The paper reviews a growing body of scientific literature on memory reconsolidation, a relatively new (and, in humans, still somewhat contentious) concept in which old information is called to mind, modified with the help of drugs or behavioral interventions, and then re-stored with new information incorporated—like a piece of metal that’s been melted down, remolded, and left to harden into a different shape.

Though different types of memories are solidified in different ways—the fear-driven memory of driving over a bomb, for example, will make its way through the brain differently than a mundane memory of yesterday’s breakfast—there are general neurological processes that all memories follow.

“In memory research, we talk about three parts,” explains Ken Paller, director of the cognitive neuroscience program at Northwestern University. “The first part is the acquisition or coding of a memory,” in which our brains process the information our senses are sending, “and the last part is retrieving a memory. And in between, we talk about consolidation,” the process by which sensory information—a sight, a taste—solidifies into fully-formed memories to be stored for the long term.

Typically, the more often memories are recalled, the stronger they become. “If you’re trying to memorize something in a book, you sit there and repeat it over and over,” explains Wendy Suzuki, a researcher at New York University’s Center for Neural Science. “That’s also an example of how things get consolidated. You repeat [them] over and over.”

But with reconsolidation, the researchers explain, consciously recalling a memory is also what allows it to be manipulated. “[Memories] are not necessarily fixed but can be changed long after storage,” they write. “Seemingly stable memories may re-enter an unstable state when they are retrieved, from which they must be re-stabilized … During reconsolidation, memories are susceptible to modification again.”

One way is by distracting the brain as it recalls, or “activates,” memories: People who were asked to memorize two separate lists of objects, for example, did worse on the second list after they were reminded of content from the first, while people who were asked to memorize a folk tale before recalling autobiographical events described their memories in less detail than those who had not seen the story.

Perhaps more compelling for the treatment of PTSD, though, are the experiments that involve tampering with fear-driven memories using pharmaceuticals. In one study, published in Nature Neuroscience, volunteers were stimulated to generate fear memories after being subjected to loud noises and images of spiders; later on, one group was given propranolol (a beta-blocker used to slow heartbeat and sometimes used in the treatment of anxiety disorders) before being made to recall the fearful experience, while the other was given placebo pills. When the two groups were again reminded of the memory days after the experiment began, those who had taken the propranolol showed markedly less fear than those who had not.