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FLEETING MEMORIES: Recall can be unraveled using current scientific techniques. But targeted memory erasure, such as that depicted in the movie Paycheck, is still highly speculative.

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In the movie Paycheck, opening Christmas Day, a crack reverse engineer helps companies steal and improve upon the technology of their rivals, then has his memory of the time he spent working for them erased. The story, based on Philip K. Dick¿s sci-fi thriller of the same name, is set in the near future, but such selective memory erasure is still highly speculative at best. ScientificAmerican.com asked neurobiologist James McGaugh of the University of California at Irvine, who studies learning and memory, to explain what kinds of memory erasure are currently possible. For more information, see his book Memory and Emotion: The Making of Lasting Memories, released in 2003.

Scientific American.com: Early in Paycheck we see the main character get several months' worth of his memories erased by having individual neurons zapped. Is that possible?

JM: No. First of all there is no evidence of memories being stored individually. And even if they were stored in individual neurons, no one would know where they were. What we know an awful lot about are the brain systems that are involved in storing memories. Your memories of this conversation, for example, are stored rather diffusely in the brain. They're not going to be stored in a couple of neurons someplace that anybody could easily identify.

SA: But haven't surgeons poked people's brains in certain spots and made them recall specific things?

JM: [In the 1950s] Wilder Penfield up at the Montreal Neurological Institute was doing surgery for people who had brain seizures, and he had to stimulate the brain and have people talk to make sure that he wouldn't eliminate speech areas, for example. He found that he could evoke some things that appeared to be like memories, but it's more likely that he was just evoking [an impression of] something, not a specific memory.

SA: Are there any ways to erase memories by stimulating the brain?

JM: The dominant evidence that goes back over 50 years is that one can block or certainly reduce memories formed within the past several hours by treating human or animal subjects with electro-convulsive shock. But it's nonselective; whatever happened in that past several hours will be gone. And that's rather gross stimulation applied to the skull. What Larry Squire at UC San Diego has shown is that if human subjects are repeatedly given electro-convulsive shocks (several times a week for several weeks), they will have impaired global memory that goes back many months, but that memory will gradually recover. He did this in the late 1980s.

SA: Are there any more selective ways to erase memory?

JM: If one work with the hippocampus, one can selectively remove animals' memories of places where they have received training. In the Morris water maze, for example, animals are trained to swim from a variety of regions [in] a six-foot tank to an invisible platform located about two centimeters below the surface of the water. That kind of learning requires the hippocampus. If the hippocampus is blocked electrically or chemically within a few hours after animals have been trained to go that spot, they will not remember it the next day. So that would be an example of a place memory that could be influenced by discrete stimulation of a specific region of the brain. This doesn't mean the memory is permanently stored there. It means that the hippocampus is involved in the processing of that information, which is ultimately stored someplace else. And it's not something that could be done by electrical stimulation applied outside of the brain except for electro-convulsive shock, which activates the entire brain.

SA: How do we know memories aren't stored in the hippocampus?

JM: If subjects are taught something and then the hippocampus is removed several weeks later (in an extreme case), the information is retained. Take the very famous case of HM. He has anterograde amnesia--he is unable to learn new knowledge about the world--because he doesn't have his anterior hippocampus. He has retrograde amnesia that goes back for maybe a year, but for earlier times--a year longer and back to childhood--the memories are pretty much intact. So that says that these regions cannot be the repositories of a long-term memory. [For more on the milder sorts of memory problems we all face, click here.]

SA: Doesn't some research indicate that every time a memory is recalled, there is a window of a few hours in which it can be erased?