Neuro-Physiology of Violence: Primer

“Physical aggression belongs to the natural behavioral repertoire of almost all mammalians. From an evolutionary point of view, it is plausible that animals and humans can exert violence under certain conditions with an appetitive component. Killing of weaker conspecifics improves the perpetrators reproduction rates. Therefore, the genes of the successful violent individual will more likely be passed on in contrast to the genes of the victim. Enjoying the act of killing increases the likelihood of this behavior and may have an evolutionary advantage by transmission of genes predisposing to appetitive aggression.

The so far common classification of aggression distinguishes between reactive/impulsive (defensive rage) and proactive/instrumental (predatory attack) aggression.

  • Individuals engage in reactive aggression to protect themselves against a real or assumed threat.
  • People engage in proactive aggression to intentionally reach a specific goal, for example, to dominate their victim or to achieve a material gain.

Both forms of aggressiveness can co‐exist. An aggressive act may include reactive and proactive as well as appetitive elements.

Appetitive aggression in animals
Investigations in animals provide indications for a relationship between physical aggression and dopaminergic activation in the reward system…aggressive behavior goes along with dopamine release in the nucleus accumbens reflecting a reward effect.

…In cats, reactive violence occurs when the medial amygdala is activated, followed by an activation of the medial hypothalamus via the stria terminalis which, in turn, leads to a subsequent activation of the periaqueductal gray in the mesencephalon.

  • Proactive violence is caused by an activation of the lateral amygdala leading to an activation of the lateral hypothalamus via ventral amygdalofugal fibers.
  • In rats, hypoactivation in the cortical orbitofrontal cortex, being involved in cognitive control, as well as hyperactivation in the subcortical nucleus accumbens lead to impulsive behavior.
  • This cortical‐subcortical imbalance of the reward‐related areas impairs the inhibition of behavior that is rewarding. In fact, this behavior might reflect not only impulsive but also appetitive violence. In primates, the amygdala shows increased activity after delivering the reward (fruit juice). The activation in the amygdala correlates positively with the reward’s amount coding reward magnitude.

Appetitive aggression in humans
So far, appetitive aggression in humans has predominantly been linked to mentally abnormal behavior. For example, people with sexually sadistic traits enjoy harming others. Functional deficits in the orbitofrontal cortex of psychopaths have been frequently described. Further investigations of subjects from the community suggest a hypersensitivity of the reward system as a functional correlate of impulsivity and antisocial behavior; subjects displaying stronger impulsive and antisocial traits show a hypersensitive dopaminergic reaction in the nucleus accumbens. Increased activation in the ventral tegmental area when watching violent videos is evident in individuals with stronger interpersonal and affective deficits.

…The detected relationship between appetitive aggression and less activation in the right parietal‐temporal area is interpreted as reduced empathy for the victim so that appetitive violence could occur. More specific neurobiological correlates of appetitively motivated perception of violence in subjects from the community have not been described so far although this form of violence is omnipresent in the media and the real world (e.g., hooliganism).

Source: neuroscience

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