A study of mice suggests that fear may blur the brain’s distinction between safe and dangerous experiences rather than sharpen it. Researchers say this survival mechanism could help animals retrieve threat-related memories quickly, but may also help explain why anxiety and post-traumatic stress disorder (PTSD) can cause intense fear in situations that are objectively safe.
Fear may make safe and dangerous experiences overlap
Scientists have long believed that fear helps the brain distinguish more clearly between threatening and benign experiences. However, research led by scientists at McGill University’s Douglas Research Center and the University of Toronto Scarborough challenges that assumption. The findings, published in Nature Neuroscience, suggest that fear may have the opposite effect.
In experiments with mice, the researchers trained the animals to associate one environment with danger and another with safety. Rather than becoming increasingly separate, the neural representations of the two locations began to overlap. In effect, the mice’s brain representations of safe and dangerous experiences became less distinct.
“Our findings suggest that fear may blur rather than clarify the line between danger and safety,” said lead author Mark Brandon, an associate professor of psychiatry at McGill and a research fellow at Douglas University. “This could change the way scientists think about anxiety and PTSD.”
Watching the brain process fear in real time
To examine how these changes occur, the research team developed an experimental system that combines augmented reality with real-time brain imaging.
The researchers placed mice in a specialized arena where the animals could be virtually “teleported” between environments associated with danger and safety. By changing the sights and sounds surrounding the mice, the scientists could make the animals experience different environments without physically moving them.
At the same time, miniature microscopes monitored hundreds of neurons in the hippocampus, a brain region essential for learning and memory. This allowed the researchers to track changes in brain activity as the mice switched between experiences associated with safety and threat.
“This gives us something that traditional fear conditioning experiments cannot easily provide: moment-by-moment observations of the brain switching between competing maps of dangerous and safe memories,” said Robert Rozeske, assistant professor in the Department of Psychology at the University of Toronto and lead author of the publication.
Why might the brain blur safety and danger?
Although overlapping memories of safe and dangerous experiences may appear problematic, the researchers believe the process could serve an evolutionary purpose.
The experiment suggested that overlapping memory representations may allow the brain to retrieve information about previous threats more quickly. This could help an animal recognize possible danger and respond before it is too late.
The strategy involves a trade-off: by prioritizing speed over accuracy, the brain may react to a potential threat even when the current situation is harmless. From a survival perspective, an unnecessary response may be less costly than failing to respond to a genuine danger.
“The ventral hippocampus may be part of a system that prioritizes rapid ‘better safe than sorry’ responses,” Rozeske said. “In other words, when the danger is significant, the brain may sacrifice accuracy for speed.”
The ventral hippocampus is involved in processing emotionally significant experiences, including fear and anxiety. The findings suggest that overlapping memory representations in this region may help the brain rapidly switch into a defensive state.
Potential implications for PTSD and anxiety
The findings may offer insight into why people with anxiety and trauma-related disorders can experience strong fear responses even when there is no immediate threat.
For people with PTSD, otherwise harmless sights, sounds or environments can trigger memories associated with traumatic experiences. The resulting fear may be immediate and overwhelming, even when the person is objectively safe.
The researchers suggest that such reactions could occur when a normally useful survival mechanism becomes overly sensitive. Rather than simply reflecting a system that can no longer distinguish between different memories, these responses may involve brain mechanisms that rapidly shift from perceiving safety to anticipating danger.
“Our study suggests that it is not necessarily a failure of the pattern separation system, but perhaps an overactive version of a normal survival mechanism,” Brandon said. “That reconfiguration could lead to therapeutic approaches to address the overly rapid transition to ‘danger mode.’”
Because the experiments were conducted in mice, further research is needed to determine whether the same mechanisms contribute to anxiety and PTSD in humans. The findings nevertheless offer a new way to understand how fear affects memory and why brain systems designed to protect against danger can sometimes generate fear in the absence of an immediate threat.
Research details and funding
The study, by Robert Rozeske and Mark Brandon, is titled “Overlapping representations in the ventral hippocampus supports rapid fear memory retrieval” and was published in Nature Neuroscience.
The research was supported by the Canadian Institutes of Health Research and the Natural Sciences and Engineering Research Council of Canada.
Source: www.sciencedaily.com