- Like the rest of us, scientists have long wondered what
exactly goes on - or, more precisely, goes wrong - in the minds of murderers.
And, like most of us, many scientists assumed that the real roots of violence
lie in bad environments and abusive parents, a view that is still scientifically
supported, as well as politically correct.
But a growing body of evidence, in particular, from studies that use modern
scanning technologies to look inside the brains of killers, now strongly
suggests that damage, or at least poor functioning, of a particular part
of the brain - the prefrontal cortex, which lies just behind the forehead
and eyes - is often involved in violence.
Though a number of research teams are exploring this, perhaps the most
compelling visual evidence for the link between brain damage and violence
is the work of Adrian Raine, a clinical neuroscientist at the University
of Southern California in Los Angeles.
Among other things, Raine has performed PET scans on 41 murderers and 41
normal people of similar age. In each group, 39 of the 41 people were male.
(PET scans, which measure glucose uptake by brain cells, show which brain
regions are most active.) The murderers, Raine said, had lower glucose
metabolism in the prefrontal cortex, a sign that this region was not functioning
as it should to inhibit aggressive impulses.
The results support previous work by researchers at the University of Iowa
showing that healthy people who suffer damage to the prefrontal cortex
can become impulsive and antisocial.
It is also consistent with years of research by Dr. Dorothy Otnow Lewis,
a professor of psychiatry at New York University School of Medicine and
clinical professor of psychiatry at Yale University's Child Study Center,
and Dr. Jonathan Pincus, chief of neurology at the Veterans Affairs Medical
Center. Based on standard neuropsychological tests, Otnow Lewis and Pincus
have also shown that prefrontal damage is linked to violent behavior.
But murders are clearly not all the same. Significantly, when Raine divided
his murderers into those who committed cold-blooded, premeditated killing
and those who killed impulsively, it was the impulsive killers who showed
the poorest functioning in the prefrontal cortex.
In addition, in murderers' brains, the corpus callosum - a band of tissue
that links the right and left hemispheres - also functioned poorly. This
makes sense, Raine said, because it may mean that the left hemisphere cannot
''talk'' to the more emotional right side, thus allowing aggressive impulses
to go uncontrolled.
Furthermore, the deep brain regions where scientists believe primitive
emotions like fear and aggression originate, were more active in the brains
of murderers than controls. For instance, Raine has used a different type
of brain scan called magnetic resonance imaging, or MRI (which looks at
the structure, as opposed to the functioning, of different brain regions),
to look at people with antisocial personality disorders.
He found that brain cells within the prefrontal cortex regions of these
people with antisocial personality disorder - who show a psychopathic lack
of remorse and a penchant for breaking rules and violent crime - were on
average 11 percent smaller than normal, yet another clue that damage or
dysfunction in this area may predispose people to hostility and aggression.
The basic model for how violence arises in the brain is that the initial
impulses originate in deep regions of the limbic system, or emotional brain.
After that, it's the job of the prefrontal cortex to decide whether to
act on these impulses or not.
Specifically, many violent impulses seem to arise in the amygdala, hypothalamus
and periaquaductal gray area of the brain, said Allan Siegel, a neuroscientist
who studies cats at the New Jersey Medical School in Newark. In fact, he
said, different parts of the hypothalamus, at least in cats, are involved
in different types of violence - the cold, premeditated type (such as stalking
prey) and impulsive acts of rage.
While damage to the prefrontal cortex may help explain impulsive violence
- sudden rage attacks - it truly can't explain violence that is premeditated,
like the Sept. 11 bombings, noted Dr. Ronald Schouten, a lawyer and psychiatrist
who heads the law and psychiatry service at Massachusetts General Hospital.
In other words, the job of prefrontal lobes is ''executive function,''
that is, planning, integrating information and generally serving as a mechanism
to control emotional impulses that originate in deeper brain regions. So,
if a burglar who meticulously planned and executed a bank robbery tried
to argue that he was not responsible because of damage to his prefrontal
cortex, he probably would - and certainly should - be laughed out of court
because it would be quite clear that his prefrontal cortex was working
just fine.
It's also true that, no matter how compelling brain scan data is, the fact
of brain injury or dysfunction by itself cannot explain violence, said
Otnow Lewis of NYU and Yale.
Otnow Lewis, who has studied hundreds of murderers, stresses that most
brain-damaged people are not violent, and that most people with serious
mental illness are not violent, either. What does create a ''cocktail of
violence,'' she said, is when a child with brain damage is raised in an
abusive environment and is also prone to psychosis, or loss of contact
with reality.
It's also worth noting that none of the emerging brain scan evidence on
violence explains the most obvious feature of violence: That, with some
stunning exceptions, it's mostly male, across cultures. Citing FBI data
from 1998, Lewis noted that, in the United States, men are eight times
more likely than women to commit murder, nine times more likely to commit
armed robbery and four to five times more likely to commit aggravated assault.
One theory for this striking gender difference involves the hormone testosterone,
which is more abundant in men than in women. But precisely how testosterone
may trigger the violence circuits in the brain is a mystery. In animals,
considerable data show that aggression is linked to high testosterone and
that castration (removal of the testicles, which produce the male hormone)
decreases aggression.
But things are not so clear with men, Lewis noted, although research does
show that male sex offenders who are castrated are less likely to repeat
their crimes and that men who take body-building steroids, which are chemically
close to testosterone, can become aggressive. Studies of prisoners - both
male and female - also suggest that aggression is linked to high testosterone
levels.
Violent people also may exhibit lower physiological arousal in general,
including more sluggish sweating and skin responses to stimuli, which raises
the intriguing possibility that violence may serve, biologically, as a
kind of rush that jolts the brain toward more normal functioning.
But it's the prefrontal cortex that is attracting the most scientific attention
these days among researchers probing the biological roots of violence.
At the National Institute of Neurological Disorders and Stroke, Dr. Jordan
Grafman, chief of the cognitive neuroscience section, has studied wounded
Vietnam veterans and found that those with penetrating head injuries that
caused damage to parts of the prefrontal cortex, as shown on CT scans,
were at increased risk for violent behavior.
In a still-unpublished study, Marina Nakic, a postdoctoral fellow in Grafman's
lab, has shown that merely viewing violent images is enough to activate
the prefrontal cortex. She showed eight men and women with no history of
violence images of violent, pleasant or neutral interpersonal interactions.
As each volunteer viewed the images, he or she lay in a functional MRI
machine to record brain activity.
Both positive and violent images triggered increases in brain activity,
she said, although in different subregions of the prefrontal cortex. Violent
images activate a region called Brodmann area 10, she said, while pleasant
images activate Brodmann area 32.
While Nakic's work doesn't suggest that just looking at violent images
leads to violent behavior, it does show that the brain responds in clear-cut
ways to perceived violence. In another study from the cognitive neuroscience
section, researchers have shown that normal people who merely imagine committing
an aggressive act show diminished activity in the prefrontal cortex, suggesting
that just thinking about violence may involve some loss of control over
violent impulses.
Some studies have also pointed to the role of serotonin, a key brain chemical.
At Harvard Medical School, Dr. Ed Kravitz, a neurobiologist, has found
that chronic administration of Prozac, which increases serotonin in the
brain, increases the willingness of lobsters that have just lost a fight
to fight again. But whether this is because the losing lobsters are less
depressed and therefore more feisty or are truly more aggressive is not
clear. In fact, it appears that either too much or too little serotonin
may increase aggression, he noted.
In humans, Kravitz said, there is some data suggesting that Prozac increases
aggression, but this hasn't held up in statistical tests.
The bottom line? Violence, even in critters as seemingly simple as lobsters,
is a complex behavior. But the more researchers learn about the neurobiology
of this impulse to harm others, the more human beings, with their huge
brains, may find ways to control it. ___
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- Judy Foreman's column appears every other week in Health-Science.
Her past columns are available on Boston.com and www.myhealthsense.com.
Her e-mail address is foreman@globe.com.
© Copyright 2002 Boston Globe Electronic Publishing Inc.
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