Many structures have been implicated with aggressive behavior. Damage to these structures or abnormal activity associated with them enhances aggression. This webpage will focus on the areas which are most important.
Click here to learn more about the structure of the brain and its components
The Limbic System
The first area discussed will be the limbic system, particularly the hypothalamus and the amygdaloid complex, which have been implicated in many situations. One of the first associations between rage and the limbic system was the appearance of inclusion bodies in the hippocampus of patients with rabies. A few years later, an attorney with substantial damage to the hypothalamus was reported as chasing visitors from his home! University of Texas mass murderer Charles Whitmann was also found to have a lesion in the median part of the temporal lobe.
Click here to see where the amygdala is located.
Click here to see where the hypothalamus is located.
Electrical recordings have shown that the amygdala is responsive to a variety of sensory stimuli - visual, auditory, tactile, olfactory, and gustatory - and its connections allow it to monitor events in the environment - external stimuli - and control neural circuits in the hypothalamus involved in aggressive behavior. The amygdala also plays a role in new learning and memory, suggesting that it may be particularly relevant for recall of the affective significance of stimuli (Bear 1991.)
A study of amygdala lesioning on 60 individuals with either epilepsy or behavior disorders was conducted. The results showed 29 patients who greatly improved (calm, obedient, social adaptation possible), 22 patients who moderately improved (calmer, easier to control), 7 patients who slightly improved (reduced excitation), and 2 whoshowed no change (Eichelman 1983,) supporting the role of the amygdala in producing aggression. The patients, through the forgetting of the affective factors of their stimuli and consequent inactivation of further neural circuits, no longer displayed aggressive behavior.
An increased activity in the amygdala would present a situation where there is an enhanced emotional response to stimuli and where the associations between stimuli and drive are fortuitous (Bear 1988.) This is evident in the display of bizarre sexual fetishes for safety pins in temporal lobe epileptics, who have increased amygdala activity (Mitchell 1954.)
Another structure in the limbic system, the hypothalamus, is involved in aggression. The hypothalamus receives input from internal sources: the mouth, heart, intestines, and stomach; and from osmotic and hormonal concentration of the bloodstream - internal stimuli, and also information from the amygdala. It in turn sends the messages on to the autonomic nervous system (that which controls internal organs when the body is in a vegetative state), the pituitary gland controlling hormone release, and midbrain and spinal motor centers that elicit stereotypic movements such as crawling and stepping.
Electrical stimulation or lesioning of the medial hypothalamus will turn animals into savage beasts. Evidence for this comes form a study by Glusman which demonstrated that shortly after the onset of electrical stimulation in this area, the response began in cats with baring of the teeth, snarling, extension of the claws, and finally repeated attacks on the observer (Glusman 1986.) Lesions in humans have been seen to have similar effects (Bear 1988.) Patients will impulsively attack attendants. The patients are not upset with the attendant, nor are they directing their actions at one specific person. They are directly simply at whoever is present at this time. Although the patients may later show remorse later on, they are unable to control their actions at the time. It is possible that lesions in this area distort sensation so that normally innocuous stimuli are perceived as aversive.
Because of its relationship to memory mechanisms and its proximity to the neocortex, it is also possible that it is the means by which volitional activity and past experience modify aggressive responses.
Localized seizure activity in this area is associated with aggressive outbursts. This abnormal activity can be elicited by angry thoughts, which is consistent with the finding that specific thoughts can alter brain activity (Fenwick 1991.)
The Prefrontal Cortex
The prefrontal cortex is located at the very front of the brain.
Click here to see where the prefrontal cortex is located.
It receives information from multiple areas: primarily the area associated with the survey of extrapersonal space for relevant stimuli, the hypothalamus, and the limbic system. This cortex then directly outputs to the neostriatum, which makes possible intricate actions such as speech and learned complex movements. "Schematically, the prefrontal cortex appears to integrate a current account of the outside world, the state of the internal milieu, and the appearance of biologically relevant stimuli in the light of previous experiences relating to reward or punishment" (Bear 1989.) It allows an individual to contemplate the effects of his actions on another, and the consequences likely of those actions: it allows an individual to possess judgement and to modulate or inhibit aggressive responses in a socially appropriate way.
Damage to the back of this cortex results in the inability to plan long-term and an apathetic state of mind. Damage to the orbital underside is associated with fast, reflexive emotional responses to stimuli in the immediate environment. This patient will predictably strike out after little or no provocation, and not consider the consequences of this or the social rules which limit aggressive behavior. For example, a woman who suffered a lesion here from multiple sclerosis became to irritated with the cries of her son that she burned him with cigarettes on frequent occasions and felt no remorse.
Integration
Multiple areas are often affected by such events as head injuries. Henry Lee Lucas, a serial killer, suffered a head injury leading to contusions of both the limbic system and the prefrontal cortex. He is known to have murdered more than ten individuals, mostly women, whom he first sexually molested (Bear 1989.) In fact, murderers have been demonstrated by PET scans to have lower prefrontal activity and higher hemispheric subcortical (limbic system area) functioning (Raine 1998.) These results support the hypothesis that emotional and unplanned impulsive murderers are less able to control aggressive impulses produced by subcortical structures due to lower prefrontal regulation.
The knowledge that aggressive behavior may be a result of lesion or stimulation to one or a combination of these areas allows us to ask a few questions as to what may have characterized the aggressive act (an important act when one considers that serial killers usually are not provoked by their victim): Were they motivated by an internal state such as hunger or fatigue or hormonal stimulation? Were they motivated by abnormally broadened emotional responses to external stimuli? Was the act a result of the failure to recognize social rules and consider long term effects?
This information indicates that specific neuroanatomical lesions may lead to violent behavior in human beings, but in no way indicates that it is the only cause for such behavior. Other factors must also be considered.
In the Mind of a Serial KillerReferences