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Memories are stored in a region of the brain called the hippocampus, shown in red in this computer illustration. (Photo Researchers, Inc.)

How Our Brains Make Memories

Surprising new research about the act of remembering may help people with post-traumatic stress disorder

For those of us who cherish our memories and like to think they are an accurate record of our history, the idea that memory is fundamentally malleable is more than a little disturbing. Not all researchers believe Nader has proved that the process of remembering itself can alter memories. But if he is right, it may not be an entirely bad thing. It might even be possible to put the phenomenon to good use to reduce the suffering of people with post-traumatic stress disorder, who are plagued by recurring memories of events they wish they could put behind them.

Nader was born in Cairo, Egypt. His Coptic Christian family faced persecution at the hands of Arab nationalists and fled to Canada in 1970, when he was 4 years old. Many relatives also made the trip, so many that Nader’s girlfriend teases him about the “soundtrack of a thousand kisses” at large family gatherings as people bestow customary greetings.

He attended college and graduate school at the University of Toronto, and in 1996 joined the New York University lab of Joseph LeDoux, a distinguished neuroscientist who studies how emotions influence memory. “One of the things that really seduced me about science is that it’s a system you can use to test your own ideas about how things work,” Nader says. Even the most cherished ideas in a given field are open to question.

Scientists have long known that recording a memory requires adjusting the connections between neurons. Each memory tweaks some tiny subset of the neurons in the brain (the human brain has 100 billion neurons in all), changing the way they communicate. Neurons send messages to one another across narrow gaps called synapses. A synapse is like a bustling port, complete with machinery for sending and receiving cargo—neurotransmitters, specialized chemicals that convey signals between neurons. All of the shipping machinery is built from proteins, the basic building blocks of cells.

One of the scientists who has done the most to illuminate the way memory works on the microscopic scale is Eric Kandel, a neuroscientist at Columbia University in New York City. In five decades of research, Kandel has shown how short-term memories—those lasting a few minutes—involve relatively quick and simple chemical changes to the synapse that make it work more efficiently. Kandel, who won a share of the 2000 Nobel Prize in Physiology or Medicine, found that to build a memory that lasts hours, days or years, neurons must manufacture new proteins and expand the docks, as it were, to make the neurotransmitter traffic run more efficiently. Long-term memories must literally be built into the brain’s synapses. Kandel and other neuroscientists have generally assumed that once a memory is constructed, it is stable and can’t easily be undone. Or, as they put it, the memory is “consolidated.”

According to this view, the brain’s memory system works something like a pen and notebook. For a brief time before the ink dries, it’s possible to smudge what’s written. But after the memory is consolidated, it changes very little. Sure, memories may fade over the years like an old letter (or even go up in flames if Alzheimer’s disease strikes), but under ordinary circumstances the content of the memory stays the same, no matter how many times it’s taken out and read. Nader would challenge this idea.

In what turned out to be a defining moment in his early career, Nader attended a lecture that Kandel gave at New York University about how memories are recorded. Nader got to wondering about what happens when a memory is recalled. Work with rodents dating back to the 1960s didn’t jibe with the consolidation theory. Researchers had found that a memory could be weakened if they gave an animal an electric shock or a drug that interferes with a particular neurotransmitter just after they prompted the animal to recall the memory. This suggested that memories were vulnerable to disruption even after they had been consolidated.

To think of it another way, the work suggested that filing an old memory away for long-term storage after it had been recalled was surprisingly similar to creating it the first time. Both building a new memory and tucking away an old one presumably involved building proteins at the synapse. The researchers had named that process “reconsolidation.” But others, including some prominent memory experts, had trouble replicating those findings in their own laboratories, so the idea wasn’t pursued.

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