Led by postdoctoral fellow Roger Redondo and graduate student Joshua Kim, a team from MIT has discovered a new technique for altering bad or fearful memories in the brains of mice by replacing them with new, pleasant memories.
To do so, they started off by establishing good and bad memories in the mice by prompting them to move into a certain area of their enclosure. Once they got there, they either received a treat, and so associated a good memory with that particular place, or they got a mild electric shock, which caused them to associate a bad memory with the place. The mice who got zapped did their best to avoid the area from that point on.
In order for the memories to be retained by each mouse, they needed to be encoded by two different areas of the brain. The memory of where the event happened - in this case that particular area in the enclosure - needs to be encoded by the brain’s hippocampus region, whereas the memory of whether the mouse experienced something good or something bad in this area - the emotional component of the memory - is encoded by the brain’s amygdala region.
To alter the memory of a male mouse who got a shock when he ventured into the designated enclosure area, the researchers tried reactivating the neurons in the hippocampus that encoded where this memory was first established. They did this while he was having a very pleasant experience in the present - spending time with TWO female mice. Once the bad memory of the enclosure area was switched to a very pleasant memory, the researchers found that the male mouse spent a lot more time visiting it, probably hoping that his female friends would someday return. While the process sounds simple, achieving it was no small feat, as Greg Miller at Wired explains:
"If the business about reactivating the memory sounds simple, it was anything but: It required a slew of clever genetic tricks to mark the relevant neurons, make them responsive to pulses of laser light, and then deliver the light to just the right spot with a surgically-implanted optical fibre."
The team was also able to reverse the technique, turning good memories to bad ones, which prompted the mice to avoid an area that they'd previously only received food rewards in, adds Miller.
Their results have been published today in the journal Nature.
While the technique sounds like the perfect treatment for people suffering from post traumatic stress disorder (PTSD), anxiety disorders, or depression, the research has so far only been performed on mice, and the process for manipulating the neurons is very invasive and unlikely to ever be performed on humans in its current form. "But this study and others like it are illuminating the neural mechanisms of memory in unprecedented detail,” says Miller, "and showing that it’s possible to activate, alter, or even create memories just by tweaking the right neurons.