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Memories surrounding a major event like September 11 might be especially susceptible, he says, because we tend to replay them over and over in our minds and in conversation with others—with each repetition having the potential to alter them.

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.

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.

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.”

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.

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.

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

In the winter of 1999, he taught four rats that a high-pitched beep preceded a mild electric shock. That was easy—rodents learn such pairings after being exposed to them just once. Afterward, the rat freezes in place when it hears the tone. Nader then waited 24 hours, played the tone to reactivate the memory and injected into the rat’s brain a drug that prevents neurons from making new proteins. If memories are consolidated just once, when they are first created, he reasoned, the drug would have no effect on the rat’s memory of the tone or on the way it would respond to the tone in the future. But if memories have to be at least partially rebuilt every time they are recalled—down to the synthesizing of fresh neuronal proteins—rats given the drug might later respond as if they had never learned to fear the tone and would ignore it. If so, the study would contradict the standard conception of memory.

When Nader later tested the rats, they didn’t freeze after hearing the tone: it was as if they’d forgotten all about it

After Nader’s initial findings, some neuroscientists pooh-poohed his work in journal articles and gave him the cold shoulder at scientific meetings. But the data struck a more harmonious chord with some psychologists. After all, their experiments had long suggested that memory can easily be distorted without people realizing it.

To Nader and his colleagues, the experiment supports the idea that a memory is re-formed in the process of calling it up. “From our perspective, this looks a lot like memory reconsolidation,” says Oliver Hardt, a postdoctoral researcher in Nader’s lab.

“When you retell it, the memory becomes plastic, and whatever is present around you in the environment can interfere with the original content of the memory,” Hardt says.

The question is whether reconsolidation—which he thinks Nader has demonstrated compellingly in rat experiments—is the reason for the distortions.

at the Douglas Mental Health University Institute. Alain Brunet, a psychologist, is running a clinical trial involving people with post-traumatic stress disorder (PTSD). The hope is that caregivers might be able to weaken the hold of traumatic memories that haunt patients during the day and invade their dreams at night.

In Brunet’s first study, PTSD patients took a drug intended to interfere with the reconsolidation of fearful memories. The drug, propranolol, has long been used to treat high blood pressure, and some performers take it to combat stage fright. The drug inhibits a neurotransmitter called norepinephrine. One possible side effect of the drug is memory loss.

Nine patients took a propranolol pill and read or watched TV for an hour as the drug took effect. Ten were given a placebo pill. Brunet came into the room and made small talk before telling the patient he had a request: he wanted the patient to read a script, based on earlier interviews with the person, describing his or her traumatic experience. The patients, all volunteers, knew that the reading would be part of the experiment. “Some are fine, some start to cry, some need to take a break,” Brunet says. A week later, the PTSD patients listened to the script, this time without taking the drug or a placebo. Compared with the patients who had taken a placebo, those who had taken the propranolol a week earlier were now calmer; they had a smaller uptick in their heart rate and they perspired less.

The treatment didn’t erase the patients’ memory of what had happened to them; rather, it seems to have changed the quality of that memory. “Week after week the emotional tone of the memory seems weaker,” Brunet says. “They start to care less about that memory.” Nader says the traumatic memories of PTSD patients may be stored in the brain in much the same way that a memory of a shock-predicting tone is stored in a rat’s brain. In both cases, recalling the memory opens it to manipulation.

Nader suggests that reconsolidation may be the brain’s mechanism for recasting old memories in the light of everything that has happened since. In other words, it just might be what keeps us from living in the past.