How does the brain form memories of life's significant events? This is the question that motivates the research in our laboratory. More specifically, our work is focused on how traumatic memories are formed, stored and retrieved. 

A fundamental assumption in this work is that the brain has multiple memory systems, each devoted to different kinds of memory functions.  For traumatic memory, two systems are particularly important.  For example, if you return to the scene of an accident, you will be reminded of the accident and will remember where you were going, who you were with, and other details about the experience.  These are explicit (conscious) memories mediated by the hippocampus and other aspects of the temporal lobe memory system.  In addition, your blood pressure and heart rate may rise, you may begin to sweat, and your muscles may tighten up.  These are implicit (unconscious) memories mediated by the amygdala and its neural connections.  They are memories in the sense that they cause your body to respond in a particular way as a result of past experiences.  The conscious memory of the past experience and the physiological responses elicited thus reflect the operation of two separate memory systems that operate in parallel.  Only by taking these systems apart in the brain have neuroscientists been able to figure out that these are different kinds of memory, rather than one memory with multiple forms of expression.  ...Learning and responding to stimuli that warn of danger involves neural pathways that send information about the outside world to the amygdala, which determines the significance of the stimulus and triggers emotional responses, like freezing or fleeing, as well changes in the inner workings of the body's organs and glands....The implication of these findings is that early on (perhaps since dinosaurs ruled the earth, or even before) evolution hit upon a way of wiring the brain to produce responses that are likely to keep the organism alive in dangerous situations.  The solution was so effective that it has not been messed with, and works pretty much the same way in rats and people, as well as many if not all other vertebrate animals.

(Overview--Emotion, Memory, and the Brain:  What the Lab Does and Why We Do it," LeDoux Laboratory, ¶ 1-2, retrieved April 30, 2002 from http://www.cns.nyu.edu/home/ledoux/overview.html)