Can Fear Be Forgotten?

Eliminating Memories

LeDoux populates his lab with kindred thinkers, resourceful polymaths who can draw from multiple disciplines to arrive at unforeseen solutions. There are people like Monfils, who explains to me how she programmed rats to forget their fears while she cradles one of the rodents in her arms, stroking its white coat as if it were a cuddly housecat. This rat, it should be noted, is one she has "modified"-the top half of its cranium looks like it has been sliced off, and in its place sits an implantable microchip that lets Monfils watch its brain activity in real time on her laptop PC.

In a study published in Nature Neuroscience last year, LeDoux's team repeated the tone experiment, except this time there were two tones: a high-pitched beep and another like a digitized cricket. The rats heard both tones 20 times and then got a shock. This sequence was repeated three times, enough for the rats to learn to fear the tones as before. Now it came time to break the memory and, hence, the fear. While only the cricket tone played, the rats were injected with U0126, a chemical that prevents long-term memories from forming. Twenty-four hours later, when the rats heard both tones again, they froze only after listening to the beep. The drug had flushed away any memory of getting shocked after hearing the cricket noise-and no memory meant no fear.

The study joined a growing chorus of research demonstrating that memories aren't immutable objects encased in museum glass. Rather, they are living, changing things and can be manipulated whenever evoked. "It sounds like science fiction, but long-standing memories are vulnerable to change," says LeDoux.

More important, it also proved that a specific memory could be altered or erased (remember, it eliminated just the rats' fear of the cricket, not the beep). The rats remembered getting a shock after hearing the cricket tone, and so they froze whenever it was played. U0126 blocked that fear memory, but only because the drug was dispensed when the rats were prepared to get the shock again. "Your memory of a specific event is only as good as your last memory of that event," LeDoux says. Thus, every time you dredge up a memory, good or bad, it's susceptible to change. (Incidentally, this is how neuroscientists account for "alien abductees" who pass lie-detector tests. The victims recall their close encounters so exhaustively and so often that the repeated recollections gradually alter the memory until the fabrications become indistinguishable, neurochemically speaking, from truths.)

News of LeDoux's experiments spread, and the neuroscience community quickly took notice. The conventional practice of "talk therapy" suddenly seemed tedious and of dubious efficacy. Why would I want to spend hours of couch sessions with my shrink when a shot of an amnesia-inducing compound into my brain at the exact moment I'm remembering my childhood spider invasion would make me fearless in an instant?

"When you recall something, you don't recall what originally happened; you recall what you recalled the last time you recalled it," explains Roger Pitman, a professor of psychiatry at Harvard University who has been experimenting with a drug called propranolol that reduces the intensity of memories in patients with post-traumatic stress disorder. "A memory is preserved in a plastic state. You can sculpt it or update it. Theoretically, any memory, including a fear or declarative memory-being able to say what you had for breakfast yesterday-is capable of being modified."