Just as dogs learn to recognize that the sound of a can opening means dinner, we too make emotional connections with sights, sounds, smells and other sensory experiences. Though sometimes pleasant, as with the association between cinnamon spice and a cozy autumn day, these connections can also be haunting.
For veterans of war, a loud bang can recall a memory of when a grenade killed their friend, or when a nearby plane exploded. For victims of sexual or physical assault, the mere touch of another human might trigger feelings of fear, helplessness and self-hatred.
These pathological associations are often signs of a deeper problem, such as post-traumatic stress disorder, which cripples its victims with inescapable memories of a traumatic past. Though therapy can help to ease the pain, a permanent cure has remained out of reach — until now.
A recent study in Japan has cracked open a new area of research about associative learning that may help release PTSD victims from the grip of their most feared memories. These researchers applied their knowledge about how memories are formed in order to artificially recreate a negative experience, which they could then manipulate.
In order to simulate a traumatic event, Kaoru Inokuchi and his colleagues allowed groups of mice to explore two different environments, one cylinder-shaped and one cube-shaped. Following their initial exploration, the mice from both groups were placed into cube-shaped enclosures and given a shock to the foot. After a couple of days, the mice were returned to their original environments, where they displayed strong behavioral preferences.
The mice that were returned to the cylinder-shaped enclosure following a shock in the cube-shaped enclosure were much less fearful of their environments than the mice that were returned to the shock-associated cube enclosure. These results were expected and simply confirm that the mice were successfully conditioned to fear a certain environment. The next step was the key.
Using a technique called optogenetics, researchers manipulated the delicate neural connections within the brains of the mice subjects by inducing production of certain light-sensitive proteins and then activating the neurons containing these proteins. By targeting specific neural groups, they were able to simultaneously reactivate memories of exploring the safe, cylindrical environment and memories of receiving the foot shock.
All of a sudden, the experimental rats became much more afraid of the cylinder-shaped environment. By artificially associating pain with an initially neutral experience, the researchers induced a fearful response to a safe situation. For the first time, neuroscience has allowed us to induce post-traumatic stress in mice by establishing novel neural connections.
Not only does this research support theories about how PTSD manifests in animals, it also suggests a possible cure. According to Inokuchi, they “will next attempt to artificially dissociate memories that are physiologically connected.” In other words, now that they have successfully fabricated traumatic memories, they want to reverse the procedure and separate these negative emotions from the experiences that they are tied to.
Inokuchi and his team know the implications of this finding. If they are successful in breaking these associations between emotions and physiological experiences, they will open the door to treatment of stress disorders. If they are successful, they may be able to cure veterans of their fear of loud noises and help rape victims rediscover physical intimacy; they may be able to change the way we relive our memories and, in doing so, change the way we experience the world.