The brain has a remarkable ability to recognize danger, but new research suggests that this survival mechanism may also explain why people sometimes feel threatened in situations that are actually safe. The discovery offers a possible explanation for the intense fear responses associated with anxiety and post-traumatic stress disorder (PTSD).

For years, scientists believed that fear helps the brain distinguish more clearly between threatening and harmless experiences. However, a study led by researchers at the Douglas Research Centre at McGill University 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 involving mice, researchers trained the animals to associate one environment with danger and another with safety. Instead of developing increasingly separate neural representations of the two locations, the animals' brain activity began to show overlapping patterns.…