The Neurophysiology of Human Aggression

 

By Cynthia Lofaso

Fear and aggression serve a distinctive purpose in survival. The primary part of the brain involved in these emotional responses is the amygdala which is located in the temporal lobe (Carlson, 2010). The amygdala is divided into twelve regions, three of which are central to the understanding the physiology of fear. These areas are the lateral nucleus (LA), the central nucleus (CE) and the basal nucleus (B). The mechanism for communication between these three areas begins with the LA which receives information from the neocortex. The LA sends information to the B which then sends information to the CE. The activation of the CE plays a critical role in many emotional responses that come about as a result of threat (Debiec & LeDoux, 2004). When stimuli are perceived to be threatening, neural activity of the CE increases and produces Fos protein. Many brain regions are affected by the activated CE and result in biological precursors to an emotional response. The diagram below shows some of these brain regions.

Conditioned responses play a significant role in the fear response. In examining the amygdala in this process, researchers have determined that the biological response comes about from the LA of the amydgala. Specifically, the sequencing from the LA to the CE stimulates regions in the hyopathalamus, pons and medulla that stimulate the autonomic, hormonal and behavioral response (Carlson, 2010). The stimulation of these areas of the brain results in heart rate changes, dilation of the pupils, and either a freeze or startle response. The cerebellum also plays an important role in the conditioned fear response. This part of the brain is significant in specific conditioned responses such as the eye blink or other somatic motor movements that are involved in avoidance of an aversive stimulus (Lavond, Kim & Thompson, 1993). Thus, in a situation in which fear has been associated with an external stimulus, the learning response originates from the LA, starting a physiological sequencing that ends with the emotional expression of fear.

While the amygdala is responsible for triggering the physiological fear response, it appears that areas of the prefrontal cortex are involved in the interpretation of the stimulus. Thus, the prefrontal cortex actively assesses the stimulus for threat and then signals the amygdala to actively respond or not. The following diagram illustrates the cycle.

Stimulus  to         Amygdala
                                                                        Prefrontal Cortex

As the information passes through the amygdala first and is then interpreted by the prefrontal cortex, it is possible to experiences some of the biological fear response even when no threat is present.

The amygdala and the prefrontal cortex also play a crucial role in aggressive behavior. These areas of the brain, as well as the hypothalamus are critical to the key circuitry that underlies emotional regulation. Abnormalities in these areas lead to a decrease in impulse control and can increase impulsive aggression (Damasio, 2007). In a study by Wasman and Flynn (as cited byTedeschi & Felson, 1994) cats that coexisted with rats had electrodes implanted in the lateral hypothalamus. Electrical stimulation caused the cats to attack and kill rats that they had previously ignored. The cats also became highly aggressive against the researchers when the medial hypothalamus was stimulated.

Similar results were found in studies of rhesus monkeys (Robinson, Alexander & Bowne, 1967 as cited by Tedeschi & Felson, 1994).  Environmental factors also played a role in the aggressive of the monkeys, however with monkeys only displaying aggression when exposed to female monkeys or a dominant male. 

Serotonin has also been found to play a significant role in aggression (Damasio, 2007). Serotonin inhibits impulsive aggression. Lows levels of this neurotransmitter are associated with a decrease in impulse control and high levels of aggression. Measurements of 5-hydroxyindoleacetic acid (5-HIAA), a chemical related to serotonin that is found in the cerebral spinal have indicated this relationship. Low levels of 5-HIAA have been found in aggressive psychiatric patients and have been found to be good predictors of aggression in boys with conduct disorder (Damasio, 2007). Low levels of this chemical have also been found to be associated with impulsive violent offenders and impulsive arsonists.

Further biological involvement stems from the ventromedial prefrontal cortex (vmPFC), an area of the brain that is important in the regulation of our emotional responses to frustration. The vmPFC includes the medial orbitofrontal cortex and the sugenual anterior cingulated cortex and receives information from the dorsomedial thalamus, temporal cortex, olfactory system and the amygdala (Carlson, 2010). The vmPFC send out information to the hippocampus, temporal cortex, lateral hypothalamus the amygdala and the dorsolateral prefrontal cortex (diPFC). The vmPFC is significant in this cycle of neural communication and plays an important role in control of emotional behavior. The famous case study of Phineas Gage was a tremendous introduction to the importance of this structure in emotional regulation. The vmPFC was destroyed in a accident leaving him alive and functional but unable to control himself. He went from a well-balanced energetic man to one who was impatient, antisocial, obstinate, unpredicatable and completely unrestrained in his behavior (Helfgott, 2008). Modern day research has brought great understanding to the neural damage suffered by Phineas Gage and the importance of the vmPFC in emotional regulation (Carlson, 2010).

There is research to suggest that androgens are associated with aggression, however cause and effect have not been determined and the aggressive actions appear to be situational (Helfgott, 2008). Rat studies have demonstrated an increase aggression for both males and female with high levels of testosterone. In humans, however, the result is not as straightforward. This relationship between testosterone appears to be dependent on and intertwined with personality and situational factors. The effect of androgens in humans may be to increase the potential for aggressive behavior, but these behaviors are only activated under certain circumstances.

Biology alone cannot determine aggression in humans. Environmental influences, including prenatal experiences and upbringing are critical in the understanding of violent behavior. Treatments, thus must not be aimed solely at the biological manipulation, but must be behaviorally focused. Early intervention for high risk youth and families is critical.

References

Carlson, N.R. (2010). Physiology of behavior (10th ed.). Boston: Pearson Education.

Damasio, A. (2007). Violence and aggression. The Dana Guide to Brain Health

Debiec, J. & LeDoux, J. (2004). Fear and the brain. Social Research 71(4), 807 812.

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Helfgott, J.B. (2008). Criminal behavior; theories, typologies and criminal justice (1st ed.) London: Sage Publications.

Lavond, D.G., Kim, J.J. & Thompson, R.F. (1993). Mammalian brain substrates of aversive classical conditioning. Annual Review of Psychology 44, 317 326.

Mohammed, M.R. & Quirk, G.J. (2002). Neurons in medial prefrontal cortex signal memory for fear extinction. Nature 420, 70 74.

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University of Iowa (2005). Scientists uncover new mechanism for the amygdala in fear recognition. Science Daily. Retrieved March 26, 2009, from http://www.sciencedaily.com/releases/2005/01/050106110624.htm