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Note II

"I'm too afraid, even to fall asleep." 

Can Fear Memories Be Removed? 

Distinguished Professor Bong-Kiun Kaang and His Team Investigates Mechanism of the Extinction of Fear Memories



 
Our experience is stored in our brains and later recalled in the form of memory. While some memories are easily forgotten, fearful memories are difficult to erase and leave long-lasting scars. Life-threatening fear memories sometimes lead to "Post-traumatic Stress Disorder" (PTSD) symptoms. Could there be a way to remove our fear memories? 



In 1949, Canadian neuropsychologist Donald Hebb presented a hypothesis on the "location of memories" and posited that memories are stored in the "synapse" of the brain. The synapse is a connection point between neurons that functions as a highway tollbooth and is used by neurons to exchange electric signals. Neuroscientists have been working for a long time to confirm Hebb's hypothesis. However, since there are tens of billions of neurons in the human brain, and each neuron has thousands of synapses, it was difficult to prove the hypothesis by checking each of the hundreds of trillions of synapses. 

Meanwhile, through decades of research, neuroscientists have also discovered that the hippocampus, located in the left and right temporal lobe, is responsible for storing memories within the brain. Scientists, however, were unable to confirm where and how memories are stored within the hippocampus. This is as if they were aware that memories are saved in USB flash drives but did not have any clue whether they are saved as a text file or as digital signals. 

However, in 2018, Professor Bong-Kiun Kaang's team at the School of Biological Sciences confirmed the existence of the synapse that stores memories through experiments for the first time in the world. This took place after 70 years since Donald Hebb presented his hypothesis. Professor Kaang's team was able to find the "memory storing synapse" thanks to the development of dual-eGRASP technology, which could identify the types of synapses in a single neuron. This technology uses fluorescent substances to distinguish the function of synapses into two colors.

For example, mice that have fluorescent substances inserted into the hippocampus neurons were subject to electric shocks for weak or strong fear conditioning, and if the neurons were activated when the fear memory was stored, the experiment was designed to have the synapses turn into bright yellow. As a result, whenever a fear memory was created, many synapses at the end of the dendrites attached to the engram cells turned yellow. The dendrites cells of the engram cells also became bigger, and the number of synapses increased. Professor Kaang's team succeeded in finding the "memory storing synapse," that is, the location where memories are stored in the brain. The cyan color was used to identify the synapses that are not involved in keeping fear memories. Professor Kaang said that the findings of this research "were the proudest accomplishment in my thirty years of research."

Last August, Professor Kaang's team even succeeded in investigating how fear memories disappear in the amygdala, which deals with fear in the brain. Professor Kaang's team played a sound with a specific frequency to the mice for 30 seconds and sent weak electric shocks to the soles of the mice for about two seconds. This was repeated three times to form a fear memory. As a result, the mice froze with fear every time the sound with a specific frequency was played, even without the electric shocks. After a few days, the team repeatedly played the same sound they used to form the fear memory, but the mice no longer displayed any fear due to the sound. The fear memory had disappeared.

In this process, Professor Kaang's team observed the systematic changes in the memory storing synapses in the amygdala using the dual-eGRASP technology they developed. The team found that when fear memory was created, the memory storing synapses were enlarged and that they later shrunk when the fear memory was weakened. After the same fear memory was learned, the synapses became bigger and regained their original sizes. The size of the synapses changed every time a fear memory was created and eliminated. 

There are two neurological theories concerning the extinction of fear memories. The first theory posits that existing fear memories are restrained as they are overwritten with new memories. The other theory is that fear memories can be weakened and deleted. The predominant view to this day is that bad memories are temporarily weakened by new memories. However, Professor Kaang and his team's discovery of evidence that bad memories could be deleted as the memory storing synapses weaken seems to provide more evidence for the latter theory. The findings of Professor Kaang's team were published in the international academic journal Neuron on August 6. 

Professor Kaang's research, which has presented the possibility of removing the seemingly permanent memories of fear and terror, would help treat disorders such as PTSD. Professor Bong-Kiun Kaang plans to study whether the memory range can be selectively controlled by physically adjusting the size of the "memory storing synapse" and what changes occur in other states of memory when the synapses are artificially adjusted. It is with the hope that we await Professor Kaang's research to be applied for improving the treatments for psychological disorders.