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Memories of concrete facts and events, which can in principle be retrieved on demand, are coordinated through the hippocampus, a crescent-shaped collection of neurons deep in the core of the brain. Other sorts of memory are handled by other areas. The amygdala, for example, an almond-size knot of nerve cells located close to the brain stem, specializes in memories of fear; the basal ganglia, clumps of gray matter within both cerebral hemispheres, handle habits and physical skills; the cerebellum, at the base of the brain, governs conditioned learning (as when Pavlov's dogs salivated at the ringing of the dinner bell) and some reflexes.
Damage to any one of these regions has an effect on the corresponding form of memory. A much studied patient known as HM, for example, lost much of his hippocampus in the course of surgery to relieve severe epilepsy. As a result, he could remember everything that happened to him before the surgery, but he was completely unable to form new memories. He was stuck forever in the 1950s. Yet HM was able to learn new skills, such as drawing while looking in a mirror.
THE POWER OF FEELINGS Physical trauma can distort memory, presumably by destroying all or part of one of these memory-processing structures. But other sorts of shock-strong emotion, for example-can do the same. Virtually everyone who was over the age of 10 when J.F.K. was shot or when Challenger exploded remembers precisely where he or she was when the news arrived. Posttraumatic stress disorder, which affects Vietnam vets like Bill Noonan, is another good example. While the intellectual memory of emotions is routed through the hippocampus, a different, gut-level sort of memory can be involuntarily revived with terrible clarity by abnormal activity in the amygdala. "It's been an eye opener to me that individuals we study who were traumatized 25 years ago still show abnormal brain function," says Dennis Charney, head of psychiatry at the VA hospital in West Haven, Connecticut. "Severe stress can change the way your brain functions biologically."
It stands to reason that humans would have a specialized region of the brain for processing emotional perceptions and memories: if our distant ancestors hadn't had an instant and violent reaction to danger, they would not have lived very long. But other parts of the brain are apparently also involved in feeling emotions. What's most surprising is the assertion by the University of Iowa's Damasio that emotion is central to the process of rational thought.
His evidence comes from nearly two dozen patients treated by Damasio, including Elliot, the businessman who started behaving irrationally after surgery to remove a brain tumor. Elliot cannot behave rationally, even though his intelligence was not affected by his tumor. The part of the brain destroyed by invading tissue was in a region of the prefrontal cortex (see diagram) essential to decision making. But what Elliot lost, psychological testing revealed, was the ability to experience emotion. While the amygdala does process fear, his doctors argue from the example of Elliot and the other patients that other parts of the brain are also critical to regulating emotion.