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Neurobiology of Impulsive Aggression

During the first half of the 20th century, studies were conducted in cats, specifically in a region of the posterior hypothalamus. It was determined that, as this region was destroyed and separated from its connections with centers, it made an aggressive behavior of rabies disappear.

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Aggression is a complex and heterogeneous construct. Mainly, we can distinguish between two types of aggression: premeditated aggression (predatory, instrumental) and impulsive aggression (affective, reactive). Here, we will focus on describing the neurobiology of impulsive aggression.

For the author Stahl, impulsive aggression may reflect “an emotional hypersensitivity and an exaggerated perception of threats. This may be linked to an imbalance between top-down cortical inhibitory controls and bottom-up limbic impulses.”

It seems that, in general, unrestrained impulsive aggression has a lot of activity in the amygdala area. In turn, it has little inhibitory activity in the area of ​​the prefrontal orbitofrontal cortex (COF). On the contrary, when a person tries to control his impulsive aggression, the activity increases in the COF. But the question is, where does the aggressive behavior in the central nervous system come from?

Hypothalamus and periaqueductal gray matter

During the first half of the 20th century, studies were conducted in cats, specifically a region of the posterior hypothalamus. It was determined that destroying this region also “destroyed” an aggressive behavior of rage (false rage) that did not seem to be associated with real anger. In addition, this behavior was not always directed towards the stimulus that had triggered it. When stimulated, said region provoked this behavior of rage.

Studies on the neurobiological basis of aggression in cats have led to the description of:

  • An affective attack, characterized by typical emotional responses of the sympathetic nervous system.
  • A predatory attack, without these typical emotional responses.

The affective attack

This can be controlled from a large extension of the medial hypothalamus. This extends to the trunk of the brain where nerve centers are located that control the expression of the attack, such as snorting and growling (6). In the affective attack may also be involved:

  • The medial amygdala. From it, the hypothalamus receives exciting information.
  • The dorsal periaqueductal gray matter of the brainstem. The hypothalamus sends you exciting information. In addition, from this substance there are excitatory connections with the locus coeruleus and the solitary nucleus that mediate the autonomous responses during the affective attack.

The predatory attack

This type of attack is controlled by the brain from the lateral hypothalamus. Also from regions of the brainstem, such as ventrolateral periaqueductal gray matter, among others. In addition, the lateral hypothalamus receives excitatory information from the central and lateral amygdala and inhibitory from the medial amygdala. In this investigation, it is determined that both circuits inhibit each other. It happens that, while the cat is performing a “predatory attack”, it can not carry out at the same time an “affective attack”.

Haller (2014) states that the mechanisms described for the cat in the hypothalamus, periaqueductal gray matter and other centers such as the amygdala can also function similarly in humans. To this, the prefrontal cortex is added as a substrate for psychological factors.

The limbic structures (amygdala, hippocampal formation, septal area, prefrontal cortex and cingulate gyrus) strongly modulate the aggression through its connections with the medial and lateral hypothalamus.

The amygdala

It seems that there is a clear implication of the amygdala in aggressive behavior. For example, in violent psychopathic subjects, a significant reduction in the volume of gray matter in the amygdala has been found in several studies (8, 9). However, it seems that other studies show the opposite (1). What seems certain is that the amygdala has a role in the aggression. However, it is not well known if its size increases or decreases when it occurs.

As for the activation of the amygdala, studies have been conducted in psychopaths that show lower levels of activity in the amygdala when they see violent images.

The prefrontal cortex in the neurobiology of aggression

What happens functionally in the CPF of violent subjects? A study by PET carried out in predatory (psychopathic) and impulsive killers and in normal neurological and behavioral subjects provided the following

it is results in relation to the type of aggression and the activity of the prefrontal cortex:

  • The impulsive assassins had lower prefrontal activity and greater subcortical activity in the temporal lobe (which contains the amygdala) compared to the control subjects.
  • The predatory killers had a prefrontal activity similar to that of the control subjects but they had an excessive succortical activity.
  • It seems, in general, that violence causes, at least, a strange functional activity in the prefrontal cortex.

In general, studies on the neurobiology of aggression indicate subcortical structures such as the amygdala and other cortical structures responsible for this behavior. It seems, then, that although the studies are inconclusive, they suggest that violent behavior could be the result of a dysfunction in cortical and subcortical activity.

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