A team of researchers at the Medical Research Council (MRC) has discovered that the human brain can intentionally forget unwanted memories via two distinct and opposite processes. By improving our understanding of the brain mechanisms underlying these processes, known as ‘memory suppression’ and ‘memory substitution’, these findings may help explain why people can have problems controlling unwanted memories, as seen in Post-Traumatic Stress Disorder (PTSD). Results from the study are published in Neuron this week.
In the first study to show the existence of two different brain mechanisms that help people to forget unwanted memories, researchers found that people could either intentionally force the memory out of their awareness (via the part of the brain known as the dorsolateral prefrontal cortex) or, instead, could direct their memories selectively by redirecting consciousness towards a substitute memory (via two regions called the caudal prefrontal cortex and the midventrolateral prefrontal cortex). These mechanisms work in opposite ways because pushing out involves shutting down regions involved in remembering; in contrast, substituting actively engages those same areas, but directs memory in a selective, motivated way towards alternative thoughts.
Improving the understanding of these two mechanisms and how they break down may ultimately enhance our understanding of which memory process can most effectively be improved through tailored treatments. A defining symptom of PTSD is involuntary, intrusive memories of trauma. Improving the understanding of these two mechanisms and how they break down may ultimately enhance our understanding of which memory process can most effectively be improved through tailored treatments.
Dr Michael Anderson, from the MRC Cognition and Brain Sciences Unit, explained: “If we compare the brain to a car, it’s the difference between slamming on the mental brakes, versus controlling the steering of remembering towards another memory to avoid the unwanted memory. Despite operating in opposite ways, both mechanisms astonishingly led to the same pattern of forgetting the suppressed memories. The only way one could tell that something different was going on was by looking at what the brain was doing.”
The team used functional magnetic resonance imaging (fMRI) to examine the brain activity of volunteers. Volunteers went through a process of learning associations between reminders and words and, as their brain patterns were monitored, were then prompted to keep these memories out of mind, either by pushing them out or by focusing on substitute memories.
Study lead Dr Roland Benoit, from the MRC Cognition and Brain Sciences Unit, said: “This study is the first demonstration of two distinct mechanisms that cause voluntary forgetting of unwanted memories: one by shutting down the remembering system, and the other by facilitating the same system to replace the unwanted memories with substitute memories. Knowing that distinct processes contribute to forgetting may be helpful, because people may be naturally better at one approach or the other.”
Professor Susan Gathercole, Director of the MRC Cognition and Brain Sciences Unit, says: “Using fMRI in this study has helped understand how the brain enables us to forget unwanted information, which can be as important in everyday life as remembering. Understanding this has great potential in helping patients with debilitating memory disorders such as PTSD, in which painful memories cannot be suppressed and repeatedly intrude and cause distress.”