In a scientific experiment that brings to mind the memory-erasing escapade in the 2004 film "Eternal Sunshine of the Spotless Mind," scientists have blocked fearful recollections in human participants, sans drugs. The results challenge the view that our long-term memories are fixed and resistant to change.

This isn't the first time science has endeavored to understand and vanquish our fears. But it's the first time using a behavioral technique has been proven to work in humans, as opposed to a pharmacological one. A similar study was carried out in rats and reported earlier this year.

"This is the first study without drugs showing what we think is the permanent alteration of the memory," Elizabeth Phelps, an NYU psychology professor, told LiveScience. Phelps and her colleagues detail their findings this week in the journal Nature.

The findings also could have implications for treating phobias in a more permanent way, say the researchers. The current therapy of choice involves exposing patients to the feared object, though in a safe environment. This so-called extinction method works, but the fear can come back when the person is under stress.

Window of opportunity

Phelps and NYU colleague Joseph LeDoux, and their colleagues, based their study on an emerging view of long-term memory. Traditionally, scientists have thought we learn something, and then that information is sealed into our long-term memory.

Now, scientists are finding our memories get consolidated over and over again each time we retrieve a certain bit of information. Let's say we see a snake: At that moment our brains pull out past information we've stored on snakes, such as a close encounter with one. By revisiting the snake memory a portal of sorts opens, and that memory is open to manipulation.

From past studies, scientists think that the window of opportunity opens up between three and 10 minutes after spotting the snake, or its equivalent. And it stays open for at least an hour, but no longer than six hours, Phelps said.

Changing memories

The research team "seized the moment" by changing the fearful information before the memory got reconsolidated or sealed up again.

In the first of two experiments on humans, Phelps and her colleagues had participants view colored boxes on a computer screen, one of which was paired with a mild electric shock. This process conditioned participants to react fearfully to a blue square. They tested the participants' skin conductance, a measure of arousal, to confirm the conditioning worked.

The next day the researchers showed participants the blue square, a reminder of the object, which was intended to reactivate their memory and initiate the reconsolidation process (memory gets brought out of long-term storage and lingers in this unstable place).

This time, however, the blue square wasn't paired with a shock, a way to teach participants that the object was now "safe." Since the researchers had a rough estimate of when the reconsolidation window opened and closed, they varied the timing of this safe information.

One group saw the blue square with no shock, and 10 minutes later they viewed the blue square again with no electrical shock. Here, the thinking was that the first observation would trigger the beginning of the reconsolidation process. After 10 minutes, that window should be open and so this new information would be written over the fearful message before being sealed into long-term memory again.

Participants in the second group saw the blue square with no shock, but this wasn't followed up 10 minutes later. The scientists were using basic extinction training to create a new now-safe memory of the blue square in addition to the older fearful memory.

The third group saw the blue square with no shock and then after six hours saw it again with no shock, with the thinking that the window of vulnerability had already closed by then.

Testing reconsolidation

All participants left that day essentially free of their fear response to a blue square. The next day, tests showed the fear had returned in the latter two groups, but not for the participants whose blue-square fears were rewritten during reconsolidation. To see if stress would cause the fearful memory to come back, in one part of this experiment, participants received a mild shock before viewing the blue square. The fear didn't come back for the reconsolidated group.

A test of some of the participants a year later showed the reconsolidation held up, with individuals showing no fear of the blue square. The fear had returned, however, for the others who only underwent the extinction therapy.

Another experiment showed this reconsolidation method is selective and targets a particular feared object without disrupting other memories.