A mechanism in the human brain which blocks unwanted memories was located by researchers from the universities of Stanford and Oregon, according to an article published in the journal Science in January 2004.

The authors said this was the first time that a neurobiological basis for memory repression had ever been demonstrated.

The study was led by Michael Anderson, of the University of Oregon, and John Gabrieli, of Stanford University.

Sigmund Freud, over 100 years ago, was convinced that humans had a “voluntary repression mechanism” that pushes memories out of consciousness.

Since Freud’s time, the idea of memory repression became a highly controversial and vague idea, partly because nobody could really imagine how such a process would occur in the brain. However, the authors believe that it may be more commonly applied than people think.

Anderson explained:

“Often in life we encounter reminders of things we’d rather not think about. We have all had that experience at some point-the experience of seeing something that reminds us of an unwanted memory, leading us to wince briefly-but just as quickly to put the recollection out of mind. How do human beings do this?”

The process of unblocking memories is not just limited to traumatic events; it is applied widely, whenever somebody is distracted by memories, be they pleasant or unpleasant ones.

Anderson added “This active forgetting process is a basic mechanism we use to exclude any kind of distracting memory so we can concentrate on our tasks at hand.”

To mimic the process that occurs in the brain in the laboratory, the researchers tested volunteers using a procedure Anderson created. The participants first learnt pairs of words, such as “jaw-gum”, or “steam-train”, or “ordeal-roach”. They were then given the first member of each word pair and asked to either suppress awareness of the second word or to think of it.

The team scanned the participants while they carried out this task using fMRI (functional magnetic resonance imaging). From these images, they could tell which parts of the brain were being used for the different tasks.

After completing this phase, Andersen tested the volunteers’ memory for all the word pairs. He confirmed that “suppressing awareness of unwanted memories resulted in memory inhibition”, replicating the findings of an earlier study that had been published in Nature.

The researchers described what the fMRI images revealed about the participants’ brain activity while recalling or suppressing memories as “astonishing”. They claim their study revealed for the first time compelling neurobiological evidence of a new idea about how memory repression occurs.

They said that what occurs is quite simple:

“Unwanted memories can be suppressed with brain areas similar to that used when we try to stop overt physical actions.

In other words, the brain systems we use to stop an arm in mid-motion can also be utilized to inhibit or stop an undesired memory retrieval.

The control processes reduce brain activation in the hippocampus, a region of the brain vital for remembering past events. Crucially, this drop in hippocampal activity caused the volunteers to forget the rejected experiences. Anderson says he relates our ability to control memory to how we control our physical actions. He gives as an example the time he knocked a plant off his windowsill: “As I saw the plant falling off the sill out of the corner of my eye, I reflexively went to catch it. At the very last second, I stopped myself, midstream when I realized that the plant was a cactus.”

The team believe that this study shows that the way we stop unwanted memory retrievals is built on the same brain mechanisms that help us achieve control over our overt behavior; they add that this provides a “very concrete mechanism that may demystify how repression occurs”.

Anderson and Gabrieli were able to predict how much forgetting their volunteers would experience by examining how active their prefrontal cortex was when they attempted to suppress memories.

The model the researchers used for exploring motivated forgetting in the lab has been described as a landmark achievement. The subject of repressing unwanted memories has been a controversial one among psychologists.

Scientists are now better able to understand the cognitive and neural mechanisms people use when dealing with the memory after-effects of a traumatic experience, and how these mechanisms break down as they do in, for example, PTSD (post-traumatic stress disorder).

Further studies are required to examine the role of these mechanisms in blocking emotional experiences, Anderson stressed. This study focused on the suppression of relatively neutral events. However, their findings provide a well-grounded hypothesis for how some individuals may come to forget unwanted memories of disagreeable life experiences.

Anderson explained:

“To me what’s most important is achieving a better understanding of how we learn to adapt mental function in response to traumatic life experience.
Survivors of natural disasters, crime, acts of terror such as 9/11, the loss of someone close all undergo a process that may continue for a very long time-a process of learning to adjust both physically and mentally to those events.

Now we have a specific neurobiological model of the mechanisms by which people normally adapt how their memories respond to the environment. My goal is to expand on this model so we can better understand these important experiences.

A study carried out at Cambridge University and published in the October 17th, 2012 edition of Neuron, identified two ways we forget unwanted memories, and which neural circuitries are involved in them.

Written by Christian Nordqvist
Original article date: 9th January 2004. Article updated: 20th October 2012.