Limbic System

Limbic System Function

Papez suggested in 1937 that the connecting circuit of the hippocampus and the limbic lobe was responsible for emotion (named the Papez Circuit).

MacLean later expanded this idea and redefined it as the limbic system. It was assumed that the processing of information is a hierarchical system: Information is received and processed first by the lower elements of the limbic system. Then the higher cognitive elements of the limbic system process the information to enable a sufficient response.

Overall, the limbic system functions can be seen as the reactionary system to a perceived threat within the environment but it is also heavily involved in other fundamental processes, such as memory.

Limbic System Structures

The higher limbic system structures comprise the following subcortical areas of the brain, amongst others:

• The Hypothalamus is important in regulating the autonomic nervous system (ANS), and regulating emotional responses. It is integral to maintaining homeostasis in the body and regulates hormones such as testosterone. It is the overseer of other brain regions, such as the pituitary gland. Damage here can cause inappropriate responses to perceived threats.

• The Hippocampus is involved in forming long-term memories and learning and spatial awareness and navigation. Neurogenesis (turning stem cells into nerve cells) occurs here. Damage here can severely impair your memory.

• The Amygdala is the emotional centre of the brain that processes fear-inducing and threatening stimuli and how they are linked/associated with forming new memories. It integrates emotions with motivational behaviours. Damage here can lead to more aggressive behaviours, as there’s a loss of control over your emotions (the brain’s emotional centre is important in emotions, it seems!)

• The Cingulate Gyrus is vital in regulating aggression, emotional responses to pain, communication and maternal bonding, amongst other functions. The anterior cingulate gyrus is linked to Broca’s and Wernicke’s areas (involved in language comprehension and production). It is thought to aid the vocalisation of emotional responses.

Limbic System Diagram

Below is a diagram of the limbic system and the other nearby structures. It is quite a complex system, but once you become familiar with it, you can begin to pick out the key areas of the limbic system.

Fig. 2: A diagram of the structure of the limbic system shows us where these important areas are.

Limbic System Parts

The limbic system is comprised of multiple areas of the brain, namely the amygdala, hippocampus, cingulate gyrus, hypothalamus, and other notable regions, such as the thalamus and septum. We will explore how areas of brain damage affect the limbic system and in turn its involvement with aggression.

First, let's consider a few notable roles parts of the limbic system play in bodily responses.

Damage and Disorders: Depression and the Limbic System

When damage to the limbic system structures occurs, several disorders can develop, depending on the specific areas of damage. Here is an overview of the potential disorders that can come about as a result of damage to the limbic system:

• Depression.

• Anxiety.

• Schizophrenia.

• Bipolar.

• Anger management disorders.

• Parkinson’s disease.

• Post-traumatic stress disorder (PTSD).

• Memory disorders (storage, forming new memories).

As we can see, a wide range of effects can occur when the limbic system is damaged.

Aggression and the Limbic System: Research

The limbic system has been linked to aggression in the following ways:

• Kluver and Bucy (1939) removed the core areas of the limbic system in rhesus monkeys. Monkeys then went on to have issues and complete absences of responses with their emotions and motor functions when researchers showed them certain stimuli (specifically, stimuli meant to induce fear and anger), and their vocal functions.

  They also lost an understanding of their places in social hierarchies and were more aggressive to try and gain dominance even when it was inappropriate/ill-advised. However, one thing to remember is that although this study shows the importance of the limbic system in affecting emotions, particularly aggression, we can not conclusively say this is comparable or applicable to humans. We deal with aggression differently (socially and culturally, as well as mediation tactics) to rhesus monkeys.

• Groves and Schlesinger (1982) surgically removed the amygdala to reduce aggression in violent individuals. However, upon removal, individuals were reported to have lost their sense of emotion altogether. This finding suggests that the amygdala is involved with aggression and other emotions, and that emotion regulation is not a direct cause.

• Gospic et al. (2011): In this study, two participants, the proposer (P) and the responder (R), played the Ultimatum Game. In this game, the proposer offers to split money either fairly or unfairly with the responder. If accepted, the money is divided as suggested. If rejected, both parties receive nothing.

  • They had their brains scanned by an MRI during the game, which found a heightened amygdala response to rejected, unfair offers, more noticeably in males than females.

  • If the participants were given anti-anxiety drugs beforehand, the amygdala had a decreased response, and unfair offers were less rejected. Overall, this suggests the amygdala plays an important role in processing and mediating emotional behaviour, particularly aggression.

• Phineas Gage: In the case of Phineas Gage, he suffered extreme frontal lobe damage after an accident occurred whilst he was working. A metal rod went through his skull and damaged most of his frontal lobe, affecting his prefrontal cortex, which is linked to the regulation of emotional behaviours.After the accident, despite surviving such an injury, Phineas lived for another 12 years. However, his friends noted he was more aggressive afterwards. With the damage, his prefrontal cortex could no longer regulate his behaviours and inhibit the amygdala, so he had an unjustified and unregulated response to what he perceived as threatening situations.

• Chang and Gean (2019) found that stress in socially isolated mice activates the ventral hippocampus (vHip) neurones, which induces attack-like behaviour (aggression). These neurones, which project into the ventromedial hippocampus, induce aggressive behaviour and can be inhibited to reduce it. This finding suggests that the above systems are involved in aggression overall.

Fig. 3: A study found regions of the brain in mice were associated with aggression.

However, issues exist in that:

• Links are only correlational: Research suggests there’s a link. However, some of the studies mentioned above only show a correlational link between aggression and the limbic system. It is not a direct cause, as seen in the case of Groves and Schlesinger. Similarly, in the ultimate game study, the amygdala only showed a faster, increased activation. It does not mean that aggression is coming from the amygdala directly, only that it is linked.

• Cause – aggression or abnormalities?: The abnormalities in the limbic system may cause increased aggression. However, increased aggression may also cause abnormalities within the limbic system. There’s no significant evidence to suggest either interpretation is correct.

• Beta bias: Multiple studies supporting the role of the limbic system, and in particular, the amygdala, have issues with beta bias (they generalise the study, which was performed on one sex, to apply to both sexes).

• Wong et al. (1997): In this study, 20 repetitive violent offenders (RVOs) were compared to 19 non-repetitive violent offenders (NRVOs) from a maximum-security mental hospital. There were asymmetric gyral patterns at the temporoparietal region (essentially, different-sized areas) that were particularly common in RVOs and absent in NRVOs. Yet, this study cannot be generalised to both males and females, as it’s a small sample size and was only conducted on men.