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Illustration by Karolis Strautniekas

Suppressing Our Most Fearful Memories

Research on mice reveals that specific memories can be weakened. Could this one day help treat the effects of trauma in humans?

Post-traumatic stress disorder is a malady of memory. Sufferers are often haunted by recurrent nightmares, distressing thoughts and flashbacks so intense in colour, smell and sound that they feel as if they are reliving the trauma.

But what if these unbearable memories could be selectively erased?

Sheena Josselyn, a U of T professor of physiology and psychology, who studies how the brain encodes, stores and uses information, is intrigued by the idea and has been investigating how to “silence” memories – make them temporarily inaccessible – in mice. She thinks it’s possible that a variation on this technique could one day help treat post-traumatic stress disorder in humans.

Studies with mice have found that although their brains contain billions of neurons, only a few are necessary to form a fearful memory.

Researchers working with mice began by teaching them to fear a tone: when it sounds, they feel a mild shock to their feet (not to hurt them, just to scare them). The next time the mice hear the tone, they crouch and freeze, signalling fear. The researchers discovered that they could trigger the memory of that fear even without presenting the tone. They did this by stimulating the small group of nerve cells holding that memory through a technology called optogenetics. Using the same technology, they found they could also suppress the fearful memory.

With optogenetics, scientists insert proteins into neurons to make them sensitive to light. Depending on the type of protein and colour of light used, these cells can then be activated or deactivated by shining pulses of the light directly into the brain: If the light activates the cells, the mice freeze as if they’ve just heard the tone. If the light deactivates the cells, the memory is suppressed.

While optogenetics is an invasive procedure and technologically not feasible with humans, Josselyn hopes that the general principles learned from these studies could eventually help scientists create new drugs for treating memory disorders such as post-traumatic stress disorder and Alzheimer’s.

But should you erase a bad memory, as Kate Winslet’s and Jim Carrey’s characters do in Eternal Sunshine of the Spotless Mind? Absolutely not, says Josselyn, who is also a senior scientist at Sick Kids. She emphasizes that this future technology should not be applied to everyday bad things, and that “these discoveries need to go hand in hand with a real thinking about the ethics involved in potentially manipulating memories in people.” Their use would only be considered in extreme cases, after all other treatment options have been explored, with patients suffering such debilitating pain that it interferes with their daily activities – in the case of a rape survivor or somebody coming home from combat, for example. The goal is not to sanitize life or make people super happy, but rather to “make everyone a functional person, capable of moments of joy.”

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