How the Brain Learns That Fear Is No Longer Needed

Fear can link cues to danger and later trigger fear when reminders appear without threat, as in posttraumatic stress disorder. Fear extinction reduces fear after repeated safe exposure to a once-threatening context, a principle used in exposure-based therapies. A study in mice examined how brain immune cells, microglia, influence this learning process. Microglia scan brain tissue, respond to inflammation and injury, communicate with neurons, and help shape connections between brain cells. In the dentate gyrus of the hippocampus, microglia monitored neuronal activity during extinction. Their activity supported the rewriting of fear responses, helping the brain learn that the cue no longer predicts harm. These mechanisms may help explain persistent fear memories in PTSD.

"Fear protects us by teaching the brain what to avoid. After a frightening experience, a sound, place, smell, or image can become linked with danger. That link can save a person from future harm. But when the danger has passed, the same system can become painful. In posttraumatic stress disorder ( PTSD), reminders of trauma can trigger fear even when a person is physically safe."

"Microglia act like caretakers of the nervous system. They scan nearby tissue, respond to inflammation and injury, communicate with neurons, and help shape the connections between brain cells. Although scientists once viewed them mainly as defenders against disease, newer work shows that microglia also help regulate learning, memory, and behavior."

"The study examined fear extinction, the process through which fear decreases after repeated safe exposure to a once-threatening situation. In therapy for anxiety and trauma, this same principle appears in exposure-based treatments. A person gradually encounters reminders of fear in a safe setting, and the brain learns that those reminders no longer predict harm."

"In this study, mice learned to associate a specific context with a mild shock. Later, the mice returned to that context without receiving a shock. Over time, their freezing behavior decreased, showing that they had learned the place no longer carried danger. Inside the brain, the team focused on the dentate gyrus, a part of the hippocampus involved in memory."

Read at Psychology Today