Like just about every one of my contemporaries, I still remember exactly where I was and what I was doing when John F. Kennedy was shot. It's so vivid, it's almost like watching a movie: I was home sick from fifth grade, lying on the couch in the living room. My mother had a talk-radio station playing. Suddenly a newscaster broke in with the news that shots had been fired in Dallas and that the President had been rushed to a hospital. Then a few minutes later came these precise words, spoken in just the tone you would imagine: "Ladies and gentlemen, the President is dead," followed immediately by funereal music. My mother burst into tears, and I, profoundly embarrassed, fled the room.
That scene, which I have replayed many times since 1963, perfectly illustrates two crucial facts that neurologists have come to understand in the past few years about the workings of human memory--facts that have important implications for the treatment of a variety of mental disorders, from post-traumatic stress to obsessive-compulsive disorder. The first is that, despite its movie-like clarity, my memory of J.F.K.'s assassination is almost certainly wrong in some details, and maybe even some significant ones. That's because I'm not simply calling up the original memory laid down in November 1963. I'm recalling the last time I thought about it. Each time we retrieve and re-store a memory, it can be subtly altered by all sorts of factors. What goes back into our brains is like the new version of a text document, overwriting the old.
The second fact: 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 at Irvine. "We remember our embarrassments, our failures, our fender benders."
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.
What happens biochemically, says McGaugh, is that when faced with an emotion-charged situation, such as a threat, our bodies release the stress hormones adrenaline and cortisol. Among other things, these signal the amygdala, a tiny, neuron-rich structure nestled inside the brain's medial temporal lobes, which responds by releasing another hormone, called norepinephrine. 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.