After reflexively reaching out to grab a hot pan falling from the stove, you may be able to withdraw your hand at the very last moment to avoid getting burned. That is because the brain's executive control can step in to break a chain of automatic commands. Several new lines of evidence suggest that the same may be true when it comes to the reflex of recollection—and that the brain can halt the spontaneous retrieval of potentially painful memories.

Within the brain, memories sit in a web of interconnected information. As a result, one memory can trigger another, making it bubble up to the surface without any conscious effort. “When you get a reminder, the mind's automatic response is to do you a favor by trying to deliver the thing that's associated with it,” says Michael Anderson, a neuroscientist at the University of Cambridge. “But sometimes we are reminded of things we would rather not think about.”

Humans are not helpless against this process, however. Previous imaging studies suggest that the brain's frontal areas can dampen the activity of the hippocampus, a crucial structure for memory, and therefore suppress retrieval. In an effort to learn more, Anderson and his colleagues recently investigated what happens after the hippocampus is suppressed. They asked 381 college students to learn pairs of loosely related words. Later, the students were shown one word and asked to recall the other—or to do the opposite and to actively not think about the other word. Sometimes between these tasks they were shown unusual images, such as a peacock standing in a parking lot.

As described in Nature Communications, the researchers found that the participants' ability to subsequently recall the peacocks and other strange pictures was about 40 percent lower if they had been instructed to suppress memories of words before or after seeing the images, compared with trials in which they had been asked to recall the words. The finding provides further evidence that a memory-control mechanism exists and suggests that trying to actively forget a particular memory can negatively affect general memory. The researchers call the phenomenon an “amnesic shadow” because it apparently blocks recollection of unrelated events happening around the time of decreased hippocampal activity. The results may explain why some people who have experienced trauma (and then tried to forget it) have poor memory of everyday events, say experts not involved in the study.

Minus the temporary amnesia, suppressing memories on demand could be a useful skill, Anderson says. That is why he and his colleague Ana Catarino are now studying whether it is possible to train people in the art of suppression: they are currently conducting an experiment in which they monitor participants' brain activity in real time and provide verbal feedback about how much hippocampal activity is dampened. They hypothesize that the cues could help someone learn how to become better at selectively forgetting the past—an ability that could especially ameliorate the pain of those with post-traumatic stress disorder.