Can 'faulty wiring' lead to impulsive violence?

Faulty brain circuitry may underlie impulsive aggression and violence, according to a research review by Richard Davidson and colleagues.

The researchers analyzed data from brain imaging studies involving more than 500 violent subjects, including convicted murderers, people with childhood brain injuries, and people with aggressive personality disorders. The evidence, they say, indicates tha at defects in neural circuitry involving the prefrontal cortex, anterior cingulate cortex (ACC), and amygdala may cause some people to commit violent assaults for little or no reason.

"Normal individuals are able to voluntarily regulate their negative affect and can also profit from restraint-producing cues in their environment, such as facial and vocal signs of anger or fear, that also serve a regulatory role," the researchers say. "W We suggest that individuals predisposed to aggression and violence have an abnormality in the central circuitry responsible for these adaptive behavioral strategies."

Davidson et al. hypothesize that the prefrontal cortex normally inhibits violent responses by modulating the activity of the amygdala, a brain region activated in response to fearful or emotional situations. The ACC plays a role as well, by recruiting oth her brain areas to respond during conflicts. Defects in any of these areas, they say, could lead to impulsive violence.

"Abnormalities in serotonin function in regions of the prefrontal cortex may be especially important," the researchers say, noting that the prefrontal cortex has a high density of serotonin receptors. Serotonin appears to exe ert inhibitory control over impulsive aggression, they say, with studies showing that reduced cerebrospinal fluid (CSF) levels of the serotonin metabolite 5-HIAA predict future aggression in conduct-disordered boys and recidivist criminal adults. (see related article, Crime Times, 2000, Vol. 6, No. 4, Page 1).

The researchers also point to studies using a fenfluramine challenge to measure serotonin function in normal subjects and those with aggressive impulsive personality disorder. Normal subjects showed increased glucose metabolism in the prefrontal cortex in n response to fenfluramine, while personality-disordered subjects did not-evidence of dysfunctional serotonin activity. "These findings," they say, "imply that an important site of serotonergic abnormality in subjects with impulsive aggression is the pref frontal cortex." Additional research implicating the prefrontal cortex, they say, includes:

• PET studies showing prefrontal abnormalities in murderers (see related article, Crime Times, 1997, Vol. 3, No. 4, Page 1).
• case studies showing that early damage to the prefrontal cortex can cause unpredictable, explosive bursts of violence or aggression (see related article, Crime Times, 2000, Vol. 6, No. 1, Page 3).
• recent research showing that individuals with antisocial personality disorder who exhibit impulsive violence have significant reductions of prefrontal gray matter volume (see related article, Crime Times, 2000, Vol. 6, No. 2, Page 1).

The researchers say that it may be possible to reduce impulsive violence in individuals with faulty neural circuitry by combining pharmacological approaches with training to help affected individuals learn to suppress their negative emotions.

"Dysfunction in the neural circuitry of emotion regulation-a possible prelude to violence," Richard J. Davidson, Katherine M. Putnam, and Christine L. Larson, Science, Vol. 289, No. 5479, July 28, 2000, pp. 591-594. Address: Richard J. David dson, Laboratory for Affective Neuroscience and W. M. Keck Laboratory for Functional Brain Imaging and Behavior, University of Wisconsin-Madison, Madison, WI 53706.