As I explained in my book's section on memory consolidation, the original learning could have been disrupted by other, non-threatening events and distractions. Unfortunately, bad experiences typically get rehearsed extensively immediately after learning and that enhances consolidation of the bad situation and the distressed emotional state at the time.
Anxiety disorders are among the most common mental health problems and are often treated with so-called extinction therapies. That is, therapy is geared toward unlearning our fears by deliberately re-living the disturbing event under safe conditions and thereby learning we can cope successfully. It is like the old idea of getting back on a horse after you fall off. But that is not a good analogy, because if you get back on a horse, you could fall off again - hardly reassuring.
Modern psychotherapy for phobias and PTSD often involves so-called "cognitive behavioral therapy (CBT)," which requires re-living the original bad event under reassuring conditions by talking about it and even writing about it. But this has to be done with re-assessment with conscious realization that the original negative emotions and fear are no longer applicable because the re-living is a simulation in a safe environment. The re-living must include dealing with the negative emotions in the light of reason. Wikipedia's description of CBT is that therapy requires "questioning and testing cognitions, assumptions, evaluations and beliefs that might be unhelpful and unrealistic; gradually facing activities which may have been avoided; and trying out new ways of behaving and reacting." Relaxation and distraction techniques are also commonly included. This re-creation of the bad event allows us to extinguish memory of the original bad situation and its negative emotions.
Extinction is something scientists know a lot about. I recently got an up-date in this area of research at a seminar by Gregory Quick from the Dept. of Psychiatry at the University of Puerto Rico. As Pavlov showed, extinction is a basic phenomenon even in simple animals. If you repeatedly ring a bell and then give foot shock to a rat, it soon learns to become distressed the next time it hears that bell, even if it does not get a shock. In the lab, this is manifest by the rat showing freeze behavior. But, if you repeat the bell enough times without foot shock,the conditioned response (CR)(freeze behavior) is extinguished. At first,scientists thought that memory of the CR is erased during extinction, but even Pavlov, the father of conditioned learning, recognized early on that extinction is a new learning experience. Think of it as learning to unlearn. Extinction creates a new memory that competes with memory of the original CR. Both memories co-exist. However, over time the extinction memory may be lost, and the CR can return. Presumably, the rules for effective consolidation apply to extinction learning as well as to CR learning. Therapy would be more effective if therapy for PTSD and other anxiety disorders was approached like a conventional learning experience whose memory needs considerable nurturing. The confirmation that extinction is new learning comes from experiments showing that traces of extinction memory remain after extinction learning seems to be forgotten. Re-learning of an extinguished response occurs much more readily than it does for an initial extinction learning.Its like re-learning a foreign language. It goes easier the second time.
As this picture shows, memory of a CR and its extinction can co-exist. The practical consequence is that these memories compete for which one is strong enough to be retrieved. Sadly, the CR memory is often stronger. As I explain in my book, cues are extremely important to both forming and retrieving and memory. It seems likely that in typical human situations, there are many more cues in CR than in extinction. Therapy should be aimed at enriching the number and variety of cues associated with extinction learning.
Here is an example applied to humans that this kind of research suggests to me: if you are afraid of heights, it could be because some frightening event happened, perhaps years ago, that involved a high place. You may not even remember what caused the fear, but clearly any high place provides plenty of cues to trigger the memory of that conditioned memory. Now, if you progressively force yourself in small steps to go to high places, under clearly safe conditions, you can learn to extinguish this memory by thinking about how irrational your fear is. For instance, walking up a staircase should not evoke fear if there is no way you can fall off. These new extinction learning experiences need to be consciously attended, rehearsed, protected from interfering stimuli, and otherwise nurtured to promote consolidation. You should strive to construct all sorts of cues that can be associated with your extinction trials. These cues should strengthen the consolidation of your extinction learning and moreover, make the extinction memory more retrievable in the face of other cues that were associated with the original CR. Like any other memory, the extinction cues and environment need to be re-experienced often at periodic intervals so the extinction memory is strengthened at the expense of the original CR that created the anxiety disorder.
Quirk, G. J. et al. 2006. Prefrontal mechanisms in extinction of conditioned fear. Biological Psychiatry. 60: 337-343,