The role of the prefrontal cortex in the expression of impulsive- and premeditated- aggression

The notion that there is a relationship between frontal lobe damage and aggressive behaviour has been recognised in the clinical literature for over 50 years. However, although there is evidence for an association between general brain dysfunction and aggression, there is little evidence pertaining to subclinical impairment and the propensity for aggressive behaviour. Further to this, given the functionally heterogeneity of the prefrontal cortex, it is vital to delineate the specific roles of the dorsolateral, orbitofrontal and medial aspects of the prefrontal cortex in the expression of aggression. Two forms of aggression are distinguished: reactive, impulsive-aggression and goal-directed premeditated aggression. While impulsive-aggression is typically described as an emotionally-charged aggressive response characterised by a lack of control, premeditated aggression is considered to be a planned and controlled aggressive display that is instrumental in nature. The qualitative differences between these subtypes of aggression suggest distinct neuropsychological differences mediating the likelihood of their display. The aim of this thesis was to clarify the role of the prefrontal cortex in subclinical impulsive-aggression and premeditated aggression. More specifically, possible executive functioning deficits mediated by the dorsolateral prefrontal cortex, and emotion recognition, impulsivity, and response reversal capabilities mediated by the orbitofrontal cortex were explored. Participants included university undergraduate students identified as having high levels of trait aggression, classified as either predominantly impulsive, or predominantly premeditated in nature. Experiment 1 (n=85) explored possible executive deficits using a battery of neuropsychological measures pertaining to dorsolateral functioning. It was found that impulsive-aggressive individuals performed significantly poorer on measures of cognitive flexibility, planning, problem-solving, and flexibility of verbal thought processes. Experiment 2 (n=87) sought to identify possible deficits in interpretations of facial expressions of emotion and hostile attribution biases. Contrary to expectations, the results indicated that while impulsive- and premeditated-aggressive individuals do not incorrectly interpret emotional expressions, premeditated-aggressive individuals attributed greater levels of aggression to neutral faces. Experiment 3 (n=87) investigated functions of the orbitofrontal cortex, namely impulsivity, response reversal, and decision-making capabilities. No differences between impulsive-aggressive and premeditated-aggressive individuals were found on any of these measures suggesting negligible involvement of the orbitofrontal cortex in subclinical aggression. Overall, the results from this thesis suggest distinct neuropsychological processes in individuals who display predominantly impulsive-aggressive behaviour compared to those who display predominantly premeditated-aggression. While impulsive-aggression may result from executive dysfunction pertaining to the dorsolateral region of the prefrontal cortex, the display of premeditated aggression is related to functioning of the orbitofrontal cortex mediating the interpretation of aggression in others. Such findings have important implications not only in the understanding of the causal features of such behaviour, but also in the development and implementation of successful treatment strategies.