Memory and emotion are intimately linked biochemically, with hormones like adrenaline actively involved in forming the neurological patterns we call memories. “Any kind of emotional experience will create a stronger memory than otherwise would be created,” says James McGaugh, a neurobiologist at the University of California.
 

On the face of it, that doesn’t seem especially surprising: we feel strong emotion at important events, which are obviously more memorable than ordinary moments. But the connection is much deeper than that and dates back to our deepest evolutionary past. “The major purpose of memory,” observes McGaugh, “is to predict the future.” An animal that can remember the significance of that large, nasty-looking thing with the big teeth and sharp claws will survive longer and produce more offspring.
 

Norepinephrine does two important things. First, it kicks the body’s autonomic nervous system into overdrive: the heart beats faster, respiration quickens, and the muscles tense in anticipation of a burst of physical exertion.
 

Second, even as it’s kick-starting the body, the amygdala sends out a crackle of signals to the rest of the brain. Some of them put the senses on high alert, ready to deal with a threat. But these signals also tell the neurons that any memories recorded in the next few minutes need to be especially robust. One piece of evidence for this scenario: Lawrence Cahill, a colleague of McGaugh’s, showed subjects emotionally arousing film clips, simultaneously gauging the activity of their amygdalae using positron-emission tomography (PET) scans. Three weeks later, he gave the subjects a surprise memory quiz. The amount of amygdala activity predicted with great accuracy how well they remembered the film clips.
 

Imaging studies also make clear that it isn’t just dangers or tragic events that cement memory formation. Positive emotions, which are also mediated through the amygdala, have the same effect. Again, that’s a perfectly reasonable evolutionary development. If eating or having sex makes you happy, you’ll remember that and do it again, keeping yourself healthy and passing on your genes as well.
 

This is an oversimplification, of course. Other neurotransmitters, and even plain glucose—the sugar the brain uses for energy—may also play a part. And then there’s the peculiar case of a woman who contacted McGaugh because she remembers absolutely everything. The stress-hormone model does not appear to apply in her case. Says McGaugh: “At one point I asked if she knew who Bing Crosby was. She’s 40, so Bing Crosby doesn’t loom large in her life, but she knew he died on a golf course in Spain, and she gave me the date, just like that.” Imaging researchers are working to determine whether the woman’s brain is structurally different from everyone else’s.