A surprising feature of how the brain stores memories could help explain why harmless situations can sometimes quickly feel threatening, a hallmark of anxiety and trauma disorders.
Scientists have long believed that fear helps the brain draw a clearer line between danger and safety. The new study from researchers at the Douglas Research Centre at McGill University and the University of Toronto-Scarborough, published in Nature Neuroscience, suggests the opposite.
In preclinical experiments, mice learned to associate one environment with danger and another with safety. Rather than creating more distinct brain maps for the two, the maps began to overlap.
"Our findings suggest fear may blur the boundary between danger and safety rather than sharpen it," said senior author Mark Brandon, Associate Professor in McGill's Department of Psychiatry and Researcher at the Douglas. "This could reshape how scientists think about anxiety and PTSD."
Augmented reality experiments
Researchers built an augmented reality arena that allowed them to "teleport" mice between an environment associated with danger and one with safety, using changes in sights and sounds. Miniature microscopes recorded the activity of hundreds of hippocampal neurons in real time.
"That gave us something conventional fear conditioning experiments cannot easily provide: a moment-by-moment view of the brain switching between competing maps of dangerous and safe memories," said Robert Rozeske, Assistant Professor in University of Toronto's Department of Psychology and lead author on the publication.
A survival mechanism in overdrive
The researchers said they believe this brain pattern may be part of a survival strategy, rather than a flaw.
In the study, overlapping memories appeared to help the brain recall past threats more quickly and respond faster to potential danger.
"The ventral hippocampus may be part of a system that prioritizes a rapid 'better safe than sorry' response," said Rozeske. "In other words, the brain may sacrifice precision for speed when danger matters."
People with anxiety and trauma-related disorders can experience intense fear when an otherwise safe situation triggers a certain memory.
"Our study suggests that's not necessarily a broken pattern-separation system, but possibly an overactive version of a normal survival mechanism gone into overdrive," said Brandon. "That reframing could guide therapeutic approaches to address an overly fast transition to 'danger mode'."
About the study
"Overlapping representations in the ventral hippocampus support rapid fear memory retrieval" by Robert Rozeske and Mark Brandon, was published in Nature Neuroscience. The study was supported by the Canadian Institutes of Health Research and the Natural Sciences and Engineering and Research Council of Canada.
