HOW THE BRAIN PURGES BAD MEMORIES?

 A brain circuit has been found that allows us to forget fear and anxiety!

 A new study confirms that a working connection between the two brain regions is necessary to do away with fear.

The brain is extraordinarily good at alerting us to threats. Loud noises, noxious smells, approaching predators: they all send electrical impulses buzzing down our sensory neurons, pinging our brain’s fear circuitry and, in some cases, causing us to fight or flee. The brain is also adept at knowing when an initially threatening or startling stimulus turns out to be harmless or resolved. But sometimes this system fails 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 PTSD and other anxiety disorders.

Like most emotions, fear is neurologically complicated. But previous work has consistently implicated two specific areas of the brain as contributing to and regulating fear responses. The amygdala, two small arcs of brain tissue deep beneath our temples, is involved in emotional reactions, and it flares with activity when we are scared. If a particular threat turns out to be harmless, a brain region behind the forehead called the prefrontal cortex steps in and the fright subsides. Our ability to extinguish painful memories is known to involve some sort of coordinated effort between the amygdala and the prefrontal cortex. The new study, led by Andrew Holmes at the National Institutes of Health, however, confirms that a working connection between the two brain regions is necessary to do away with fear.

Normally mice that repeatedly listen to a sound previously associated with a mild foot shock will learn that on its own the tone is harmless, and they will stop being afraid. Using optogenetic stimulation technology, or controlling specific neurons and animal behavior using light, the authors found that disrupting the amygdala–prefrontal cortex connection prevents mice from overcoming the negative association with the benign tone. In neurobiology speak, memory “extinction” fails to occur. They also found that the opposite is true—that stimulating the circuit results in increased extinction of fearful memories.

Until now investigators were unsure whether the amygdala–prefrontal cortex communication pathway could on its own control fear extinction; both structures interact with many other brain regions, and so isolating their effects of on behavior was a challenge. Optogenetics made the discovery possible, allowing the NIH group to precisely assess only the connection between the two brain regions in real time, providing a more accurate correlation between neuronal activity and behavior.

http://www.scientificamerican.com/article/how-the-brain-purges-bad-memories/