Altering a person’s memories may be as easy as suggesting new or different information, according to a new study published in the Proceedings of the National Academy of Sciences.

The key, said lead researcher Jason Chan of Iowa State University, is the timing and recall of that memory.

“If you reactivate a memory by retrieving it, that memory becomes susceptible to changes again,” he said in a news release. “And if at that time you give people new contradictory information, that can make the original memory much harder to retrieve later.”

In order to come to this conclusion, participants watched a 40-minute episode of the TV show “24” in which a terrorist uses a hypodermic needle to attack a flight attendant. Viewers were tested to reactivate their memory of the show, after which  some participants listened to an audio recap that included different details, such as the terrorist using a stun gun instead of a needle.

Sure enough, those same participants had a harder time remembering the needle when asked about it on the test, but only if they had recalled the needle before hearing about the stun gun.

However, Chan said, there is a short amount of time - roughly 6 hours - in which the memory is unstable, at which point it resumes a more concrete place in the mind.

“During that reconsolidation period, that’s when the memory is easy to be interfered with. Once that window closes and that memory is stable again, if you get new information it should not interfere with that original memory,” Chan said. “We found support for that idea in a number of experiments in which we varied the delay between the interfering memory or the misinformation and when people took that initial test.”
In addition, he and his colleague Jessica LaPaglia, a graduate student, found no effect if the information was presented in a different context than the original memory.

Beyond the lab, Chan believes the implications of the study could have a number of implications, including in the context of an eyewitness to a crime.

For example, a person who watches a bank robbery and later recalls it while watching a movie with a similar event could confuse details from the two.

“One thing we know about how memory works is that you don’t need something to be exactly the same for new things to interfere with your old memory,” Chan said.

Going forward, Chan said he wants to pin down the exact timing and context necessary to manipulate a memory, as well as identifying ways for using the technique in such a way that traumatic memories could be altered in order to treat victims of, for example, PTSD.