There is much yet to be discovered about the complex process of human memory. All I can hope to do in this brief section is acquaint the reader with some of the ongoing investigations and hints that we have gathered thus far about how memory and traumatic memory work. We know that there are at least two different kinds of memory and we know that memory is often profoundly altered as a result of overwhelming stress. The exact mechanisms, however, are still under active investigation (Squire, 1987).
The memory we think of as "normal" memory is called declarative or explicit memory. This memory system is grounded in our use of language. Our brain automatically categorizes every new experience. We begin creating these categories at birth and continue making new categories of information until we die. These categories comprise a system of mental schemata that are verbally encoded and become integrated into the already existing knowledge base. Most of this activity occurs in the part of the brain known as the hippocampus. This language-based system is a relatively time¬ consuming mechanism which does not lend itself to emergency situations in which immediate response is necessary. Thinking takes time. Additionally, the declarative memory system is open. to change and distortion, both by previous experience, by new information, and by the state of arousal that the person is in at the time of recall.
But throughout every day we are drawing upon another kind of memory that does not require words, or even thoughts. This system is known by various terms including nondeclarative, implicit, or procedural memory (Southwick, Bremner, Krystal, & Charney, 1994; Squire, 1987; B. A. Van der Kolk, 1994, 1996; B. A. Van der Kolk, Burbridge, & Suzuki, 1997). This is the memory of habits, skills, conditioned emotional responses, and conditioned sensorimotor responses. This is the memory we automatically draw upon when we go out to drive our car in the morning, or start riding a bicycle flawlessly for the first time in decades. This memory system is not language¬based and appears to be controlled largely by areas of the brain separate from our normal memory, most importantly by a structure called the amygdala. The implicit memory system developed earlier in evolution than our more recently evolved verbal, explicit memory system and may also be operational far earlier in our development than the verbal system that does not become available until later in development when the child begins to under¬ stand speech (J. E. LeDoux, 1994; B. A. Van der Kolk, 1994). Children may, in fact, have nonverbal memories long before they have memories that can be given any kind of verbal form. These memories may then linger as vague physical sensations, emotions, and sensory images and not as verbal messages.
The implicit memory system functions much faster than our verbally based memory system and has been called a "quick and dirty reaction mechanism" (J. E. LeDoux, 1994). It is the amygdala that appears to attach emotional meaning to our experience, even before we have recognized what we are reacting to or what we are feeling and then the higher critical areas elaborates that experience and imbues it with meaning. This aspect of memory is especially important in the processing of "fear conditioning" and therefore centrally involved in how we manage fear. This is important because so many psychiatric disorders, including posttraumatic stress disorder, appear to involve some malfunction in the brain's ability to control fear (J. E. LeDoux, 1994).
Under normal conditions, both memory systems are available for optimal functioning. We are able to freely access information and draw upon biographical material. Emotional experience is attached to these memories, but our day-to-day life is not dominated by powerfully emotionally charged memories or intrusive experiences from the past. The past remains in the past and is not experienced as the present. Under normal circumstances these two memory systems work in parallel and their activities are seamlessly interconnected at the level of our conscious experience. We are consciously aware of the information from the hippocampal system and the other system remains unconscious, out of our awareness but always exerting a powerful influence (J. E. LeDoux, 1994).
Our way of remembering things-storing new memories, and drawing upon old memories- may be dramatically changed when we are under stress. Studies have shown that emotional memories with personal relevance tend to be quite accurate and long-lasting in contrast with memories of meaningless events that are measured in laboratory memory experiments (Yuille & Cutshall, 1989). For the past century, many observers have noticed that the imprint of traumatic experiences is very different from the memories of normal events.
Bremner and his colleagues recently reviewed the differences between "normal forgetting" and traumatic amnesia (Bremner, Krystal, Southwick, & Charney, 1995). They point out that from an evolutionary point of view, the efficient recall of memories associated with previous danger is crucial for survival. If you are in a forest and hear a loud growl behind you, it is far better for your brain to flash up an image of a threatening beast, or just a feeling of intense fear that impels you to run, than it is for your brain to stop and ponder in words a series of alternative explanations, options, or actions. After all, we are evolved from animals who lacked verbal capacities, and yet their survival depended on remembering danger. But it is difficult to explain how the over-remembering of danger associated with PTSD is adaptive or why so many people develop under-remembering, or amnesia, in the face of danger.
People who have experienced a variety of differing traumas are noted to have a wide range of memory problems with vivid intrusive memories of a past event (flashbacks) often alternating with partial or total amnesia for the traumatic events. These intrusive experiences appear to be triggered by emotions and sensations in the present that are associated with the traumatic past and the person often has amnesia for the occurrence of the flash¬ back as well as the original trauma (B.A. Van der Kolk, McFarlane, & Weisaeth, 1996).
People under severe stress secrete neurohormones that affect the way that their memories are stored. In animals-and there is a growing body of evidence to support a similar effect in humans-high levels of glucocorticoids secreted during stress impair the functioning of the hippocampus and neuroimaging techniques indicate that changes in the very structure of the hippocampus may be secondary to prolonged stress (Bremner et al., 1995; B. A. Van der Kolk et al., 1997). This may partially or totally disable the ability of the brain to verbally categorize incoming information. At the same time, during states of high fear, the amygdala is extremely active and interferes with hippocampal functioning (B. A. Van der Kolk, 1996). The result is a partial or complete loss of the ability to assign words to incoming experience, the biological equivalent of "speechless terror" (B. A. Van der Kolk, 1994, 1996; B. A. Van der Kolk & Fisler, 1995). Dependent upon words, our capacity to logically think through a problem is diminished or entirely shut down and our minds shift to a mode of consciousness that is characterized by visual, auditory, kinesthetic images, and physical sensations as well as strong feelings.
Evidence also exists that the massive secretion of neurohormones at the time of the trauma may deeply imprint the traumatic memory (B. A. Van der Kolk, 1994, 1996) . The neuroscientist LeDoux has termed this "emotional memory" (J. E LeDoux, 1992). In studying the influence of fear in particular, he has shown that emotional memory appears to be permanent and quite difficult, if not impossible, to eliminate although it can be suppressed by higher centers in the brain. This "engraving" of trauma has been noted by many researchers studying various survivor groups (B. A. Van der Kolk, 1994; B. A. Van der Kolk & Van der Hart, 1991).
But these memory effects do not happen to everyone who is exposed to overwhelming stress indicating that there must be much individual variation in the way the mind and body responds to trauma. According to a recent review, much depends on the extend to which traumatic memories are integrated with conceptual knowledge about the self (Brewin, 2011). People who suffer from PTSD often have both intrusive recollections and amnesias. The evolving model of trauma-related memory indicates that there is a complex interaction between the event, the individual, and the context of the event. Some people are more reactive to external events than others and appear to be that way from birth. Some people more readily develop associations to different stimuli than others. Some people tend to ruminate more than others. Others are comparatively more distractible than other people. All of these factors, and probably others, may influence how memory is processed at the time of the trauma and subsequently (R. Yehuda & Harvey, 1997).
There may also be significant differences between acute and chronic stress exposure. Whereas acute stress triggers an increased level of steroids (i.e. cortisol), chronic stress produces decreased steroid levels, decreased responsiveness due to a new acute stress, and other changes that may help to explain the very complex and interactive nature of memory problems after trauma (R. Yehuda et al., 1995).
Traumatic memory often poses the greatest problems for people who have suffered repeated or severe traumatic experience. The intrusive symptoms of post-traumatic stress disorder-the nightmares, and the sensory, emotional, and physical flashbacks-all appear to be a result of disordered memory functioning. But traumatic experience has also been noted for the predominance of amnesia in the clinical picture as well. For the last century clinicians and researchers have been reporting the presence of traumatic amnesia in many different survivor groups (Bremner et al., 1995). This alternation between hyperamnesia and amnesia is one of the most problematic aspects of stress disorders.
On the one hand, the traumatic memories are vivid and intrusive. These memories do not fade, nor do they seem to be altered by ordinary experiences. They are state-dependent-they tend to intrude into consciousness when they are triggered by a state that resembles the state experienced at the time of the original traumatic event. Flashbacks are likely to occur when people are upset, stressed, frightened, or aroused or when triggered by any association to the traumatic event (B. A. Van der Kolk, 1996). In a study by Van derKolk and Fislerall subjects, regardless of the age at which the trauma occurred, reported that their initial memory was not in the form of a narrative, but was instead a somatosensory or emotional flashback experience. Seventy-five percent of the subjects with childhood trauma had external confirmation of the traumatic experience (B. A. Van der Kolk & Fisler, 1995).
Sleep may be seriously disturbed by vivid and horrifying nightmares that replay the events of the traumatic experience until the victim wakes up screaming in terror. Combat veterans have been known to assault their spouses during a nightmare, mistaking the body in bed next to them as the enemy in their dream. Some nightmares are the replay of the actual traumatic event and some are disguised representations of the trauma, hidden behind frightening symbols. Some nightmares decrease in frequency over time and just seem to fade away until triggered by a new stress. Others appear to become incorporated into other thematic experiences of life, often of earlier trauma or symbolized conflicts (Lansky & Bley, 1995).
There is now some data available from positron emission tomography that provides more information about these intrusive phenomenon. Traumatic memories "happen" principally in the emotional areas of the brain's right hemisphere, and are accompanied by an increase in activity in the visual areas of the brain, signifying that people with PTSD actually "see" their flashbacks, while there is a decrease in the area of the brain in charge of the translation of emotional states into language (Rauch et al., 1996; B. A. Van der Kolk et al., 1997; B. A. Van der Kolk & Fisler, 1995). This is experienced by the person as a total or partial reliving of the traumatic experience. It can be a sensory fragment of the trauma or the entire traumatic sequence running like a virtual reality movie. In such a state the traumatized person has difficulties distinguishing reality from flashback. The sensory experience is often quite vivid, feeding a vicious cycle of autonomic arousal that increases the sense of reality of the flashback even more.
When people experience intrusive flashbacks as visual, olfactory, affective, auditory, or kinesthetic sensations, although we term this "traumatic memory" it bears little if any relationship to the normal process of remembering. Remembering in our normal terms is based on language while traumatic recall is nonlinguistic. Gradually, as people begin to sort out these intrusive images, they begin to form a narrative as a means of explaining their experience. Once such an experience enters the narrative sphere it may be open to many of the distortions and changes related to "normal" memory processing, the distortions so highlighted by the false memory advocates. This is one of the difficulties in the public discourse about true memory and "false memory." Linguistic difficulties have helped to muddy this entire issue. The same words-"memory;' "forgetting;' and "remembering"-are used to describe two entirely different phenomena which appear to have different neuroanatomical and neurophysiological bases, as well as entirely different clinical presentations .
Dr. Chris Brewin recently summarized the key points about memory and PTSD in a critical review of the existing literature: 1) There is little evidence that single traumatic events are associated with memory disturbance in healthy individuals; 2) In healthy individuals, traumatic events are often forgotten if they are not personally signiﬁcant or consequential; 3) Extensive trauma in childhood can produce complexity and fragmentation in conceptual knowledge concerning the self; 4) PTSD is associated with widespread disturbance in general memory capacity, in the contents of trauma memories, and in a variety of memory processes, a number of which predict the course of the disorder; 5) In PTSD, the trauma is usually strongly associated with the person’s sense of identity; 6) The most distinctive element of memory disturbance in PTSD is the combination of impairment in voluntary trauma memories and facilitation of involuntary trauma memories; 7) The nature of both voluntary and involuntary trauma memories may change over time, as evident in the phenomenon of delayed-onset PTSD (Brewin, 2011).
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