The findings, by lead researcher Michael Anderson, associate professor of psychology at the University of Oregon, and his colleague, John D.E. Gabrieli, professor of psychology at Stanford, will be published Jan. 9 in Science.
The research provides compelling evidence that Freud was on to something 100 years ago when he proposed the existence of a voluntary repression mechanism that pushes unwanted memories out of consciousness. Since then the idea of memory repression has been a vague and highly controversial idea, in part because it has been difficult to imagine how such a process could occur in the brain. Yet, the process may be more commonly applied than was previously thought.
"Often in life we encounter reminders of things we'd rather not think about," Anderson explains. "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?"
Anderson says that this process isn't restricted to traumatic experiences, but is applied widely, whenever we are distracted by memories, pleasant or unpleasant.
"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 brain's process in the lab, Anderson and Gabrieli tested subjects using a procedure Anderson devised. Subjects first learned pairs of words such as ordeal-roach, steam-train and jaw-gum. Then they were given the first member of each word pair and asked either to think of the second word, or to suppress awareness of the second word.
Subjects performed this task while being scanned in a functional magnetic resonance imaging (fMRI) machine that produces images of brain tissue and function. From these images, researchers can determine which parts of the brain are in use for different tasks.
After this phase was completed, Anderson tested the students' memory for all of the word pairs and confirmed that suppressing awareness of unwanted memories resulted in memory inhibition, replicating a finding he reported earlier in the journal Nature.
The fMRI images of the subjects' brain activity during this procedure yielded astonishing results. This study revealed for the first time strong neurobiological evidence for a novel idea about how memory repression occurs that is quite simple: unwanted memories can be suppressed with brain areas similar to that used when we try to stop overt physical actions.
Put simply, the brain systems that permit one to stop an arm motion midstream can be recruited to inhibit or stop an unwanted memory retrieval. Instead of inhibiting activity in brain regions having to do with physical action, however, these control processes reduce brain activation in the hippocampus, a structure known to be involved in conscious memories of the past. Crucially, this reduction in hippocampal activity led the subjects to forget the rejected experiences.
Anderson relates the ability to control memory to the ability to control our physical actions, like the time he knocked a plant off his windowsill at home.
"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."
Anderson's research indicates that stopping unwanted memory retrievals build on the same brain mechanisms that help us to achieve this control over our overt behavior, providing a very concrete mechanism that may demystify how repression occurs. Intriguingly, Anderson and Gabrieli could predict how much forgetting people in their experiment would experience, simply by examining how active their prefrontal cortex was when attempting to suppress memories.
Anderson and Gabrieli's clear, straightforward neurobiological model for exploring motivated forgetting in the laboratory is a landmark achievement. Until now the idea that unwanted memories can be repressed has been a controversial issue among psychologists.
The UO researcher and his associates have provided a way to scientifically investigate and map the cognitive and brain process in the laboratory. Among the immediate benefits may be the ability to better understand the cognitive and neural mechanisms by which people deal with the memory aftereffects of a traumatic experience, and the breakdown of these mechanisms in post-traumatic stress disorder.
Anderson emphasizes, however, that future research is needed to examine the role of these mechanisms in suppressing emotional experiences, as the current study focused on the suppression of relatively neutral events. Nevertheless, they also provide a well-grounded hypothesis for how some people may come to forget unwanted memories of unpleasant life experiences.
"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," Anderson explains. "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."
For more information: http://darkwing.uoregon.edu/~blevy/lab/homepage.htm