“Neural Mechanisms in Dissociative Amnesia for Childhood Abuse: Relevance to the Current Controversy Surrounding the ‘False Memory Debate,’” a 1996 paper by Drs. J. Douglas Bremmer, John Krystal, Dennis Charney, and Steven Southwick, examined a number of studies that strongly indicated the presence of neurological conditions that affect the brain’s normal processes of encoding and retrieval under periods of high stress (or as some researchers refer to it, “high arousal”).* In turn, these events “modulate” or alter the usual ways in which the brain encodes and stores memory. After briefly citing the history of dissociation as a psychological/psychiatric construct, the authors propose that advancements in neuroscience offered new insights not just to the fact that dissociation occurs, but how it occurs. This would have not only a direct bearing on the issue of posttraumatic stress disorder (PTSD), but on the issue of childhood sexual abuse.

The authors stipulate early on that some childhood sexual abuse victims really cannot recall the abuse, although it’s often documented in police investigations and hospital records. The real disagreement was whether this is a case of simply forgetting or permanently disregarding a memory, or whether the memory still lies dormant in the brain: They write:

The fact that many individuals forget episodes of childhood abuse is well established. As many as 38% of trauma victims who experience abuse severe enough to result in a visit to a hospital emergency room had no memory of the event 20 or more years later....Some investigators have explained the loss of memory of abuse as secondary to ‘repression’ or dissociative amnesia. They maintain that memories of abuse may not be available to consciousness for many years, or perhaps even the individual’s lifetime....The opposite viewpoint holds that the loss of memories of abuse is a process of ‘normal forgetting.’...These authors argue that there is a popular misconception that forgotten memories exist somewhere in the brain and are only awaiting the proper stimulus or means to bring them to consciousness.

The authors then establish some points about how memory normally operates. They define memory retrieval as the process of taking a memory from storage in to conscious forethought. If you can easily recall the memory datum from storage to the forefront of your consciousness, the authors would call that “explicit memory.” This is what you exhibit when you play trivia, fill out an application, or reminisce with an old friend about the good times.

Some memories, however, don’t lend themselves to conscious recall, but are still quite evident. Researchers call these “implicit memories.” Oftentimes they “embed” themselves in skill sets. For example, if you’re chopping vegetables, you don’t really have to bring the memory of how to do that to the forefront of your conscious mind in order to get the job done. Implicit memory also occurs when emotions become associated with explicit memories of important events. If you watch interview shows, for example, then you well know that respondents who are reciting the particulars of an event will often exhibit strong emotion (e.g., they’ll break down and cry if the circumstances or consequences are horrific), even if the events that they are bringing to conscious memory happened fifty years in the past. In popular discourse, we call this “reliving the moment.” Bremmer et al would describe this as an association between an implicit memory (the horror, the shock, the betrayal, etc.) and an explicit memory (the sequence of actions).

The authors also address the issue of memory “integrity” or accuracy, and the ability of investigators or therapists to implant false memories into subjects. Reviewing studies analogous to “The War of the Ghosts” example and Lost in a Shopping Mall, they acknowledge that the findings “...led to the conclusion by some authors that misleading information ‘overwrites,’ or replaces, the original memory trace.”** On the other hand, they noted the belief among other researchers that “...if the subjects remember the original information [or perhaps never noticed it the first time around], they may make a guess based on the recall of the misleading information.

Between these two extremes (and I use the term loosely), Bremmer et al advance their own position, which is this: the studies, criticisms and arguments on both side are probably irrelevant to the issue of childhood sexual abuse. Why? Because this line of inquiry only examines normal memories under normal conditions and circumstances. Memories originating in moments of stress–especially stress at the level of trauma–are encoded, stored and processed differently from normal memory.

Figure 1. Diagram of a human brain

Figure 2. Another diagram of a human brain

Figure 3. Still yet another diagram of a human brain***

The authors explain that memories are initially located in the part of the brain known as the ‘hippocampus.’ After a period of time, say several weeks, the storage gets, well, sort of outsourced to other areas of the brain, quite often the various cortexes (or corticies). Some forms of memory have a tendency to cluster in a specific area. For example, visual memory could head for the occipital cortex, while tactile memories (which I seem to be experiencing a lot of, lately) could set up shop in the sensory cortex. Here, Bremmer et al suggest that the hippocampus serves as a node, of sorts, bringing together all these memories from their storage areas, located generally in the neocortex, during the time of explicit memory recall.

Central to the discussion here is the area of the brain known as the ‘amygdala,’ which the authors characterize as “an important mediator of emotional memory.” As an example, they mention the startle response–that thing you do when someone sneaks up behind you and says “Boo!” In their words, this represents, “...a primitive reflex that is part of the animal’s response to threat.” Organisms that suffer lesions or other physical damage to that part of the brain can completely stifle the startle response. The authors thus contend, “The amygdala integrates information that is necessary for the proper execution of the stress response, including (internal) emotion and information from the external environment.”

Part of the brain’s stress response includes the substantial release of neuropeptides and neurotransmitters. A number of studies compellingly show that these biochemicals “richly innervate,” or highly stimulate, the hippocampus and the cortexes involved with memory encoding, storage and retrieval. An abundance of these chemicals can enhance some forms of memory, but only at certain levels. For example, epinephrine boosts the avoidance response, which is brought about because of unpleasant emotional memories. Researchers in one experiment shocked labs rats as they crossed a specific threshold in a maze. That made them really skittish about crossing another threshold. When scientists removed the adrenal medulla, the place where epinephrine is typically stored, they could shock the rats all they wanted. The rodents would still not exhibit fear from the memory, or more importantly, avoid another threshold.

Adrenocorticotropic hormone (ACTH) and glucocorticoids can both enhance memory, but only at low levels. A lot of either or both has the opposite effect. The authors also cite studies that show a definite correlation between high stress and physical damage to two specific regions within the hippocampus. Other studies suggest that the flood of glucocorticoids produced during the stress response damage the hippocampus.

Because of the pivotal role of the hippocampus in memory, abnormalities within it could explain why some people have access to traumatic memories, and others don’t. In 1995, Bremmer et al participated in a study in which they compared the hippocampal region of twenty-six Vietnam vets suffering from PTSD to a control group of twenty-two individuals who had never suffered from posttraumatic stress.**** The hippocampuses of the veteran subjects were, on average, 8% smaller in comparison to the rest of their brains compared to the control group. Among those same vets, they found “Lower IQ scores in combat veterans with PTSD than in comparison subjects.”

That makes for some interesting questions. For example, one could guess that damage to the hippocampus would make it more difficult to store memories of learned tasks initially, and coordinate what memory is there. Could this mean that people with lower IQs have a greater risk for PTSD because they have a relatively smaller hippocampus? Could it be that the stress response actually created the PTSD by damaging that area of the brain in the first place?

Bremmer et al imply these questions, but provide no answers for them. They do point out, however, that people who suffer from PTSD have greater difficulty in learning and in explicit memory recall during their lives. They also exhibit other behaviors that directly tie into the issue of childhood sexual abuse.


_________________
*This paper appeared in the American Journal of Psychiatry, vol. 153, n. 7 (July 1996 Festschrift Supplement). The affiliated institutions: Yale University, School of Medicine; the Division of Clinical Neurosciences, National Center for Posttraumatic Stress Disorder; and the Psychiatry Services of the West Haven, CT Veterans Administration.

**In one study, for example, children who watched an episode of The Six Million Dollar Man, in which the title character became so weak that he couldn’t lift a can of paint, remember him lifting the paint can three weeks later. Like the “War of the Ghosts” stories, the memory adapted to personal or cultural expectations during the recall process. In another study cited here, respondents witnessed a film in which researchers staged an automobile accident. Many of the subjects recalled seeing broken glass, when there was none. In this case, the researchers deliberately misled the subjects by using such words as “smash” as opposed to “collided,” or “made contact with,” and so on.

***I could not find a single diagram that shows all of the areas of the brain discussed here, but I thought between several images that should give a better idea of where these areas are located.

****Bremmer, J. Douglas, Penny Randall, Tammy Scott, Richard Bronen, John Seibyl, Steven Southwick, Richard Delaney, Gregory McCarthy, Dennis Charney and Robert Innis. 1995. ”MRI-Based Measurement of Hippocampal Volume in Patients with Combat-Related Posttraumatic Stress Disorder.” American Journal of Psychiatry, vol. 152, n.7 (July).