Suppressing bad memories from the past can block memory formation in the here and now, research suggests.

The study could help to explain why those suffering from post-traumatic stress disorder (PTSD) and other psychological conditions often experience difficulty in remembering recent events, scientists say.

Writing in Nature Communications, the authors describe how trying to forget past incidents by suppressing our recollections can create a “virtual lesion” in the brain that casts an “amnesiac shadow” over the formation of new memories. “If you are motivated to try to prevent yourself from reliving a flashback of that initial trauma, anything that you experience around the period of time of suppression tends to get sucked up into this black hole as well,” Dr Justin Hulbert, one of the study’s authors, told the Guardian.

“I think it makes perfect sense because we know that people with a wide range of psychological problems have difficulties with their everyday memories for ordinary events,” said Professor Chris Brewin, an expert in PTSD from University College, London, who was not involved in the study. “Potentially this could account for the memory deficits we find in depression and other disorders too.”

The phenomenon came to the attention of the scientists during a lecture when a student admitted to having suffered bouts of amnesia after witnessing the 1999 Columbine high school massacre. When the student returned to the school for classes after the incident she found she could not remember anything from the lessons she was in. “Here she was surrounded by all these reminders of these terrible things that she preferred not to think about,” said Hulbert.



The student’s account raised an intriguing possibility. Previous studies had revealed that the suppression of memories is linked to a decrease in activity in the hippocampus - the area of the brain involved with making new memories and retrieving old ones. “It occurred to us that this student’s experience may very well be a side-effect of the very thing we had discovered only a couple of years earlier,” said Professor Michael Anderson, a co-author of the research. “There is a trade-off: you get rid of the memories you don’t want, but you also make your memory less effective for short windows of time.”

To probe the theory, researchers at Bard College, New York, the UK’s Medical Research Council and the University of Cambridge subjected 381 healthy participants to a series of memory tests.

In the first test, participants were shown to pairs of words which had a loose relation to each other, such as “ballet” and “leap” and were asked to learn them. They were then shown one of the words, and asked either to recall or suppress the memory of the second word. Such suppression, however, required practice. “One simple strategy is focusing on the cue word on the screen,” Hulbert explained. “So they might look at how many vowels are in the word or whether the word is pleasing to look at or not.”

Later in the task, participants were then shown an image of an object in an unusual setting, such as a peacock in a car park, and asked to construct a story in their minds about the scenario, before being asked again to either recall or suppress the memory of a word associated with a cue.

Following a string of such exercises, participants were then asked to recall the object in each image. Their ability to do so was found to be worse when they had been instructed to suppress a memory before or after seeing the picture - an effect that was more pronounced for the later stages of the task, suggesting that amnesia increased the more participants practised suppressing their memories.

The most dramatic amnesiac effect was found for the instances in which participants were asked to suppress memories both before and after the image was shown. In these instances, the participants’ ability to recall the object was found to be 42% lower than for the occasions on which they had not tried to suppress their memories. What’s more, scans taken using functional magnetic resonance imaging revealed that activity of the hippocampus decreased when participants engaged in suppressing memories.

However, further experiments revealed that when participants employed thought-substitution rather than thought suppression, the unwanted memory was dodged without evidence of amnesia.



That, says the researchers, offers new insights into the ways in which we block distressing recollections. “By learning more about the mechanisms that allow us to remember when we want to remember or forget when we want to forget, we can identify memory control strategies that help maximise the benefits and minimise the side effects,” said Hulbert.

Brewin believes the study supports current approaches to tackling PTSD, which advises against suppressing recollections. “We know that is harmful in other ways, not just to their memories, because it prevents them from really coming to terms with the event, and desensitising themselves to the horrific experiences they’ve had” he says. “So all the things that therapists already are doing are the right things according to this theory as well.”

“We now have quite convincing evidence, from two independent laboratories, that you can, just by performing a cognitive task, produce a temporary virtual lesion in the hippocampus, during which time healthy, young undergraduate students show characteristics typically observed in amnesic patients,” added neuroscientist Dr Sinead Mullally from Newcastle University, whose previous research focused on a similar idea. “And that has real knock-on consequences as a tool to understand memory better.”