By Ingrid Wickelgren on October 19, 2022
Credit: Francesco Carta fotografo/Getty Images
In the mid-1980s scientists conducted a famous experiment in which they asked participants to try to avoid thinking of a white bear. Over the course of five minutes, the experimental subjects were to ring a bell if a white bear came to mind. They rang the bell more than once per minute on average. And later, when the same people were told to think of white bears, the animals came to mind more often than they did for a control group that was instructed to think about white bears from the outset.
The findings suggested that blocking out unwanted thoughts triggered rebound effects, making it harder to keep them at bay. Many people interpreted the results as support for the Freudian notion that repressed memories persist in the subconscious, where they may haunt us. The idea that thought suppression is harmful became conventional wisdom and influenced the practice of clinical psychology. Even today established forms of therapy offer antidotes to the dangers of quashing a memory by guiding patients to repeatedly revisit and elaborate on difficult experiences.
But accumulating research on “active forgetting” in which people block memories using environmental reminders directly challenges the long-standing dogma. Imaging and other psychology experiments from several academic groups show that memory suppression is not only possible but adaptive, possibly protecting people against anxiety and depression. It may even help cleanse the mind of intrusive memories in the aftermath of trauma, warding off post-traumatic stress disorder (PTSD). “There is enough evidence now for us to believe that there is an effect” of suppression on memory, says Daniel Schacter, a memory researcher at Harvard University. “People who have PTSD have problems suppressing,” he adds. “That, to me, is evidence it has some potential clinical utility.”
Support for the idea that people can successfully suppress memories emerged about 20 years ago when cognitive psychologist Michael Anderson, then at the University of Oregon, devised an alternative to the white bear test called the “think/no-think” task. He and his then student Collin Green taught 32 college students to memorize 40 pairs of words such as needle-doctor, journey-pants and lawn-beef. The first word was a reminder, or cue, for the other word. The researchers then showed the students only the cue and told them to either think about and say the other word out loud or not think about (suppress) it. The suppression led to forgetting. And the more suppression trials the students engaged in, the worse their memory for the words that had come second.
The task involved “retrieval suppression,” or the suppression of memories elicited through associations with environmental cues. It reflects the common experience of encountering reminders of unwanted thoughts and either letting them in or pushing them away. The white bear method, by contrast, requires people to think directly about a white bear whenever they remind themselves of the task’s instructions. “In white bear, you are trying to suppress ‘white bear’ itself, so you are thinking about ‘white bear.’ That may prime it for later recovery [in memory] in a way that doesn’t happen in think/no-think,” Schacter says. The think/no-think task enables suppression because the use of cues means that people are not reminded directly of the memories they are trying to shut out.
The white bear exercise may produce a real effect—a persistent memory, experts say. But “clinicians have overgeneralized,” says Anderson, now at the University of Cambridge. “You can do retrieval suppression, and it operates in a very different way.”
In the mid-2000s Anderson and a team at Stanford University, among others, began to unravel the biological basis of this type of forgetting. Imaging experiments showed that the brain’s prefrontal cortex, long known as the engine of behavioral self-control, sends inhibitory signals to the hippocampus, a memory hub. But it was another decade before researchers learned that the effects on the hippocampus may actually persist for a small window of time—at least 10 seconds but potentially longer, casting what Anderson calls an “amnesic shadow” that reflects a temporary degraded functional state in the hippocampus. This degraded state is a result of suppression and leads to forgetting, Anderson says. In the past several years, researchers have found additional clues to the mechanism in the brain that enables people to suppress their thoughts.
In a 2022 functional magnetic resonance imaging (fMRI) study, Anderson’s team found that people use the same prefrontal brain structures for both inhibiting actions and blocking thoughts—but that for action-based tasks, the brain’s motor cortex is shut down instead of the hippocampus. The brain activity patterns in the prefrontal cortex are so similar that a computer taught to recognize the pattern associated with suppressing an action could use it to predict whether a thought had been suppressed, the researchers found.
In another study published this year, scientists at the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany, demonstrated that this process physically disrupts memories rather than simply making them less accessible. The scientists taught 37 people to associate neutral clues with pictures of disasters, accidents or injuries and then had them practice suppressing those associations. During the suppression task and in a later memory test, the researchers monitored the participants’ brain activity using fMRI. Suppressing memories of these upsetting scenes made the scenes less vivid in people’s mind. What’s more, a computer analysis indicated that the patterns of brain activity that had originally represented memories of those scenes were in many cases virtually unrecognizable. The less vividly a person remembered a scene, the more distorted that recollection’s representation in the brain was.
The findings run counter to the idea that suppressed memories lurk in the subconscious. “It’s not that the memory is there, but for some reason, [a person] can’t verbalize it anymore,” says cognitive neuroscientist Roland Benoit, one of the study’s investigators. “The brain is no longer [capable] of reactivating the activity pattern that it would need to support a vivid recollection.”
Also, mounting evidence suggests that memory suppression seems to be an important coping skill. Some of the first support came in 2003 when Trinity University cognitive psychologist Paula Hertel, along with her then student Melissa Gerstle, showed that people with depression had trouble with memory suppression and that the greater the deficit was, the more a person tended to ruminate or experience unwanted thoughts. Memory control problems, Hertel concluded, might explain why people ruminate. And such habits can be difficult to reverse. “If you’ve been practicing thinking about events from your past that bother you or worrying about things that might happen, then practice makes perfect,” Hertel says. “One of the best ways you can remember something is to practice bringing it to mind.”
Over the years, scientists have replicated this research and extended it to other mental illnesses in which intrusive thoughts abound. For instance, in 2016 Anderson and his colleagues linked people’s ability to suppress their thoughts to their resilience after watching disturbing videos that reliably produce traumatic reactions. The people who could forget had fewer intrusive memories in the following days. And a 2020 meta-analysis of 25 studies showed that people without mental health conditions can successfully suppress unwanted memories, whereas people with anxiety, PTSD or depression cannot. The ability to willfully forget is correlated with better mental health. “This suppression mechanism may protect us from developing these disorders,” Benoit says.
In 2020 Pierre Gagnepain, a cognitive neuroscientist at the French National Institute of Health and Medical Research and his colleagues reported something similar in real trauma survivors. In the aftermath of the 2015 deadly terrorist attacks in and around Paris, Gagnepain’s team put 102 of the attack’s survivors along with 73 controls in a brain scanner while they performed retrieval suppression. Of the survivors, 55 had PTSD, but the rest did not. The people who remained stress-free, the researchers found, were better at suppressing memories than those with PTSD. Their brain activity—in the prefrontal cortex, the hippocampus and other areas—showed that resilient survivors had more effective cognitive control mechanisms. “The disruption of the mechanisms that you use to inhibit memories may actually be involved in the development of the traumatic memory,” Gagnepain says.
Retrieval suppression is not the only way to dim traumatic or upsetting memories. Another is “memory reconsolidation.” In this technique, a person is prompted to recall a memory, which “reactivates” it, making it vulnerable to change. Then the person does something—say, take a drug—to interfere with the memory. Researchers have tried giving people propranolol, which is thought to block memory reconsolidation based on studies of related compounds in rodents, with mixed results, Schacter says. Behavior can also interfere with memory. In a 2020 trial, neuroscientist Emily Holmes of Uppsala University in Sweden and her colleagues tried to meddle with visual aspects of memories of a film with traumatic scenes by having participants play the game Tetris shortly after being reminded of that film. The researchers found that their procedure reduced the number of intrusive memories people experienced afterward.
Exactly how to use memory suppression as therapy is not yet clear. One way is to just teach people to practice it, Anderson says. In experiments, participants’ performance on the think/no-think task reliably improves with repeated trials. Individuals might, for example, practice stopping their thoughts while staring at cues related to their own upsetting memories or fears.
Whether no-think training would work for people with significant deficits in memory control is less certain. Some of Hertel’s research suggests that work-arounds are necessary for depressed people, such as providing them with a new thought to associate with the cue as a replacement for the troublesome one.
Another helpful adjunct might be sleep. Scott Cairney, a cognitive neuroscientist at the University of York in England, has found that sleep deprivation significantly worsens people’s ability to suppress unwanted memories. In a 2021 study, Cairney’s team showed that when people were sleep-deprived, they experienced many more intrusive memories than people who had slept. “Over chronic sleep deprivation, you’ll have more intrusive memories, and it will increase your likelihood of developing mental health problems,” Cairney says. In addition, more intrusive memories can affect the ability to sleep, “so it becomes a vicious cycle.”
In classic cognitive-behavioral therapy, people are encouraged to generate alternative interpretations for difficult experiences. Research on retrieval suppression, however, suggests that such reanalysis may often be unnecessary. “You can reduce the frequency of thoughts by simply stopping them,” Anderson says, “and they don’t tend to come back.”
Leave a Reply