In the sci-fi movie Men in Black, Tommy Lee Jones could erase a person's memory with a flash from his hand-held "Neuralizer." Neuroscientists aren't quite there yet. Their most successful attempts at memory wiping have involved genetic engineering or injecting drugs into the brains of rodents. Now researchers report a less invasive approach: a form of exposure therapy that makes rats forget a fearful association. The findings may eventually help improve treatments for people with phobias or post-traumatic stress disorder.
The new approach combines two methods previously shown to weaken memories of fear, says first author Marie Monfils, a neuroscientist at the University of Texas, Austin. One method, called extinction training, involves repeated exposures to a previously scary stimulus--a tone previously paired with a shock, for example--to dull the stimulus's impact. Extinction training is the basis of exposure therapy for people with debilitating phobias, but its effects aren't permanent.
Another method, called reconsolidation blockade, seems to have more lasting effects. It involves prompting a rat (or person) to recall a scary memory and then administering a drug to weaken it (ScienceNOW, 25 October 2004). Researchers think this works because a memory becomes briefly vulnerable to manipulation each time it's recalled. But many of the drugs that have proven effective in rodent studies are toxic, limiting their therapeutic potential in people. Monfils hypothesized that combining elements of both methods might yield lasting effects without the use of toxic drugs.
To test this idea, she and colleagues at New York University trained rats to expect a shock when they heard a certain tone. Rats remember this experience indefinitely and freeze anytime they hear the tone. The next day, the researchers played the tone once to reactivate the fearful memory and then, after a delay of 10 minutes or an hour, initiated extinction training, playing the tone 18 more times without delivering any shocks. Other rats got the extinction training without a delay period or after a much longer delay of 6 or 24 hours after the initial tone.
All the rats stopped freezing in response to the tone, at least for a while. The benefit lasted at least a month--the longest the researchers checked--for those that had been primed with the initial, memory-reactivating tone and received the training within an hour. But after a month's time, the rats who got extinction training with no delay or with a longer delay had regained their fear of the tone. The bottom line, as reported online today in Science: Extinction training is much more powerful when done within a narrow time window after reactivating the memory.
"It's really elegant" work and a "robust effect," says Karim Nader, a neuroscientist at McGill University in Montreal, Canada. Even so, he and Monfils both say many questions will have to be answered before the method can be applied to people. For one thing, Monfils says, there might be a risk of going too far and eliminating healthy fear responses. Weakening the grip of fear in people who won't go outside due to a fear of snakes is all well and good, she says by way of example, but "you don't want to turn them into snake chasers."