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Why Painful Memories Linger

Posted by Brian Bigelow on

Why Painful Memories Linger

As we’ve seen in the media lately, memories of traumatic events can be hard to shake, and now it seems scientists are now beginning to understand why. 

For the first time, research performed on laboratory animals has revealed the brain mechanism that translates painful and unpleasant experiences into long-lasting memories. 

These recent findings also support a 65-year-old theory referred to as Hebbian plasticity. This theory essentially states that in the face of trauma, whether it be emotional, physical or a combination of both more neurons in the brain fire simultaneously to create stronger connections to each other than under normal circumstances. 

The Stronger The Connections
The Stronger The Memory

These new findings are not only important for the advancement of brain research and a better understanding as to how Hebbian plasticity works, but they may also help scientists create new treatments for those suffering from PTSD or simply those suffering with painful memories. 

The study completed by researchers at New York University in collaboration with Japan’s RIKEN Brain Science Institute was presented at the National Academy of Sciences. 

The concept of Hebbian plasticity is such that when a region of the brain called amygdala allows sensory stimuli to become associated with either positive or negative outcomes, in turn this produces emotional memories. 

This goes along with the old saying in the field of neuroscience that “neurons that fire together, wire together.” In turn this creates a much stronger neural connection. 

One of the study’s lead authors, Joshua Johansen says, "these processes for triggering aversive memory storage may represent a general mechanism controlling memory formation that is shared across other learning systems in the brain.”

In previous studies, research teams led by Joseph LeDoux, director of NYU's Emotional Brain Institute, found evidence of Hebbian plasticity using brain cells that had been removed from animals. 

However, this new study marks the first time the process was documented and observed in a real brain with real memories. 

During the study, researchers studied rats who were conditioned to associate an auditory stimulus with a mild electric shock to their feet. Using a new technique called optogenetics, the researchers could track and control the path of electrical impulses in the amygdalas of each rat.

By blocking or weakening the signals among neurons, the researchers discovered that the memory linked to the sound with shock failed to form. Leading the researchers to conclude that these findings support the idea of Hebbian plasticity. 

However, they also found that Hebbian plasticity cannot completely explain the entire process. For example, using lasers the scientists directly stimulated the neurons in the amygdala without delivering the electric shock. When they did, they were unsuccessful at forming the negative memory. This was in spite of the strong stimulation. 

Surprisingly, learning did occur when the receptors for noradrenaline, a brain molecule that is important for attention, were activated at the same time as the laser was on.

What this appeared to confirm was that while Hebbian mechanisms are important, they are not enough to form a memory. Instead, the presence of neuromodulators like noradrenaline are also required. And the fact that these types of chemicals are abundant in stressful situations may be why these types of memories are so strong. 

"Our results not only show that we are able to artificially manipulate memory, but also that this manipulation is correlated with long-lasting changes in the brain," said Lorenzo Diaz-Mataix, a postdoctoral fellow at NYU and also lead author on the report. "Basic findings like this one will potentially help to understand and treat many psychiatric conditions that share aberrant memory processing," he continued.

Remembering scary events, such as a past assault or an emotionally scarring event, clearly has advantages from an evolutionary perspective. 

But these memories can also be extremely painful. For people who have these types of memories, the new findings offer hope.

"Because of the importance of forgetting aversive memories for PTSD, many labs, including my own, are trying to understand how these types of memories can be forgotten," Johansen said. "One possibility is that instead of tapping into 'forgetting' mechanisms, we try to reverse what happened during memory formation. Our findings in this paper are important in this regard and may enable novel approaches to enhance the forgetting or reversal of learning of aversive experiences."

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