A particular neuronal circuit between the two regions of the brain: the amygdala and the prefrontal cortex, is found to be responsible for alerting us when a particular threat has been eliminated.
The many neurological circuits between these two areas of the brain are the reason we experience and are able to regulate our fear response.

How Our Brain Purges Bad Memories

<ul>
<li>We all have a fear memory, where we feel scared of remembering a tragic event from the past.</li>
<li>The brain has the ability to make new neuronal connections and remove older ones.</li>
<li>The new findings on how the brain eliminates bad memories open up opportunities to treat Post Traumatic Stress Disorder(PTSD) and other anxiety-related issues.</li>
</ul>

Our Fear Memory

<img src="https://static.scientificamerican.com/sciam/cache/file/928A5583-B3E8-4787-A87766E04460D75E.jpg" alt="SA Mind" width="86" height="40">A brain circuit has been isolated that allows us to forget fear, suggesting a treatment target for anxiety disordersThe brain is very good at alerting us to threats—and it is also adept at letting us know when a threat no longer exists. Sometimes this system fails, however, and unpleasant associations stick around—a malfunction thought to be at the root of post-traumatic stress disorder (PTSD). New research has identified a neuronal circuit responsible for the brain's ability to purge bad memories, findings that could have implications for treating a broad range of anxiety disorders, including PTSD.Previous work has consistently implicated two areas of the brain as contributing to and regulating fear responses. The amygdala is involved in emotional reactions, and it flares with activity when we are scared. The prefrontal cortex steps in to calm us down when a particular threat turns out to be harmless. A large body of work implicates the two areas in fear memory, but because of their connections to many other parts of the brain, it was unknown whether their joint effort was truly at the root of overcoming fear. The new study, led by Andrew Holmes of the National Institute on Alcohol Abuse and Alcoholism, confirms that a working connection between the two brain regions is necessary to do away with fearful associations.The researchers worked with mice trained to fear a sound paired with a foot shock. Typically if such mice are later exposed to the repeated sound without a foot shock, they will learn that the noise is harmless and will stop being afraid. In the new study, the researchers disrupted the mice's amygdala–prefrontal cortex connection using optogenetics, which controls specific neurons with fiber-optic lights. The authors found that disrupting this key connection prevented the mice from overcoming the negative association with the benign tone—they continued to fear the sound long after the foot shocks disappeared. They also discovered that the opposite was true: stimulating the circuit resulted in faster extinction of fearful memories.<figcaption>Advertisement</figcaption>As Holmes explains, the amygdala and prefrontal cortex are two major hubs in a complex communications network. In the case of impaired fear extinction such as PTSD, however, it appears that just the one connection between the two regions is faulty, not the hubs themselves. As such, the efforts of previous experiments to treat PTSD by altering activity in one of these major brain areas have probably been overkill.The new finding suggests that researchers should explore medications that act on this specific fear circuit. Holmes believes that healthy fear extinction relies on “neural plasticity,” the brain's ability to make new neuronal connections, which is in part influenced by the brain's native cannabinoids, compounds that regulate neurotransmitters. Drugs that alter the cannabinoid system, such as THC, the active component in marijuana, could temporarily make the fear circuit more plastic—perhaps allowing clinical techniques, such as exposure therapy, to better alleviate anxiety.This article was originally published with the title "How the Brain Purges Bad Memories" in SA Mind 26, 6, 11 (November 2015)doi:10.1038/scientificamericanmind1115-11<h3>ABOUT THE AUTHOR(S)</h3><img src="https://static.scientificamerican.com/sciam/cache/file/C68AC95A-DB7F-4D0E-A96C432C0DEC6C5F_small.png" alt="author-avatar" width="56" height="56">Bret Stetka is a writer based in New York City and editorial director of Medscape Neurology (a subsidiary of WebMD). His work has appeared in Wired, NPR and the Atlantic. He graduated from the University of Virginia School of Medicine in 2005.Credit: Nick Higgins

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A brain circuit has been isolated that allows us to forget fear, suggesting a treatment target for anxiety disorders

The brain is very good at alerting us to threats—and it is also adept at letting us know when a threat no longer exists. Sometimes this system fails, however, and unpleasant associations stick around—a malfunction thought to be at the root of post-traumatic stress disorder (PTSD). New research has identified a neuronal circuit responsible for the brain's ability to purge bad memories, findings that could have implications for treating a broad range of anxiety disorders, including PTSD.

Previous work has consistently implicated two areas of the brain as contributing to and regulating fear responses. The amygdala is involved in emotional reactions, and it flares with activity when we are scared. The prefrontal cortex steps in to calm us down when a particular threat turns out to be harmless. A large body of work implicates the two areas in fear memory, but because of their connections to many other parts of the brain, it was unknown whether their joint effort was truly at the root of overcoming fear. The new study, led by Andrew Holmes of the National Institute on Alcohol Abuse and Alcoholism, confirms that a working connection between the two brain regions is necessary to do away with fearful associations.

The researchers worked with mice trained to fear a sound paired with a foot shock. Typically if such mice are later exposed to the repeated sound without a foot shock, they will learn that the noise is harmless and will stop being afraid. In the new study, the researchers disrupted the mice's amygdala–prefrontal cortex connection using optogenetics, which controls specific neurons with fiber-optic lights. The authors found that disrupting this key connection prevented the mice from overcoming the negative association with the benign tone—they continued to fear the sound long after the foot shocks disappeared. They also discovered that the opposite was true: stimulating the circuit resulted in faster extinction of fearful memories.

As Holmes explains, the amygdala and prefrontal cortex are two major hubs in a complex communications network. In the case of impaired fear extinction such as PTSD, however, it appears that just the one connection between the two regions is faulty, not the hubs themselves. As such, the efforts of previous experiments to treat PTSD by altering activity in one of these major brain areas have probably been overkill.

The new finding suggests that researchers should explore medications that act on this specific fear circuit. Holmes believes that healthy fear extinction relies on “neural plasticity,” the brain's ability to make new neuronal connections, which is in part influenced by the brain's native cannabinoids, compounds that regulate neurotransmitters. Drugs that alter the cannabinoid system, such as THC, the active component in marijuana, could temporarily make the fear circuit more plastic—perhaps allowing clinical techniques, such as exposure therapy, to better alleviate anxiety.

This article was originally published with the title "How the Brain Purges Bad Memories" in SA Mind 26, 6, 11 (November 2015)

doi:10.1038/scientificamericanmind1115-11

Bret Stetka is a writer based in New York City and editorial director of Medscape Neurology (a subsidiary of WebMD). His work has appeared in Wired , NPR and the Atlantic . He graduated from the University of Virginia School of Medicine in 2005.

How the Brain Purges Bad Memories

How Our Brain Purges Bad Memories

Our Fear Memory

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