Aggression is a complex and heterogeneous behavior. We can essentially distinguish two types of aggression: premeditated aggression (predatory and instrumental) and impulsive aggression (affective and reactive). We will focus here on the neurobiology of impulsive aggression.
For author Stahl, impulsive aggression may reflect “ emotional hypersensitivity and exaggerated perception of threats. This may be related to an imbalance between top-down cortical inhibitory controls and bottom-up limbic impulses. ”
Roughly speaking, it would appear that unrestrained impulsive aggression has great activity in the amygdala area. It also exerts a weak inhibitory activity in the area of the orbitofrontal cortex (COF). But when a person tries to control their impulsive aggression, activity increases in the area of the COF. So the question we are asking ourselves here is: within the central nervous system, where does aggressive behavior come from?
Impulsive aggression: hypothalamus and periaqueductal gray matter
During the first half of the 20th century, studies on the subject were carried out on cats. More particularly at the level of their posterior hypothalamus. The researchers concluded that the destruction of this area destroyed aggressive rage-like behavior (false rage) that did not appear to be true anger. Moreover, this behavior was not always directed towards the stimulus causing the rage. A stimulus in this area caused this raging behavior (2, 3).
Studies on the neurological basis of aggression in cats have led to the following description (4, 5):
- An affective attack characterized by emotional responses typical of the sympathetic nervous system
- A predatory attack without these typical emotional responses
Affective attack, an expression of impulsive aggression
The affective attack can be controlled from a large extension of the medial hypothalamus. This extension extends to the brainstem where the nerve centers that control the expression of the attack (bitching, grumbling) are located (6). These two other areas can be involved in the emotional attack:
- The medial amygdala
- It sends stimulating information to the hypothalamus
- The dorsal periaqueductal gray matter of the brainstem
- Hypothlamus sends stimulating information to this substance. Moreover, from this substance there are stimulating connections with the locus cœruleus and the solitary nucleus which mediate the autonomic responses during the affective attack (6)
The predatory attack
It is the brain that controls this type of attack from the lateral hypothalamus and from other regions of the brainstem such as the ventrolateral periaqueductal gray matter, among others. In addition, the lateral hypothalamus receives stimulating information from the central and lateral amygdala, and inhibitory information from the medial amygdala. As part of this research, the researchers concluded that these circuits inhibit each other ; when the cat was performing a predatory attack, it could not perform an emotional attack at the same time.
Haller (2014) argues that the mechanisms described in cats at the level of the hypothalamus, periaqueductal gray substance and other centers such as the amygdala can function similarly in humans. It adds to these areas the prefrontal cortex as a substrate for psychological factors.
Limbal structures (the amygdala, hippocampal formation, setum, prefrontal cortex, and convolution of the cingulum) strongly modulate stress through their connections to the medial hypothalamus and lateral hypothalamus (7)
Aggressive impulse: the role of the amygdala
Obviously, the amygdala is clearly involved in the aggressive behavior. For example, in violent psychopathic subjects, several studies show a significant decrease in the volume of gray matter of the amygdala (8, 9). But it is true that other studies show the opposite (1). However, it seems certain that the amygdala plays a role in the aggression. However, it is not yet clear whether it increases or decreases in size when the assault occurs.
As for the activation of the amygdala, several studies have been carried out on psychopaths. These studies have shown lower activity levels in the amygdala while viewing violent images (1).
The prefrontal cortex in the neurobiology of aggression
On the relationship between type of aggression and prefrontal cortex activity, a positron emission tomography (PET) study of predatory (psychopathic) and impulsive assassins, as well as subjects normal from a neurological and behavioral point of view (10), brought the following results:
- Impulsive killers exhibited less prefrontal activity and greater subcortical activity in the temporal lobe (location of the amygdala) compared to other subjects
- Predatory assassins exhibited similar prefrontal activity to other subjects, but also excessive subcortical activity
In general, therefore, it would appear that violence causes at least one strange functional activity in the prefrontal cortex.
Studies on the neurobiology of impulsive aggression show that subcortical structures such as the amygdala are responsible for this behavior. Although these studies are inconclusive, they suggest that the violent behavior is the result of dysfunction in cortical and subcortical activity.