A newfound brain mechanism erases memories on purpose to help make way for new ones. Scientists suggest it could lead to the development of memory-erasing drugs that make a person forget certain things.

Researchers have often debated about the reasons we forget — for instance, why newly acquired short-term memories are fleeting. One theory suggests that such memories are simply unstable, fading over time. Others contend interference causes short-term memories to be overridden as new data comes in.

Both notions suggested that forgetting is a passive mechanism, but now it seems "it's not that at all — it's an active system to erase memory, completely independent from the mechanisms to form memories," researcher Yi Zhong, a neurogeneticist at Tsinghua University in Beijing and Cold Spring Harbor Laboratory in New York, told LiveScience.

Zhong and his colleagues made their discovery by training fruit flies with two odors, one of which was paired with a foot shock to the insects as they smelled it. That experience normally leads flies to avoid the shocking odor in favor of the alternative.

In the first set of experiments, the researchers left the flies alone after their training session was over, later testing them at specific points in time as their memory weakened. In a second experiment, the researchers disrupted the odor-shock memories by exposing flies to a new pair of odors. Finally, they reversed the flies' lesson, delivering the foot shock in conjunction with the opposite odor.

In all cases, the flies forgot what they learned previously, which the researchers suspect was due to a small protein known as Rac that switched on with the passage of time. This molecule switched on faster when the insects either got distracted by new experiences or confused by conflicting information.

When Rac was blocked, flies held on to newly acquired memories for longer than they otherwise would have, extending their life from a few hours to more than a day. When the researchers artificially increased Rac in fly neurons, new memories were erased more rapidly.

Scientists have yet to deeply understand what molecules are involved in the formation of memories. "By studying what is being erased or altered with this new mechanism, we may be able to identify the material basis of memory," Zhong said.

Zhong suspected the forgetting mechanism they uncovered would apply to other animals, with some hints in that direction in mice. Intriguingly, he noted mutations in other molecules involved with Rac have been linked to mental retardation in humans.

"We're now developing experiments to see if this also works on long-term memory," Zhong said.

Rac or molecules linked with it "could serve as targets for drugs to erase memory," Zhong added. Although such amnesia drugs could serve nefarious purposes, they could also help treat disorders where people remember trauma against their will, such as post-traumatic stress disorder.

The scientists detail their findings in the Feb. 19 issue of the journal Cell.