Moreover, just as fear can interfere with reason in the presence of an imminent threat, it can also exercise the same power over reason in the realms of memory. We mistakenly assume that memory is the exclusive province of reason, but in fact those regions of the brain that give us our capacity for fear have their own memory circuits. Over the course of our lives, we emotionally tag traumatic experiences as memories that are especially accessible to recall -- either consciously or unconsciously -- and they are constantly being retrieved to guide us in new situations, especially when a rapid response is required.
Most of us are familiar with the phenomenon of post-traumatic stress disorder (PTSD), which is common to rape victims, child abuse victims, and combat veterans, among others. Normally, when an experience is translated into memory, it's given a sort of "time tag," a mechanism that gives us the ability when we recall those experiences to sense how long ago the events we recall occurred and a rough understanding of their temporal sequence. You can sense that the remembered experience was before this and after that. Or that it was ten weeks ago or eleven weeks ago.
However, when traumatic events -- those involving anxiety or pain -- are stored in memory, the process is different. All bets are off. The amygdala is activated, and that memory is coded and stored differently. In effect, the "time tag" is removed -- so that when the traumatic experiences are later recalled, they feel "present." And the memory has the ability to activate the fear response in the present moment -- even though the trauma being remembered was a long time ago -- because the intensity of the memory causes part of the brain to react as if the trauma were happening again right now. PTSD is the constant intrusion into the mind of traumatic memories and a reexperiencing of the events as if the event had just happened. As Dr. Ramachandran has pointed out, it is this preoccupation with the trauma that can be so disabling. Even if we know intellectually that the events were long ago, the specialized and robust memory circuits in the fear centers of the brain reexperience the traumatic events when they are remembered and drive the same kinds of responses -- such as a faster heartbeat and increased feelings of fear -- that would be driven if the experiences were actually occurring at the time.
Structural similarities between previous experiences and subsequent ones can cause the fear centers of the brain to pull memories forward and force them into the present moment. If a subsequent experience is even superficially similar to a traumatic memory, it can wield incredible power over emotions and can trigger the same fear responses evoked by the original trauma.