Recurrent intrusive memories lie 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 and gently re-expose patients to feared stimuli or simulations—from reminders of active combat to germs on a toilet—teaching the brain to become accustomed to the stimuli and to decouple them from danger.
But exposure therapy has drawbacks. “Facing these traumatic memories is painful to patients,” says Yingying Wang, a cognitive psychologist at Zhejiang University in China. “These treatments suffer from a very high dropout rate.” Wang and her colleagues have taken a first step toward developing a more benign way to dim traumatic memories. Their proof-of-concept study involves subliminal exposure to cues to those memories after putting the brain in a state in which it is likely to forget.
The new findings present a new spin on a form of active forgetting in which people learn to suppress memories by practicing not thinking about them in the presence of reminders. In various studies, participants have memorized pairs of words such as needle-doctor or jogger-collie and then practiced either thinking or not thinking about the second word when the first word (the reminder) appears. Practicing not thinking about the second word has led to forgetting.
The mechanism for this effect centers on the brain’s main memory hub, the hippocampus. Psychologists have discovered that suppressing memory retrieval puts the hippocampus in a degraded functional state. This state lasts for a small window of time—at least 10 seconds but potentially much longer—casting what researchers have dubbed an “amnesic shadow” that leads to poor memory for other things that happen within it. So when people suppress neutral word pairs, they put their brain into a state in which they are likely to forget new experiences.
This state also enables 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 totally neutral and then remind people of that memory nearby in time,” says Michael Anderson, a cognitive psychologist at the University of Cambridge, who has done much of the work on memory suppression and is an author of the new paper. In that study, Wang, Zhu and Anderson showed that the process could degrade memories of unpleasant scenes even when the reminder of those scenes was presented in a way that made people unaware that it was there.
When Wang and Zhu floated the idea for this experiment, however, Anderson was skeptical. “They bounced the idea off me by e-mail, and I thought, ‘That sounds really cool. I’m sure it’s not going to work,’” Anderson recalls. But Wang was confident that it would because she had dug up studies showing that certain invisible reminders would recruit the hippocampus, she says.
So the researchers asked 88 healthy young adults across two experiments to memorize pairs of neutral two-character Chinese words. To establish traumalike memories, the participants studied disturbing images, along with pictures or words that would serve as reminders of those images. The upsetting pictures depicted themes such as physical or sexual abuse, injuries, death, natural disasters and serious accidents. The reminder images showed objects like those that appeared in the scenes, akin to real-world visual reminders of upsetting events. In one photograph, for example, a woman and her daughter were lying dead on the ground, having been shot, and a doll was next to the girl. The reminder, in this case, was an image of a doll.
Later the participants were shown one of the two-character words they had studied and asked to either think about the other word in the pair or not to think about it (to suppress it). Between the trials, the participants were shown the object reminders for some of the disturbing scenes. In some cases, the objects were invisible to participants, because they showed up only very briefly and were further masked by flashing an image of “noise” akin to visual static on a television before and after the object. (Two types of “consciousness checks” confirmed that participants did not see the masked images in almost all cases.)
On a later memory test, participants remembered the scenes they had not been reminded of 65 percent of the time on average. By contrast, their memory of the disturbing scenes for which reminders had appeared between two suppression trials hovered around 55 percent, showing that the amnesic shadow dimmed the recollection of those scenes. This degree of forgetting is not huge, but it’s significant. “This is proof of concept that this can be done,” says Charan Ranganath, a cognitive neuroscientist at the University of California, Davis, who was not involved in the study. “Frankly, I’m amazed that it worked out so well.”
Subconscious reminders worked as well as conscious ones. “When you ask them to identify what that thing was, they say, ‘I don’t know,’” Anderson says. “Nevertheless, when you do that over and over and over again within amnesic shadow window, they are more likely to forget the unpleasant scene.”
The result also held when the researchers later triggered people’s memory of the scenes with the words rather than the pictures, showing that forgetting is independent of the specific reminder. “[This] indicates that the effect is likely on the target memory itself” rather than on just its association with a particular cue, Wang says.
The result opens up the possibility that patients who are haunted by a painful past may not have to relive it to get better. “The idea that you could edit out disturbing memories without having to be reexposed to them is just fascinating as a possibility,” says John Gabrieli, a neuroscientist at the Massachusetts Institute of Technology, who previously collaborated with Anderson but was not involved in this study.
This cognitive editing occurred in typical young adults, however, so no one knows whether the procedure can produce meaningful benefits for people with mental health conditions. Given that the participants had no such conditions, the researchers also had to artificially establish upsetting memories with a set of disturbing images rather than use realistic reminders for people haunted by traumatic memories.
In addition, the researchers did not address whether this subliminal procedure could lessen the emotional fallout from bad memories. “We don’t necessarily want to forget bad things that have happened to us. We just don’t want to be crippled by it,” Ranganath says. It would be interesting to know if this method could reduce the visceral impact of traumatic memories, as measured by responses such as a racing heart or sweating, he says.
For these reasons, experts caution that the findings are unlikely to yield a therapy anytime soon. “The notion that you can do something nonconsciously to help reduce memory is exciting,” says Elizabeth Phelps, a cognitive neuroscientist at Harvard University, who was not involved in the work. “But I do think the clinical translation is a long way away.”