Abstract:

Aggression can be defined as any behavior directed toward another with the intent to cause direct harm. It can be divided into reactive aggression and proactive aggression, which differ in precipitating factors, motivations and emotional characteristics. Aggression has an underlying neurophysiological basis. As a key region of emotion processing and learning, the amygdala is closely associated with aggression. The fight-flight mechanism, the violence inhibition mechanism and the fear dysfunction hypothesis emphasize the importance of the amygdala for aggression. Structural deficits and dysfunctions of the amygdala have been observed in individuals who display aggressive or violent behaviors, with two different abnormal manifestations. In groups with high risk of reactive aggression, the amygdala’s response to threatening stimuli is enhanced (a “hot” response). However, in proactive aggression, exhibited by individuals with psychopathic and callous-unemotional traits, researches have reported diminished amygdala responses to threatening stimuli and others’ distress cues as well as insufficient activation of the amygdala during fear conditioning learning and moral decision-making (a “cold” response). These dysfunctions might impair the normal function of individuals in terms of threat response, empathy, punishment avoidance and moral decision-making. Future research should investigate the following four aspects. First, it seems that population-based studies are more important than process-based studies in previous researches. However, the two types of aggression are not opposites; thus, a distinction based on these populations will not provide a true comparison. Therefore, studies should directly explore the neural response during aggression with relative paradigms. At present, brain imaging paradigms used to investigate reactive aggression are well developed; however, proactive aggression paradigms still need further development and innovation. Second, the amygdala may exhibit different activations according to the functions of its substructures, which have not been deeply explored. Previous brain imaging studies have usually investigated the amygdala as a single structure. However, the substructures of the amygdala differ in function. More detailed studies of the substructures can facilitate to accurately locate the neural targets associated with the two types of aggression. Third, the abnormalities may be caused by deficits in other brain regions that provide input to the amygdala or abnormal anatomical connections with the amygdala. It is necessary to interpret the role of the amygdala in the context of the brain functional network. According to dual system theory, further research should consider the stage characteristics of neural development during the investigation of the above network. Finally, from the perspective of prevention, the structural deficits and dysfunctions of the amygdala can serve as potential indicators of violence recidivism; however, further clarification of the role of amygdala substructures in aggression and their connectivity with other brain regions is needed. From the perspective of intervention, studies should explore appropriate interventions based on the characteristics of the aggressive population. For example, research may incorporate deep brain stimulation and other technologies to treat aggressive behavior in individuals with psychotic traits, while exploring noninvasive interventions such as oxytocin for violent offenders. In addition, the social factors that influence the brain-aggressive behavior associated should not be overlooked.

Key words: amygdala, reactive aggression, proactive aggression, psychopathy