In any kind of emotionally arousing experience, the brain takes advantage of the fight-or-flight reaction, which floods cells with two powerful stress hormones, adrenaline and noradenaline. "We believe that the brain takes advantage of the chemicals released during stress and powerful emotions," says James McGaugh, "to regulate the strength of storage of the memory." These stress hormones stimulate the heart to pump faster and the muscles to tense; they also act on neurons in the brain. A memory associated with emotionally charged information gets seared into the brain. We owe our very lives to this: a dangerous, threatening, or exciting event needs to be recalled well so that we may take precautions when meeting similar danger again.
Scientists are now beginning to understand just how emotional memory works and why it is so powerful. According to Joseph Ledoux, Ph.D., of the Center for Neural Science at New York University, the hormones associated with strong emotion stimulate the amygdala, an almond-shaped organ in the brain's cortex.
It's long been known that when rats are subjected to the sound of a tone and a shock, they soon learn to respond fearfully to the tone alone. The shocker is that when the auditory cortex--the part of the brain that receives sound is completely destroyed, the rats are still able to learn the exact same fear response. How can a rat learn to be afraid of a sound it cannot hear?
The tone, it appears, is carried directly back to the amygdala, as well as to the auditory cortex. Destroy the amygdala, and even a rat with perfect hearing will never learn to be afraid of the sound. As neurologist Richard Restak, M.D., notes, this "implies that much of our brain's emotional processing occurs unconsciously. The amygdala may process many of our unconscious fear responses." This explains in part why phobias are so difficult to treat by psychotherapy. The brain's memory for emotional experiences is an enduring one.
But the ability of the brain to utilize stress hormones can go badly awry--and a memory can become not simply permanent but intrusive and relentless. "Suppose somebody shoots you and years later you're still waking up in a cold sweat with nightmares," says McGaugh. "The hormonal regulation of memory, when pushed to an extreme in a traumatic situation, may make memories virtually indelible."
Such memories seem so powerful that even an innocuous stimulus can arouse them. Roger Pittman compares the inescapable memories of PTSD, where flashbacks to a nightmarish trauma intrude relentlessly on daily life, to a black hole, "a place in space-time that has such high gravity that even light cannot pass by without being drawn into it."
So with ordinary associations and memories in PTSD: "As all roads lead to Rome, all the patient's thoughts lead to the trauma. A war veteran can't look at his wife's nude body without recalling with revulsion the naked bodies he saw in a burial pit in Vietnam, can't stand the sight of children's dolls because their eyes remind him of the staring eyes of the war dead."
The tragic twist is that, Pittman believes, each time a memory floods in again, the same stress hormones are released, running the same neural paths of that particular memory and binding the victim ever tighter in the noose of the past. Yet in response to the stress of recalling trauma, the body releases a flood of calming opiates. These neurochemicals, which help us meet the immediate demands of stress and trauma, might create a kind of unfortunate biochemical reward for the traumatic memory. "This whole question of an appetitive component to trauma is really fascinating and as yet unexplored," notes Pittman. "It may explain the intrusive, repeating nature of these memories. Maybe, however horrible the trauma, there's something rewarding in the brain chemicals released."
A solution, then, to treating the kind of PTSD we see in war veterans and victims of rape and child abuse, might lie in blocking the action of some of these stress hormones. And perhaps a key to enhancing ordinary learning is to create a judicious amount of stress--excitement, surprise, even a healthy dose of fear (like the kind one may feel before cramming for a demanding final exam).
A landmark study recently reported by James McGaugh and Larry Cahill, in Nature, indicates that any emotion, even ordinary emotion, is linked to learning. They gave two groups of college students a drug that blocks the effects of adrenaline and noradrenaline, then showed the students a series of 12 slides that depicted scenes such as a boy crossing the street with his mother or visiting a man at a hospital. A control group was told an ordinary story (son and mother visit the boy's surgeon father) that corresponded to some of the slides. The experimental group heard a story of disaster (boy is hit by car; a surgeon attempts to reattach his severed feet).
Two weeks later, the volunteers were given a surprise memory test. Students who heard the ordinary story recalled all 12 slides poorly. The second group, however, recalled significantly better the slides associated with the story of disaster.
Then, in an ingenious twist, McGaugh and Cahill repeated the experiment with new volunteers. Just before the slide show, the experimental group was given a beta blocker--a drug that acts on nerve cells to block the effect of stress hormones. Two weeks later they could not be distinguished from the control group. They similarly remembered all 12 slides poorly.
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