Recurrent intrusive memories are a key component of some mental illnesses such as post-traumatic stress disorder (PTSD) and obsessive compulsive disorder (OCD). These conditions are often treated by “exposure therapy” which involves gradually exposing patients to feared stimuli or simulations, such as reminders of active combat or germs on the toilet. This teaches the brain to become more comfortable with the stimuli and decouple it from danger.
Exposure therapy has its drawbacks. “Facing these traumatic thoughts is difficult for patients,” says Yingying Wang (a cognitive psychologist at Zhejiang University, China). “These treatments have a high dropout rate.” Wang and her collaborators have taken the first step towards developing a more benign method to dim traumatic memories. The proof-of-concept study involved subliminal access to cues that were directed to the memories after putting it in a state where it is most likely to forget.
The new findings offer a new perspective on active forgetting, in which people learn to suppress their memories by practicing not thinking of them in the presence reminders. Participants were asked to memorize pairs of words, such as needle-doctor and jogger-collie. Then they had to practice thinking or not about the second word (the reminder). It has been shown that forgetting the second word is possible by not thinking about it.
This effect is caused by the brain’s main memory center, the hippocampus. Psychologists discovered that memory retrieval suppression can lead to a reduced functional state of the hippocampus. 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. When people suppress neutral words pairs, it puts their brain in a state that makes it more 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. Michael Anderson, a cognitive psychologist from the University of Cambridge who has done much of this work on memory suppression, is the author of the new paper. Anderson, Zhu, and Wang showed that this process can degrade memories of unpleasant scenes, even if the reminder was presented in a manner that made people forget it was there.
When Zhu and Wang suggested the idea for this experiment they were skeptical of Anderson. Anderson was skeptical when Zhu and Wang emailed me with the idea. I thought it was really cool. Anderson says that she is certain it won’t work. Wang was certain it would, however, because she had found studies that showed that certain invisible reminders could recruit the hippocampus.
So the researchers asked 88 healthy young adults across two experiments to memorize pairs of neutral two-character Chinese words. Participants were shown disturbing images and words to help them remember those images. The disturbing images depicted themes like physical or sexual abuse, deaths, injuries, natural disasters, and serious accidents. The reminder images featured objects similar to those seen in the scenes. They were similar to real-world visual reminders about upsetting events. One photograph showed a mother and her daughter lying on the ground after being shot. A doll was next to the girl. In this instance, the reminder was an image of a doll.
Later, participants were shown one the two-character words that they had studied. They were asked to think about the other word in a pair and to suppress it or not. Participants were also shown reminders of some disturbing scenes. Participants were sometimes unable to see the objects because they appeared briefly. In these cases, participants were shown “noise” images that resembled static on a TV before and after the object. Two types of “consciousness tests” confirmed that participants didn’t see the masked images almost in 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 is a significant amount of forgetting, although it isn’t huge. Charan Ranganath, a cognitive neuroscientist from the University of California, Davis, said that “this is proof of concept” that it was possible. “Frankly, it’s amazing that it worked out .”
Subconscious reminding worked just as well as conscious ones. Anderson says that when you ask them to identify the thing, they will say “I don’t remember.” “But, if they do it over and over in amnesic shadow windows, they are more likely not to remember the unpleasant scene .”
The researchers also found that people remembered the scenes more easily if they were prompted with words than pictures. This shows that forgetting can occur independently of the reminder. Wang states that this indicates that the effect is likely to be on the target memory, rather than just on its association with a specific cue.
This result suggests that patients who are haunted from a past trauma may not need to go through it again in order to heal. John Gabrieli, a neuroscientist from the Massachusetts Institute of Technology, said that it was fascinating to think that you could erase disturbing memories without having them reexposed. He previously worked with Anderson, but was not part of this study.
This cognitive editing was performed in young adults. However, no one knows if the procedure can have meaningful benefits for people with mental disorders. Researchers had to create upsetting memories using a series of disturbing images, as the participants did not have such conditions. This was in order to avoid creating traumatic memories that are real and reliving them.
The researchers also did not consider whether the subliminal procedure could reduce the emotional fallout from bad experiences. “We don’t want to forget the bad things that happened to us.” Ranganath states that we don’t want it to make us feel weak. Ranganath says it would be interesting to see if this method could reduce the visceral effects of traumatic memories as measured by responses like a racing heart and sweating.
Experts warn that the findings will not lead to a treatment. “The idea that you can do something nonconsciously in order to reduce memory is exciting,” said Elizabeth Phelps, a cognitive neuroscientist from Harvard University who was not involved with the research. “But I do think the clinical translation is a long way away.”