To remember the things we want, forget the things we don’t, and then be able to access all those memories when they’re needed, the brain has to do some pretty fast maneuvering. Unfortunately, it doesn’t always work out, and sometimes we forget the things we really wish we could recall.

Now we may understand why: The brain may not only lose the memories that we don’t seem to need, they may be actively pushed out of the way by the memories that we do.

The idea that the very act of remembering can cause forgetting is surprising, and could tell us more about selective memory and even self-deception.

People were asked to look at a number of images and then recall some of them later. British researchers simultaneously scanned the participants' brains with MRI, and divided the scans of areas central to memory up into tiny 3D sections. They were able to watch how tiny areas of the brain responded as memories were recalled or left alone.

When a particular image was recalled over and over again, the memory and response of certain cells strengthened. At the same time, other, unrequested and therefore unrecalled memories were “pushed” below baseline. The researchers concluded that there was an active form of suppression going on, where the needed memories pushed away the less required ones.

“People are used to thinking of forgetting as something passive,” said study author Michael Anderson in a statement. “Our research reveals that people are more engaged than they realise in shaping what they remember of their lives. The idea that the very act of remembering can cause forgetting is surprising, and could tell us more about selective memory and even self-deception.”

The new findings may also have important consequences in court settings, where people are desperately trying to remember details of critical events — or when they are put under extreme pressure to admit to crimes they may not have committed.

“[Our research] has significance for anything that relies on memory, but a really good example is that of eyewitness testimonies,” said author Maria Wimber.

“When a witness is asked to recall specific information about an event, and they are quizzed time and time again, it could well be to the detriment of associated memories — giving the impression that their memory is sketchy. In fact, the repeated recall is causing them to forget these details.”

On the other hand, sometimes forgetting is actually what you hope will happen — for instance if you are plagued by unpleasant memories, or have been the victim of a crime.

Reliving a memory over and over again involuntarily — as with post-traumatic stress disorder (PTSD) — may serve to strengthen a painful memory when what you want is exactly the opposite. If researchers could develop strategies to help fade memories by using the brain’s natural “forgetting” mechanisms, it has potential to help those suffering from haunting or destructive memories.

The researchers say the results are applicable to many types of memory, from newly acquired short-term memory to long-term autobiographical memories such as for the birthday party you had at age 6.

More research will be needed, of course, but this study is exciting since it’s one of the first to really lay out the mechanism by which some memories persist and others fade. Its many potential applications in clinical psychology and the judicial system are fascinating. And it probably applies to everyday life, too.

If you have a important memory that you realize may not be quite as crisp as it once was, start giving it a little more air time, and maybe talk with someone else who remembers it to bring back those details. Otherwise, it might be pushed out of the way by other, newer memories that are demanding brain space of their own.

The study was carried out by a team at the Medical Research Council’s Cognition and Brain Sciences unit in Cambridge, England and published in Nature Neuroscience.