The limbic system consists of a number of brain parts, and is involved in emotion and motivation. The brain parts that are part of the limbic system are the hippocampus, amygdala, fornix, septum and parts of the thalamus.
Researchers assume that there is not just one brain region responsible for all types of emotions. This is supported by a well-known case study with a patient referred to in the literature as SM. This patient scored well on several cognitive tasks, and no obvious abnormalities were found on emotional tests either. However, when she had to recognize emotions in facial expressions and spoken fragments, she failed to distinguish the emotion of fear. When she was asked to describe this emotion she succeeded, which shows that she understands the concept of fear, but cannot recognize it. From this case, we can infer that fear is probably processed in a different way than other emotions (because this is the only emotion that is affected in SM).
Emotions are subjective experiences, and are therefore difficult to examine. The easiest emotion to examine is fear, because obvious physiological changes can be used to measure this emotion. You can think of an increased heart rate, or sweating. It appears that the amygdala has a very important role in experiencing fear. This can be tested by applying fear conditioning. This involves showing a stimulus that is immediately followed by an unpleasant experience (for example, an electric shock). After a number of trials, this leads to the body getting ready for a shock when only the stimulus is presented (heart rate changes, etc.). When there is damage to the amygdala, these physiological changes do not occur even though the person knows that an electric shock is coming. Thus, it has become difficult to react fearfully. The hippocampus is actually involved in learned emotions. This is because this area is also important for memory and associations in general. In people where the hippocampus is damaged, physiological changes do occur in the above experiment. However, these individuals cannot indicate that the stimulus is followed by a shock. Thus, they only have an unconscious response of fear, but they cannot name why they are afraid.
The hippocampus and the amygdala can influence each other, and in doing so, generate or enhance an emotion. The hippocampus can cause the amygdala to become active based on a memory. You can think of the following situation; you have been bitten by a dog, and you have stored that in your memory. The next time you see a dog you retrieve this memory, and the hippocampus sends a signal to the amygdala that the body must be ready to flee (you experience this as fear). It is very difficult to say exactly how emotions are created in the brain. This is partly because the different parts of the limbic system all become active while experiencing different emotions. Only the emotion disgust is an exception, because here activity is found mainly in the insula.
Author: Myrthe Princen (translated by Joyce Burger)
Images: Marcel Loeffen