How we can intentionally forget unwanted memories

Long criticized for his discredited theories on the meaning of dreams, Sigmund Freud at least got one thing right - the human mind can block out bad memories.

A Dartmouth- and Princeton-led brain scanning study showed that people can intentionally forget past experiences by changing how they think about the context of those memories.

The findings have a range of potential applications centered on enhancing desired memories, such as developing new educational tools, or diminishing harmful memories, including treatments for post-traumatic stress disorder.

Researchers designed a functional magnetic resonance imaging (fMRI) experiment to specifically track thoughts related to memories' contexts and put a new twist on a centuries-old psychological research technique of having subjects memorize and recall a list of unrelated words. They showed participants images of outdoor scenes, such as forests, mountains and beaches, as they studied two lists of random words, manipulating whether they were told to forget or remember the first list prior to studying the second list.

"Our hope was the scene images would bias the background, or contextual, thoughts that people had as they studied the words to include scene-related thoughts," noted lead author Jeremy Manning. "We used fMRI to track how much people were thinking of scene-related things at each moment during our experiment. That allowed us to track, on a moment-by-moment basis, how those scene or context representations faded in and out of people's thoughts over time."

The study has two important implications. "First, memory studies are often concerned with how we remember rather than how we forget, and forgetting is typically viewed as a 'failure' in some sense, but sometimes forgetting can be beneficial, too," said Manning.

The second implication is more subtle but also important. "It's very difficult to specifically identify the neural representations of contextual information," added Manning. "If you consider the context you experience something in, we're really referring to the enormously complex, seemingly random thoughts you had during that experience. Those thoughts are presumably idiosyncratic to you as an individual, and they're also potentially unique to that specific moment."

Manning continued, "So, tracking the neural representations of these things is extremely challenging because we only ever have one measurement of a particular context. Therefore, you can't directly train a computer to recognize what context 'looks like' in the brain because context is a continually moving and evolving target."

"In our study, we sidestepped this issue using a novel experimental manipulation - we biased people to incorporate those scene images into the thoughts they had when they studied new words. Since those scenes were common across people and over time, we were able to use fMRI to track the associated mental representations from moment to moment."

The study appears in the journal Psychonomic Bulletin and Review.