Source: October 1995 issue of Scientific American. Reproduced with permission. Copyright 1995 by Scientific American, Inc. All rights reserved. Link to Scientific American.
Many women and men who have been subjected to severe physical or sexual abuse during childhood suffer from long-term disturbances of the psyche. They may be invaded by nightmares and flashbacks--much like survivors of war--or, conversely, may freeze into benumbed calm in situations of extreme stress. Two recent studies find that survivors of child abuse may also have a smaller hippocampus relative to control subjects. If substantiated, the discovery could fill out the profile of an abuse survivor and help define what constitutes abuse.
Changes in the hippocampus--the part of the brain that deals with short-term memory and possibly the encoding and retrieval of long term memory-- could, researchers suggest, be wrought by hormones flooding the brain during and after a stressful episode. Such alterations are presumably reflected in the psychological aftermath of trauma. Between 10 and 20 percent of adult survivors of abuse are believed to suffer from dissociative disorders or from post-traumatic stress disorder (PTSD): the estimate is uncertain because survivors who do not seek counseling are hard to identify.
Dissociation and PTSD are not sharply separated and often alternate in the same individual. Dissociation, often employed by children who cannot escape from the threat of abuse, is a means of mentally withdrawing from a horrific situation by separating it from conscious awareness. The skill allows the victim to feel detached from the body or self, as if what is happening is not happening to her or him. People with PTSD tend to relive violent memories. They are easily startled, avoid cues that remind them of the original experience and become intensely agitated when confronted with such stimuli.
The two studies of brain changes associated with abuse both used magnetic resonance imaging to measure hippocampal volumes and found the most significant deficits on the left side. Murray B. Stein of the University of California at San Diego compared 22 women who reported severe childhood sexual abuse with 21 control subjects and detected an average volume reduction of 5 percent of the left hippocampus. PTSD and dissociative symptoms were more pronounced in those abuse survivors with a smaller hippocampus.
J. Douglas Bremner and Dennis S. Charney of Yale University matched a control with each of 12 men and five women who had experienced severe abuse and suffered from PTSD. The researchers found a 13 percent reduction in left hippocampal volume. Given the small number of subjects in the studies, and the disparity in their psychological profiles and genders, the similarities in the results came as more of a surprise to the scientists than did the differences. Neither study has yet been peer reviewed.
Bremner also found that the abuse survivors had impaired short-term verbal memory. The result echoes his earlier finding showing impairment of verbal memory in Vietnam veterans with PTSD; the veterans had smaller hippocampal volumes as well. Tamara Gurvits and Roger Pitman of the Veterans Administration Medical Center in Manchester, N.H., reported recently that the left hippocampus was smaller by 26 percent and the right hippocampus by 22 percent in seven Vietnam veterans with PTSD.
The neurochemical mechanisms that might alter the hippocampus remain far from transparent. The brain responds to intense stress by causing adrenaline, noradrenaline, cortisols, opiates and several other hormones to be released into the bloodstream. The chemicals alter neuronal connections and seem to mediate psychological reactions: enhanced noradrenaline levels cause PTSD sufferers to experience flashbacks. The hippocampus is particularly sensitive to high levels of cortisols, which circulate for hours or days after stress. Robert M. Sapolsky of Stanford University has found that in rats, glucocorticoids circulating for months kill neurons and reduce hippocampal volume.
But prolonged stress leads, if anything, to chronically depleted cortisol levels in humans. John W. Mason of Yale has demonstrated that PTSD patients have extreme levels of key hormones: anomalously low cortisol coupled with high adrenaline, noradrenaline and testosterone. Low cortisol is linked with emotional numbing; spasms of high cortisol coincide with disturbing memories. Nevertheless, argues Frank W. Putnam, Jr., of the National institute of Mental Health, childhood stress may lead to initially high and damaging cortisol levels. His ongoing study of about 80 girls, recruited in 1987 within six months of disclosing sexual abuse, reveals initially high plasma cortisol. Although the mean cortisol levels are decreasing from year to year, the total amount of cortisol the victims are subject to may be above average.
"The thermostat is broken," explains Rachel Yehuda of the Bronx Veterans Affairs Medical Center: the feedback systems that control hormone levels appear to be dysfunctional. Putnam suggests that stress floods the brain with cortisol; the brain, in turn, resets the threshold at which cortisol is produced, so that it ultimately circulates at a dramatically low level. But the system remains hypersensitive.
There is, however, one other explanation for the observed hippocampal volume deficits. Both MRI studies were dominated by survivors who suffered from PTSD or dissociation. Therefore, the results strictly apply only to those victims in whom these disorders developed. In particular, Stein emphasizes, those born with a smaller hippocampus could be more vulnerable to acquiring PTSD or dissociation if subjected to extreme stress. (Prior child abuse, it turns out, is a risk factor for development of war-related PTSD in Vietnam veterans.)
If the neurophysiology is mysterious, its interface with psychology is more so. David W. Foy of Pepperdine University notes that within days or weeks of a traumatic experience, therapy seems beneficial in dispelling PTSD. This period, Bremner speculates, could reflect the timescale over which the hippocampus organizes experiences into a person's worldview. Although some functions of the hippocampus are known, its mechanics are poorly understood.
Psychiatrists contend that if repeatedly invoked in childhood, dissociation prevents memories from being integrated into consciousness and can lead to an altered sense of self. Many normal children play with imaginary companions; abused children can use such creative resources to a pathological extent, in extreme cases falling prey to multiple personality disorder (MPD). Adults may continue to use dissociation as a coping mechanism. Once dissociation or PTSD develops, the majority of psychological symptoms and the hormonal profile are very resistant to treatment.
Ninety-seven percent of psychiatrists believe in dissociative disorders, which have a strong presence in the Diagnostic and Statistical Manual of Mental Disorders. Fourth Edition (DSM-VI). But their link with MPD, and their implicit provision of a mechanism for memory suppression, has made them controversial. PTSD, too, has detractors: the condition was defined for diagnosing Vietnam veterans. "It's not the same clinical picture in l0-year-old girls," Putnam points out. Several clinicians argue for a classification for dissociation and PTSD as related specifically to child abuse.
Thus, the findings, although helping to ground psychology in biology, raise more questions than they answer. "The last thing we want is for clinicians to be telling patients. 'You have a smaller brain,'" Yehuda warns. "There is a knee-jerk reaction: big brains good, small brains bad." The real story is more complex, but no more so than humans themselves.