Role of the amygdala

Structure and function of the amygdala

• The amygdala is a structure in the brain made up of grey matter, a collection of neuron cell bodies densely packed together into a cluster of 13 nuclei located in the medial temporal lobe of the brain, and is a part of the limbic system.

• There are in fact two amygdalae, one per brain hemisphere. The amygdala was first implicated in emotional behaviour by James Papez (1937) and later Paul Maclean (1952).

• It is highly connected: it is neurally linked to the hypothalamus, the hippocampus, and the prefrontal cortex as well as the other parts of the brain. Therefore it has a widespread influence on brain functioning and ultimately on behaviours associated with emotion, motivation and social interaction in both human and non-human animals. It plays a major role in how we assess and respond to environmental threats, hence its importance in determining aggressive behaviour.

Amygdala and aggression

• Coccaro et al  investigated the effects of the amygdala on aggression by studying people with intermittent explosive disorder (IED), a common symptom of which is outbursts of reactive aggression.

• Each participant viewed images of faces at the same time as having an fMRI scan of their brain. There was a key difference between these participants and non-IED participants. The IED participants showed high levels of amygdala activity when they viewed angry faces.

• So, this finding demonstrates an association between amygdala activity and processing of aggressive emotions – and the study had high realism as an angry facial expression is an everyday signal of threat.

Amygdala and fear condtioning

Yu Gao et al (2010) - we learn as children to inhibit aggressive and antisocial behaviours through fear conditioning – we learn that aggressive behaviour leads to punishment or other negative outcomes. We know that the amygdala is involved in processing fear information and fear conditioning. A dysfunctional amygdala means that the child cannot identify the social cues that indicate threat and therefore does not link punishment to their aggressive behaviour. Fear conditioning is disrupted and the outcome is that individuals with amygdala dysfunction seem fearless, overly aggressive and antisocial. A longitudinal study where 1,795 participants were tested for fear conditioning at the age of three years. The measure used was psychological arousal (indicated by sweating) in response to a painful noise. Twenty years later, the researchers found out which participants were involved in criminal behaviour. Those who had committed crimes at the age of 23 years had shown no fear conditioning when they were three years old. They were effectively ‘fearless’. This suggests there may be a causal relationship between amygdala dysfunction and antisocial/criminal behaviour.

Evaluation - Supporting evidence

Gospic et al (2011) - Ultimatum game, a laboratory based method of measuring aggressive behaviour. It involves two players, the Proposer and the Responder, and a sum of money. The proposer offers to split the money in a fair or unfair way. An unfair offer is considered to be a social provocation. If the responder accepts then the money is split accordingly. However, if the responder refuses the offer then they both get nothing. A rejection is considered aggressive behaviour. Participants played the role of responder while undergoing fMRI scans. It was found that when responders rejected unfair offers, amygdala activity was heightened and quicker. Furthermore, a sedative drug given before the game had two effects: it reduced aggression by halving the number of rejections and at the same time reduced amygdala activity. This is strong evidence of an association between reactive aggression and increased amygdala activity.

Evaluation - support from longitudinal studies

Pardini et al (2014) selected 503 males who had been part of an earlier study when they were six or seven years old. Twenty years later the researcher identified a subgroup of 56 men who had shown aggressive behaviour since childhood, including involvement in serious criminal violence. fMRI brain scans were used to measure amygdala volume in these men. It was found that high levels of aggression over the twenty year period were associated with lower amygdala volumes. This association persisted in a follow up study three years later. This finding cannot be explained by potential confounding variables or by earlier levels of aggressive behaviour, all of which were controlled for. This is powerful support for the role of the amygdala in aggressive behaviour and it is evidence of predictive validity of the explanation. It shows that differences in amygdala volume may predict future aggressive behaviour and involvement in crime.

Evaluation - Other brain areas

• The amygdala does not operate on its own to determine aggressive behaviour. It functions together with the orbifrontal cortex (OFC) which is thought to regulate self control, impulsive behaviour and inhibit aggression.

• Raine investigated murderers who had shown a large degree of reactive aggression in their crimes. These individuals had a higher glucose metabolism in the prefrontal cortex (including the OFC).

• This finding shows just how complex the regulation of aggressive criminal behaviour is. It involves at least three brain structures: the amygdala, the OFC and the neural connection between them. Therefore, dysfunctions in the amygdala may not be enough on their own to account for criminal behaviour.

Evaluation - effects of the amygdala are indirect

• The amygdala has a role in regulating fear and anxiety related behaviours, for example of physiological arousal that accompanies the flight or fight response.

• Damage to the amygdala affects the ability to process fear and anxiety related information normally which in turn affect social functioning (for example reducing empathy). This makes aggressive behaviour more likely but not inevitable.

• Therefore amygdala dysfunction is not necessarily a direct cause of aggressive criminal behaviour but is instead a risk factor for it. Whether an individual becomes involved in criminal behaviour depends on other biological and environmental factors. This is a more complex explanation than the conventional amygdala dysfunction theory.