did not prove that the fruitflies have memory; the flies could simply have been continuing their walk towards the vanished object much like a mountaineer who continues climbing a peak obscured by clouds. In order to demonstrate that flies have spatial memory, the team needed to show that the insects can still find their way to an invisible object after being temporarily distracted en route.

To this end, Strauss and his colleagues built a virtual reality fly 'arena'. The flies' wings were clipped to prevent them from flying away and they were surrounded by a water 'moat'. Flies cannot discern between water obstacles and the “end of the world”, says Strauss, so faced with such a situation, most flies choose to stay inside the moat.

Building on the efforts of former graduate student Josef Pichler, Strauss and co-author Markus Mronz designed a way to lure the flies off course by means of a visual distraction. Strauss has long advocated the use of electronics to investigate brain function — as a teenager, he designed an electronic device to simulate the visual system of the horseshoe crab, for which he won a national prize.

To investigate the flies' spatial memory, the team designed a fly arena studded with more than 5,000 light-emitting diodes, and used these to project vertical stripes onto a cylindrical screen around the arena perimeter. In this situation, flies are drawn to vertical stripes that emerge from the floor because they perceive them to be an escape route. The authors monitored how flies reacted when the stripe they were walking towards disappeared and a vertical 'distracter' stripe appeared to their side, then disappeared after one second.

The team found that flies stored memory about their orientation and would turn back towards their original — still invisible — target after between one and four seconds. The researchers went on to untangle some of the key biochemical pathways involved in orientation and memory in flies (see page 1244).

Strauss says that flies' capacity for short-term memory is comparable to that of humans. If placed in an unfamiliar room and briefly pointed toward the exit before having the lights switched off, he explains, we would have a few seconds' worth of memory to guide our decision about how to procede before having to rely on search behaviour. The same decision-making processes, says Strauss, enable us to “learn from the past to improve the basic behaviour of the future”.