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How the Brain Hides Memories

Study Reaches Repressed Memories for New Mental Health Therapies

Scientists believe suppressed memories are created by a process called state-dependent learning. When the brain creates memories in a certain mood or state, particularly of stress or trauma, those memories become inaccessible in a normal state of consciousness. Suppressed memories can then best be retrieved when the brain is back in that state. For the first time, a new study from Northwestern Medicine has discovered how the brain locks those memories away.

What is a Hidden Memory?

The brain will sometimes hide particularly stressful, traumatic or fear-related memories. This can be protective in the short term, when the emotional pain of recalling the event is still profound. However, in the long term, suppressed memories can create serious emotional health concerns such as anxiety, depression, post-traumatic stress disorder and dissociative disorders.

Because these memories are often formed through state-dependent learning, resolving the ensuing conditions, even with the help of a therapist, can be difficult. When patients are unable to remember their experiences, therapists can struggle to help them identify and treat the root cause of their symptoms.

The new study suggests the best way to access hidden memories is to return the brain to the same state of consciousness it was in when the memory was formed. Jelena Radulovic, MD, PhD, Dunbar Professor in Bipolar Disease in Psychiatry and Behavioral Sciences and Pharmacology at Northwestern University Feinberg School of Medicine, finds the radio to be a useful analogy.

“The brain functions in different states, much like a radio operates at AM and FM frequency bands,” Dr. Radulovic said. “It’s as if the brain is normally tuned to FM stations to access memories, but needs to be tuned to AM stations to access subconscious memories.”

Finding the Mechanism in Mice

Northwestern Medicine scientists, led by Dr. Radulovic, used mice in their study of repressed memories. The team gave the mice gaboxadol, a drug that stimulates receptors associated with hidden memories, to change their brain state.

The mice were then put in a box and given a brief, mild shock. The next day, when the mice were returned to the same box, they moved freely and without fear — indicating to the scientists that they did not recall the earlier shock. However, when the scientists gave the mice the drug again, returning them to the previous mental state, the mice froze in fear when in the box. The scientists concluded that only when the mice were returned to the brain state in which they experienced stress did they remember that experience.

Multiple Pathways Hide Memories

Chemical receptors in the brain control emotional tides, excitement and calm. Normally, the system is balanced, but certain receptors, called extra-synaptic GABA receptors, are independent agents: they work outside the system to adjust brain waves and mental states according to the levels of internal chemicals.

However, the study revealed that these receptors are also involved when the brain encodes and hides memories of a fear-inducing event. If a traumatic event occurs when these receptors are activated in the brain, the memory cannot be accessed unless those same receptors are activated once again.

Specifically, the scientists discovered that when the drug activated the receptors in mice, it changed the way the stressful event was encoded. In the drug-induced state, the brain used completely different pathways to store the memory. On a genetic and molecular level, entirely different systems exist to store traumatic memories and normal memories separately.

New Insight for Mental Health Therapies

The scientists believe that this different system may be a protective mechanism in the brain for when an experience is overwhelmingly stressful. Memories are usually stored in networks that make them easily accessible to consciously remember. However, in the drug-induced brain state, the drug had rerouted the processing of stress-related memories so that those memories couldn’t be consciously accessed.

The findings suggest that when faced with traumatic stress, the brain can activate a different system to form and suppress memories. Moreover, the study shows that there are multiple pathways of storing memories. While the Northwestern Medicine scientists only identified the first of these pathways, this research could some day lead to new treatments for patients for whom conscious access to memories is integral to recovery.