Neural Mechanisms

Question 1

Limbic System Info

Answer 1

• Papez (1937) and Maclean (1952) linked the limbic system – which they defined as the hypothalamus, amygdala, parts of the hippocampus, and other structures – to emotional behaviours like aggression
• The reactivity of the amygdala in humans and other mammals is an important predictor of aggressive behaviour (the more responsive the amygdala, the more aggressive a person is), so it has a key role in how mammals assess and respond to threats

Question 2

Pardini (2014)

Answer 2

• Did a longitudinal study following 56 male participants (with varying histories of violence) from childhood to adulthood, who got a brain MRI aged 26
• Results showed the people with lower amygdala volumes exhibited higher levels of aggression and violence

Question 3

Gospic et al (2011)

Answer 3

• Some participants were subjected to mild provocation – when participants reacted aggressively, fMRI scans showed a fast and heightened response by the amygdala
• A benzodiazepine drug (reduces arousal of autonomic nervous system) taken before provocation led to the decreased amygdala activity and halved the number of rejections – reduced aggression

Question 4

Supporting Evidence - Wong et al (1997)

Answer 4

• Undertook MRI scans of 19 violent male criminals in Broadmoor hospital and compared the size of amygdala with 20 ‘normal’ control subjects
  – He found that the volume of the amygdala was significantly smaller in the 19 violent criminals, thus supporting the role of the amygdala and limbic system in aggression
• This supports the role of neural mechanisms in aggression as it identifies specific areas of the brain that are directly linked to producing/moderating aggressive responses

Question 5

Evidence Against

Answer 5

• Recent studies suggest that the amygdala alone does not operate by itself in aggression
• It appears to function with the OFC (see Coccaro 2007), which isn’t part of the limbic system; combined with Gospic’s findings, this indicates that the regulation of aggression is highly complex and involves at least 3 neural structures – amygdala, OFC, and connection between the two

Question 6

Hippocampus-Related Study - Raine et al (2004)

Answer 6

• Studied two groups of violent criminals (one had faced conviction – unsuccessful psychopaths – and the other had evaded the law – successful psychopaths)
• MRI scans revealed asymmetries in the hippocampus in the unsuccessful group – the asymmetry might impair the ability of the hippocampus and the amygdala to work together
• The hippocampus in either hemisphere of the brain in these individuals differed in size, an imbalance presumed to have arisen early in brain development

Question 7

OFC & Serotonin

Answer 7

• Serotonin is a neurotransmitter that has widespread inhibitory effects in the brain
• Normal levels of serotonin in the OFC are linked with reduced firing of neurons, which is associated with greater behavioural self-control
• Decreased serotonin disrupts this, reducing self-control and leading to an increase in impulsive behaviour including aggression (Denson et al in 2012)

Question 8

Duke et al (2013) - Serotonin-deficiency Hypothesis

Answer 8

• Meta-analysis of 175 studies involving 6500 people, and found a small inverse relationship between serotonin levels and aggression, anger, and hostility
• Also found the magnitude of the relationship varied with the methods used to assess serotonin functioning
• Suggests the relationship between serotonin and aggression is more complex than previously though

Question 9

Virkkunen et al (1994)

Answer 9

• Compared levels of a serotonin breakdown product in the cerebrospinal fluid of violent impulsive and violent non-impulsive offenders – the levels were significantly lower in impulsive offenders, and they also suffered from sleep irregularities
• Serotonin regulates sleep patterns – disturbance of this pattern implies disruption of serotonin functioning and showing the support for the role of serotonin in reactive aggression

Question 10

Research Into Effects Of Drugs

Answer 10

• Drugs that increase serotonin (like paroxetine) have been found to reduce levels of aggressive behaviour
• Berman et al (2009) gave participants a placebo/dose of paroxetine, with all the participants taking part in a lab-based game that involved giving/receiving electric shocks in response to provocation – the paroxetine group consistently gave fewer and less intense shocks than the placebo group
• This is evidence of a causal link between serotonin function and aggression

Question 11

Research Showing Non-Limbic Brain Structures Are Involved In Aggression

Answer 11

• Limbic structures (like the amygdala) function together with the OFC (not part of the limbic system), which is involved in impulse regulation and inhibition of aggressive behaviour
• According to Coccaro et al (2007), OFC activity is reduced in those psychiatric disorders that feature aggression – this reduced activity disrupts the OFC’s impulse-control function, which causes aggressive behaviour
• This shows that the neural regulation of aggression is more complex than theories focusing on the amygdala suggest