How the brain fights off fears that return to haunt us
April 1, 2019
Science Daily/University of Texas at Austin
Neuroscientists at The University of Texas at Austin have discovered a group of cells in the brain that are responsible when a frightening memory re-emerges unexpectedly, like Michael Myers in every "Halloween" movie. The finding could lead to new recommendations about when and how often certain therapies are deployed for the treatment of anxiety, phobias and post-traumatic stress disorder (PTSD).
In the new paper, out today in the journal Nature Neuroscience, researchers describe identifying "extinction neurons," which suppress fearful memories when they are activated or allow fearful memories to return when they are not.
Since the time of Pavlov and his dogs, scientists have known that memories we thought we had put behind us can pop up at inconvenient times, triggering what is known as spontaneous recovery, a form of relapse. What they didn't know was why it happened.
"There is frequently a relapse of the original fear, but we knew very little about the mechanisms," said Michael Drew, associate professor of neuroscience and the senior author of the study. "These kinds of studies can help us understand the potential cause of disorders, like anxiety and PTSD, and they can also help us understand potential treatments."
One of the surprises to Drew and his team was finding that brain cells that suppress fear memories hid in the hippocampus. Traditionally, scientists associate fear with another part of the brain, the amygdala. The hippocampus, responsible for many aspects of memory and spatial navigation, seems to play an important role in contextualizing fear, for example, by tying fearful memories to the place where they happened.
The discovery may help explain why one of the leading ways to treat fear-based disorders, exposure therapy, sometimes stops working. Exposure therapy promotes the formation of new memories of safety that can override an original fear memory. For example, if someone becomes afraid of spiders after being bitten by one, he might undertake exposure therapy by letting a harmless spider crawl on him. The safe memories are called "extinction memories."
"Extinction does not erase the original fear memory but instead creates a new memory that inhibits or competes with the original fear," Drew said. "Our paper demonstrates that the hippocampus generates memory traces of both fear and extinction, and competition between these hippocampal traces determines whether fear is expressed or suppressed."
Given this, recommended practices around the frequency and timing of exposure therapy may need revisiting, and new pathways for drug development may be explored.
In experiments, Drew and his team placed mice in a distinctive box and induced fear with a harmless shock. After that, when one of the mice was in the box, it would display fear behavior until, with repeated exposure to the box without a shock, the extinction memories formed, and the mouse was not afraid.
Scientists were able to artificially activate the fear and suppress the extinction trace memories by using a tool called optogenetics to turn the extinction neurons on and off again.
"Artificially suppressing these so-called extinction neurons causes fear to relapse, whereas stimulating them prevents fear relapse," Drew said. "These experiments reveal potential avenues for suppressing maladaptive fear and preventing relapse."
https://www.sciencedaily.com/releases/2019/04/190401115757.htm
Exposure to trauma impacts ability to squash bad memories
March 4, 2019
Science Daily/Boston University School of Medicine
People exposed to trauma are less able to suppress unwanted emotional memories due to neural and behavioral disruptions in their brain that may contribute to the development of post-traumatic stress disorder (PTSD).
PTSD is characterized by intense reliving of the trauma that is repetitive, intrusive and incapacitating. The intrusive nature of these hallmark symptoms suggests that the inability to suppress unwanted memories may be a strong contributor to the behavioral manifestation of PTSD.
Previous work has shown that healthy individuals can actively suppress emotional memories while individuals with PTSD frequently experience unwanted memories of their traumatic experiences, even when making explicit efforts to avoid them. However, little is known regarding the behavioral and neural effects of memory suppression among individuals with PTSD.
Using functional magnetic resonance imaging, the researchers examined memory suppression in three groups: those with PTSD; those who experienced trauma without PTSD and controls with no trauma exposure or PTSD. They found trauma-exposed participants (regardless of PTSD status) were less likely to successfully suppress memory than non-trauma-exposed controls.
"Neuroimaging data revealed that trauma-exposed individuals showed reduced activation in the right middle frontal gyrus, a critical region for memory suppression, during a memory suppression task and were less likely to successfully suppress memory compared to non-trauma exposed individuals. These results suggest that trauma exposure is associated with neural and behavioral disruptions in memory suppression and point to the possibility that difficulty in active suppression of memories may be just one of several likely factors contributing to the development of PTSD," explained lead author Danielle R. Sullivan, PhD, affiliated with Boston University School of Medicine and the National Center for PTSD, VA Boston Healthcare System.
These findings appear in the Journal of Psychiatric Research.
https://www.sciencedaily.com/releases/2019/03/190304105445.htm