Why do we remember some but not all the events in our lives? Answering this, most of us imagine the brain as a confined storage space able to contain only so much. Now, a new brain imaging study from the University of Birmingham and the MRC Cognition and Brain Sciences unit in Cambridge suggests that clumsy and ugly metaphor may be accurate.

"When you repeatedly recall the same details from a scene over and over again, you will start to forget other details from the same scene," Dr. Maria Wimber, lead author of the study, told Medical Daily in an email. Whenever you intentionally recall the past, your brain will suppress competing memories and by doing so forever alter which recollections will remain accessible to you in the future, according to Wimber's research. In storage space terms, your brain sorts through the accumulated memories, chooses one, pushes similar memories to the back, and in this way makes them even harder to reach in the future.

“By simply using our memory system via selective retrieval, we adapt the landscape of memory,” wrote the authors in their conclusion. In fact, their work supports many recent research papers which have drawn similar portraits of a tyrannical memory that imperiously favors some recollections only to oppress others.

Darwinian Struggle

How do we retrieve a single experience from the past when it is locked away with countless overlapping events? Why do some experiences almost instantly transform into stable memories while others are forgotten just as fast? These are the kinds of questions that Wimber, a cognitive neuroscientist at the University of Birmingham, traffics in daily.

For her most recent study, Wimber and Dr. Michael Anderson from the MRC Cognition and Brain Sciences Unit Cambridge led a team of researchers in making an attempt to decode how the brain performs one of its most common activities: remembering. They asked participants to recall memories of images they had been shown, yet all the while they monitored each participant's brain activity patterns via an MRI scan.

Sounds simple enough. However, this study, like so many others, needed to overcome a major obstacle. It is very difficult for scientists to locate and identify the brain activity linked to an engram — the collection of neural fingerprints and other traces left behind by experience. In this case, the research team chose to capitalize on the relationship between perception and memory. The very design of the experiment encouraged participants to create a cluster of memories so that when the researchers later triggered one memory, several competing memories awoke all at once. All the while the researchers, using an MRI, observed the mental processes in play.

In fact, by dividing the brain into tiny voxels — separate values on a 3D grid — the team could precisely track the brain activity induced with each memory. And so, by watching activation patterns unfold across these voxels, the researchers discovered how the brain would reactivate all the voxels associated with similar memories yet quickly suppress those unlinked to the requested memory.

Over the course of four retrievals, the participants would excavate a target memory, which became more and more vivid each time. Yet, with each successive trial, competing memories became less easy to see and eventually began to fade. In voxel terms, the oppressed memories were soon pushed below baseline expectations of neural activity, they were drowned by the greater detail and richness of what was remembered.

But is this always the case? When memories are integrated extremely well, they tend to not compete any more, but instead be activated as a whole," Wimber explained to Medical Daily in an email. "In the case of well learned vocabulary (or semantics in general), my guess is that this semantic integration prevents against retrieval-induced forgetting. Having said this, there is evidence that acquiring a new language can inhibit word meaning in your native language."

“I think a lot of people are surprised to hear that recalling memories has this darker side of making us forget others by actually suppressing them,” Wimber stated in a press release. Dark, perhaps, maybe simply necessary: In order to remember we must forget, and in this way we shape a narrative from the raw experiences of our lives.

Source: Wimber M, et al.  Retrieval induces adaptive forgetting of competing memories via cortical pattern suppression. Nature Neuroscience. 2015.