Implanting and Erasing Memories: Life-Changing, or Taking Science Too Far?

Most people who have experienced emotional trauma — such as war veterans, sexual assault survivors, or those whose lives have been threatened — are able to heal emotionally within weeks and months of the distressing event.

Breakthrough: Decoding the Brain (National Geographic Channel)

But for some individuals, the anxiety associated with the event never quite goes away with the passage of time. Recurring and intrusive flashbacks, nightmares, feelings of numbness or hopelessness, and avoidance of places, people, and activities that remind you of the traumatic event are common symptoms. At some point in their lives, around 7.8% of Americans will experience post-traumatic stress disorder, or PTSD.

Cognitive behavioral therapy, exposure therapy, and antidepressant medication are the current treatments for PTSD, but they're not successful in everybody.

But what if doctors and researchers could attack PTSD at the source: actually implanting or erasing specific memories in a person's brain?

It may sound like science fiction — not unlike Lord Voldemort luring Harry Potter to the Ministry of Magic by creating false images in Harry's mind, or the entire premise of the movie Inception — but science is actually getting close. In mice, neuroscientists have found ways to not only identify the location of certain memories, but to actually manipulate those memories.

But can we do this in humans — in patients with PTSD? And perhaps the bigger question: should we?

Steve Ramirez. Breakthrough: Decoding the Brain

(National Geographic Channel)

In a 2012 paper published in Nature, Xu Liu* and PhD student Steve Ramirez at MIT employed a relatively new technique called optogenetics to identify small clusters of neurons, called "engrams," that are activated during learning. It had long been hypothesized that specific memories — like losing your first tooth or riding your first roller coaster — are stored in engrams.

Typically, when a mouse is startled, it freezes in place for a second or two before moving around again. Scientists perform these "fear conditioning" experiments in mice by putting them in boxes that provide a brief shock to their feet.

For their Nature study, Liu and Ramirez were able to identify a population of neurons in the dentate gyrus region of the hippocampus that were activated when mice first learned to fear the footshock. They labelled these neurons with channelrhodopsin-2, a special type of protein originally found in algae that is activated by light.

Interestingly, when Liu and Ramirez took mice out of the scary environment — the box where the footshocks occurred — and simply shone light directly onto the engrams, the mice would still freeze. This suggests that simple activation of these engrams, no matter the context, is enough for the animal to recall the memory. This may explain how a certain smells or hearing particular songs are able to stir up old memories for us.

Optogenetics; a light directly applied to the brain to activate specific neurons

labelled with a light-sensitive protein. U Chicago 

In July 2013, the group published another paper in Science where they did something a little different. This time, they let a mouse explore a plain old box for a few minutes, allowing them to encode an engram for this new location. The next day, in a different box, Liu and Ramirez applied a footshock; at the same time as the shock, they activated the engram for the "safe" box with light. On the third day, the mouse was placed back in the original, boring box.

But fascinatingly, once put in the old box, the mouse froze in fear, thinking they'd receive a footshock. In other words, a "false memory," created on the second day, caused the mouse to fear something that had always been harmless.

There's still a long way to go before these techniques can be applied to any type of therapy in humans, though.

In the lab, optogenetics is still a rather invasive technique. Also, fear is very fast-acting, evolutionarily-ingrainted instinct — how can we target much more elaborate, complex memories, such as a particularly horrifying experience for a combat veteran combining sight, smell, sound, and other sensations? And even if we are able to develop the technology to precisely target and manipulate these memories, should we, from an ethical perspective? Let me know your thoughts in the comments.

Check out the new series Breakthrough:  Decoding the Brain premiering this Sunday, November 15 at 9 pm ET on National Geographic Channel. The show will examine the work by Liu and Ramirez, as well as many more fascinating neuroscience-y topics.

*Sadly, Dr. Liu passed away earlier this year at the young age of 37.