Subliminal cues, well timed, can help people forget bad experiences

Recurring intrusive memories are at the heart of certain mental illnesses, including post-traumatic stress disorder and obsessive-compulsive disorder. Clinicians often treat these conditions with “exposure therapy.” They gradually re-expose patients to feared stimuli or simulations—from memories of active combat to germs on a toilet—teaching the brain to acclimate to the stimuli and detach them from danger.

However, exposure therapy has disadvantages. “Confronting these traumatic memories is painful for patients,” says Yingying Wang, a cognitive psychologist at Zhejiang University in China. “These treatments suffer from a very high drop-out rate.” Wang and her colleagues have taken a first step to develop a gentler method to dampen traumatic memories. Their proof-of-concept study involves subliminal exposure to cues to these memories after the brain has been placed in a state where it is likely to be forgotten.

The new findings represent a new variant of a form of active forgetting in which people learn to suppress memories by practicing not thinking about them in the presence of memories. In various studies, participants memorized pairs of words, such as needle doctor or jogger collie, and then practiced either thinking about the second word or not thinking when the first word (the memory) appeared. Practicing not to think about the second word has led to oblivion.

The mechanism for this effect centers on the brain’s main memory center, the hippocampus. Psychologists have discovered that suppressing memory retrieval puts the hippocampus in a deteriorated state of functioning. This state lasts for a small window of time — at least 10 seconds, but possibly much longer — and casts what researchers have called an “amnesiac shadow,” resulting in poor memory for other things happening within it. So when people suppress neutral word pairs, they put their brains in a state where they are likely to forget new experiences.

This state also allows for the forgetting of established memories, according to a 2021 study by Wang and psychologist Zijian Zhu of Shaanxi Normal University in China. “All you have to do is suppress something completely neutral and then give people a timely reminder of that nearby memory,” says Michael Anderson, a cognitive psychologist at the University of Cambridge who did much of the work on memory suppression has and is the author of the new paper. In this study, Wang, Zhu, and Anderson showed that the process can worsen memories of unpleasant scenes, even when the memory of those scenes is presented in a way that makes people unaware that they are there.

However, when Wang and Zhu presented the idea for this experiment, Anderson was skeptical. “They emailed me the idea, and I was like, ‘That sounds really cool. I’m sure it won’t work,’” Anderson recalls. But Wang was confident that would be the case because she unearthed studies that showed certain invisible memories would recruit the hippocampus, she says.

So, in two experiments, the researchers asked 88 healthy young adults to memorize pairs of two-character neutral Chinese words. To construct trauma-like memories, participants examined disturbing images along with images or words that would serve as a reminder of those images. The harrowing images featured subjects such as physical or sexual abuse, injury, death, natural disasters and serious accidents. The memory images featured objects like those that appeared in the scenes, similar to visual reminders of disturbing real-world events. For example, in one photograph, a woman and her daughter lay dead on the ground after being shot, and a doll lay next to the girl. The memory in this case was the image of a doll.

Later, participants were shown one of the two-letter words they had learned and asked to either think about or not think about (to suppress) the other word in the pair. Between trials, participants were shown object memories for some of the disturbing scenes. In some cases, the objects were invisible to the participants because they appeared only very briefly and were additionally masked by flashing an image of “noise” on a television before and after the object, resembling visual noise. (Two types of “awareness tests” confirmed that the participants did not see the masked images in almost all cases.)

On a subsequent memory test, participants recalled the scenes they were not remembered an average of 65 percent of the time. In contrast, their recall of the disturbing scenes for which memories had surfaced between two processes of suppression was about 55 percent, showing that the amnesiac shadow clouded recall of those scenes. This level of forgetting is not huge, but it is significant. “This is proof of concept that this is possible,” says Charan Ranganath, a cognitive neuroscientist at the University of California, Davis, who was not involved in the study. “Honestly, I’m amazed it worked out so well.”

Subconscious memories worked just as well as conscious ones. “If you ask them to find out what the thing was, they’re like, ‘I don’t know,'” says Anderson. “However, if you do that over and over again within the amnesiac shadow window, you’re more likely to forget the uncomfortable scene.”

The finding held up even when the researchers later triggered people’s recall of the scenes with the words rather than the images, showing that forgetting is independent of specific memory. “[This] indicates that the effect is likely due to the target memory itself,” says Wang, and not just its association with a particular cue.

The finding opens up the possibility that patients haunted by a painful past may not need to relive it to get better. “The idea that you could filter out disturbing memories without being exposed to them again is just an intriguing possibility,” says John Gabrieli, a neuroscientist at the Massachusetts Institute of Technology who previously worked with Anderson but was not involved in this study was .

However, this cognitive processing occurred in typical young adults, so no one knows if the procedure can bring meaningful benefits to people with mental illness. Because the participants did not have such conditions, the researchers also had to create artificially disturbing memories using a series of disturbing images, rather than using realistic memories for people haunted by traumatic memories.

Additionally, the researchers did not address whether this subliminal procedure could reduce the emotional impact of bad memories. “We don’t necessarily want to forget bad things that happened to us. We just don’t want to be paralyzed by it,” says Ranganath. It would be interesting to know if this method could reduce the visceral effects of traumatic memories, as measured by reactions like tachycardia or sweating, he says.

For these reasons, experts warn that the results won’t provide any therapy any time soon. “The idea that you can unconsciously do something to reduce memory is exciting,” says Elizabeth Phelps, a cognitive neuroscientist at Harvard University who was not involved in the work. “But I think the clinical translation is still a long way off.”