Something horrible happens. A child is lost. A bomb goes off. A car goes out of control.
And deep in the brain, in the lateral amygdala region, a scattered set of neurons come to life and begin to vibrate with fear.
Through an ingenious set of experiments, a group of researchers at Toronto's Hospital for Sick Children have not only located these terror-laden brain cells in mice, but erased them " along with the frightening memories they stored.
The study, which appears today in the journal Science, may hold out the hope that terrifying memories one day might be erased before they can fester into such conditions as post-traumatic stress disorder.
While the sights and sounds of a blast or crash would stay intact, the memory of the fear it caused could conceivably be removed, the researchers suggest.
"You wouldn't want to completely get rid of all aspects of a memory," says Dr. Michael Salter, head of the Neurosciences & Mental Health program at the hospital.
"To help people with these kinds of post-traumatic stress disorders ... you might just want to minimize the emotional association between the memory and the highly disruptive and negative emotions that people have in this context."
Eternal Sunshine of the Murine Mind
By Haley Stephenson
ScienceNOW Daily News
12 March 2009
Fearful memories are housed within a region of the brain called the lateral amygdala (LA). When something scary happens, LA neurons produce higher levels of a protein called CREB (cyclic adenosine monophosphate response element-binding protein). Previous studies have erased fear memories by blocking enzyme action in this brain region (ScienceNOW, 23 October 2008), however this study took another approach. Scientists suspected the CREB-making neurons were doing the actual "remembering" of fear, acting as the key to removing fearful memories, so they decided to destroy them and see what happened.
First, Sheena Josselyn, a neuroscientist at the University of Toronto in Canada and colleagues put mice through fear training. When a tone played, the mice received an electrical shock to their feet. After several rounds, the mice froze in fear at the sound. Then the team flipped a genetically engineered switch that killed CREB-making neurons. When the researchers played the tone again 2, 5, even 12 days later, the mice didn't freeze--they forgot their fear.
No other memories seemed to be affected, the team reports tomorrow in Science. The mice could store new memories, such as how to find cheese in a maze, and even relearn the foot-shock fear response, but only with training. Even when researchers killed a random assortment of LA neurons, fear-memory formation remained intact. "By destroying specific neurons, instead of deleting an entire brain region, our findings show for the first time which neurons store a [fearful] memory," says neuroscientist and co-author Jin-Hee Han of the University of Toronto.
"This is a remarkable validation ... of the contribution of the lateral amygdala to fear memory but also a major discovery about memory itself," says Joseph LeDoux, a neuroscientist at New York University. "This ... approach is a powerful way of selectively targeting neurons that are involved in storing fear memories," adds Sarina Rodrigues, a neuroscientist at the University of California, Berkeley.
Still, Han says that people may want to think twice about using the approach, assuming it ever becomes available for humans. Forgetting your fears sounds appealing, he says, but these unpleasant memories exist for a reason. "If we didn't remember that the last time we touched a hot stove we got burned, we would be more likely to do it again." On the other hand, he says, erasing detrimental memories like those post-traumatic distress disorder could be therapeutic.