We all have memories that we would rather forget. Yet too often they surface in our consciousness. That gaffe at work or during an interview? A faceplant after sliding on the ice on the first date? An accidental response to the whole family? (Embarrassing).
For the most part, a quick shot of embarrassment, anger or fear is all we feel and it quickly dissipates. But for people with post-traumatic disorders (PTSD) or depression, unwanted memories of their trauma can seriously derail their life.
So how is it that these memories only sometimes invade unsuspecting minds?
A new study in the Journal of Neuroscience has some answers. By scanning the brains of 24 people who actively suppress a particular memory, the team discovered a neural circuit that detects, inhibits and ultimately erodes intrusive memories.
A trio of brain structures make up this warning system. At the heart is the dACC (for “anterior dorsal cingulate cortex”), a scarf-like structure that wraps around the deepest brain regions near the forehead. It acts like an intelligence agency: it checks the neural circuits for intrusive memories and, once discovered, alerts the “executive” region of the brain. The executive then sends an interrupt signal to the brain’s memory center, the hippocampus. Like an emergency stop button, this prevents the hippocampus from recovering memory.
The whole process takes place below our consciousness, suppressing unwanted memories so that they never emerge into awareness.
But what if memories break into our thoughts? Here, the dACC has another task. When proactive surveillance fails, the brain region raises its alert signal to the executive think DEFCON1, probing it to further dampen activity in the hippocampus.
“Preventing unwanted memories is a human adaptability,” wrote the authors, led by Dr Michael C. Anderson of the University of Cambridge and Dr Xu Lei of Southwest University in Chongqing, China.
Meet the Dynamic Trio
The three regions of the brain are familiar to memory researchers. Each functions as a government agency in a spy novel, with multiple tasks and extensive intercommunication. Recovering, or dampening, a memory is similar to an intelligence operation.
The hippocampus is the “operative” with the boots on the ground to fish a memory from the neural networks. Buried deep within the brain, the structure encodes, temporarily stores and retrieves the memories that capture the stories of our lives: when, where and what.
Another player is the brain’s “command center”, the prefrontal cortex (PFC). Across vast neural networks to various regions of the brain, including the hippocampus, this “executive” controls the operations of the brain and is at the center of cognitive control. If the actions of the hippocampus are out of control, one part, the rDLPFC, sends out an electrical “cut off” signal and dampens the activity of the hippocampus.
But what gives rDLPFC intelligence?
Meet the enigmatic dACC, a C-shaped structure that activates through multiple brain functions. Previous studies using computational modeling suggest that it closely monitors ongoing neural processes. As an intelligence agent looking for signs of potential danger, it acquires information “indicating the need to intensify cognitive control,” the authors explained. The dACC then broadcasts the question to the command center, urging the executive to implement control, at least in non-memory contexts.
The new study asked: Does the dACC also spy on offensive memories?
Brain Scan Tag Team
How do you discover a neural hub for memory control?
The trick is to track brain activity with multiple types of scans, each capturing unique aspects of brain processing. One is EEG (electroencephalogram), which uses electrodes placed on the scalp to detect brain waves, the cumulative electrical activity of neurons. Like a wide-angle surveillance camera, the EEG excels at acquiring electrical schematics in relatively large areas of the brain in real time, but sacrifices resolution.
fMRI is the perfect cop friend. Compared to the EEG it is slow to react, but offers a much higher resolution. Simultaneous use of the two methods offers the best of both worlds, allowing the team to get a glimpse of changes in neural activity like in an IMAX movie.
Once they get the data, they can match precise timestamps of the activity changes, which they get from the EEG, to their precise location on the fMRI scans.
For the study, they recruited 24 volunteers, equally divided between males and females, with no history of neurological problems or mental health problems. The volunteers then learned 68 word pairs. For example, “gate” combined with “train”; “meadow” with “beef”. One word in each pair was used as a cue; when asked, participants would do their best to remember the associated word.
Subsequently, the volunteers entered the fMRI scanner. For some tests, after receiving the signal, for example “gate”, they were asked to remember the associated word, “train”. In other tests, they had to actively not think about the answer. Aptly named, the test is called the Think / No-Think, or TNT, paradigm.
During the activity, the team monitored and analyzed the interactions between the trio of brain regions using EEG and fMRI. Finding patterns in neural network activity, they then focused on two specific brain wave signatures (theta power and N2 amplitude) in the dACC, which is often associated with cognitive control.
A two-stage dance
The dACC activity came in two bursts.
The first came on at about 400 milliseconds, or so in the blink of an eye, and generally before a memory entered consciousness. The dACC relayed the information to the rDLPFC commander, who in turn ordered the hippocampus to reduce its activity and stop recovering its memory.
We can see this with a decrease in theta brain waves in the hippocampus, which is needed to recover memories, the authors explained.
Mission completed, the entire neural circuit dulled during the remainder of the test, suggesting that the neurons were happy to relax with a job well done, there was no need to continue working to inhibit an already suppressed memory.
Conversely, if the dACC signal did not activate in time, for example if the person remembered the associated word even when he tried not to, the region went into a state of maximum alert. This “reactive alarm” increases activity in the commander, rDLPFC. The region then further suffocates the theta waves in the hippocampus in an attempt to stop the intrusive thoughts. People who excelled at actively suppressing the associated word, for example, had a much stronger flow of information from the rDLPFC command center to the hippocampus for words they had forgotten, compared to those they remembered despite trying to crush memory.
Overall, the brain has a two-step, proactive and reactive internal mechanism that helps suppress intrusive thoughts, the authors explained. They both have the dACC as an intelligence agent. When you encounter a reminder, for example, the toy of a beloved pet who recently passed away, the dACC detects the neural network signals generated by the signal. In two waves, therefore, it prevents recovery or pushes the painful memory out of awareness.
For now, the study is limited to visual cues. Still others will need to see if other powerful signals, such as hearing the voice or smelling the scent of a deceased loved one, also activate the dACC. But for now, we’ve found a guardian angel built into the brain that can “clear the mind of unwanted thoughts and speed the disappearance of memories we’d rather not have,” the authors wrote.
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