The Limbic System is a collection of parts of the brain that are responsible for generating and regulating emotions1. The limbic system's 9 major components are the (1) thalami bodies, (2) hypothalamus, (3) mamillary bodies, (4) septum pellucidum, (5) cingulate gyrus, (6) hippocampus, (7) amygdala, (8) fornix and (9) olfactory bulbs.2,3 Richard Gross notes that when these are seen from the side they seem to encircle the brainstem in a "wishbone" shape4. "Emotions arise in the limbic system, travelling along neural pathways to the frontal lobes of the cortex, where feelings are monitored and interpreted [which] next influence the hypothalamus, which transmits the messages that trigger appropriate physical responses"1. Physical disruption or disease of the Limbic System causes major behavioural and emotional changes in a person.
The limbic system is "fundamentally similar in all mammals" in terms of its neural functions and connections5, and "the human limbic system is very similar to that of primitive mammals and so it often called 'the old mammalian brain'"4. What separates us further from other animals is the complex associations to the neocortex of the frontal lobes4. Our 'higher functions' reside within the cortex, our emotions within the limbic system, and our experiences and consciousness reside in the interaction between the two.
Some neurologists do not like the concept of the "limbic system" and think that the terminology has become misleading, and that it is best to focus on the specific properties of each component of the brain individually6.
The Amygdala is "concerned with attaching emotional significance to sensory information, and therefore has a role in memory. It also has close ties with the hypothalamus, which influences many endocrine and autonomic responses"3.
"Damage to the amygdala alters aggressive responsiveness, but this area of the brain has been particularly implicated in fear and the control of fear responses. [...] It gives emotional content to memories".2
“The amygdala [is] particularly important for many emotions, including aggression. Removing the amygdala of high-ranking monkeys results in a decrease in aggression. The animals become very submissive and rapidly slip down to a low-ranking status. Additional evidence that the amygdala is involved in aggression comes from electrical stimulation of different areas of the amygdala that can either inhibit or induce aggression, depending on the location. In some humans with anger problems, their very violent behaviour has been thought to be triggered by seizures activating the amygdala. Recognizing that aggression can be reduced by brain operations in animals, some people have had their amygdala surgically removed. This type of operation, termed psychosurgery, was relatively common in the early part of the 20th century. [...] Amygdala removal successfully reduced aggressive antisocial behaviour, and in addition increased the ability to concentrate. However, these benefits come at a cost. Amygdala damage in humans is associated with a general reduction in emotionality as well as a reduced (or absent) ability to recognize emotion in others. [...] Socially extremely handicapping.”
"Emotions and Mind" by Toates, Mackintosh and Nettle (2004)2
The Hippocampus is "involved in memory; someone whose hippocampus is damaged is very easily distracted and they will be unable to carry out an intended sequence of actions (e.g. making a cup of tea) because they have forgotten what they had planned to do"4.
Over-activation of the limbic system can result in depression (as well as religious phenomenon). In the case of depression, symptoms can be removed by medicating with drugs that affect certain parts of the limbic system, in particular through quantitively altering the action of certain neurotransmitters. In Abnormal Psychology7 it says "because of its relevance to the so-called vegetative symptoms of depression, such as disturbances in appetite and sleep, the hypothalamic - pituitary - adrenocortical axis is thought to be overactive in depression. Various findings support this proposition"8.
“[The Limbic System is] a network of ring-shaped structures in the center of the brain's neocortex perched on top of the brainstem, associated with control of emotion and behavior - especially motivation, gratification, memory, and thought. [...] The most common symptoms of damage to this area of the brain include abnormalities of the emotions, including inappropriate crying or laughing, easily provoked rage, unwarranted fear, anxiety and depression, and excessive sexual interest.”
“These areas are deep within the brain and their involvement in emotion has been known for a long time. In the 1930s, Kluver and Bucy discovered that removal of the temporal lobes (including the amygdala and parts of the hippocampus) had a dramatic effect on emotional responses in monkeys. [...] They seemed to see but not to recognize the (emotional) meaning of [things.] Many of these characteristics have also been seen in humans unfortunate enough to have suffered temporal lobe damage. In addition to the visual recognition problems, they report 'flattened' or diminished emotional experiences.”
"Emotions and Mind" by Toates, Mackintosh and Nettle (2004)2
If these nine parts of the limbic system together form the chair on which consciousness sits, the focus that sits in the chair is the interaction between these nine parts and the neocortex, which is the house of our higher functions.
“The cerebral hemispheres are the two largest structures at the top of the brain which enfold (and, therefore, conceal from view) most other brain structures. [...] The top layer of the cerebrum (about 1cm at its deepest) is the cerebral cortex (usually just called 'cortex' which means 'bark'). [...] About three-quarters of the cortex does not have an obvious sensory or motor function and is known as the association cortex; this is where the 'higher mental functions' (cognition) - thinking, reasoning, learning, etc. - probably 'occur'. [...] There is no doubt that the cortex is not necessary for biological survival [...] as some species do not have one to begin with (e.g. birds) and in those that do, surgical removal does not prevent the animal from displaying a wide range of behaviour, although it becomes much more automatic and stereotyped. [...] The limbic system as a whole serves as a meeting place between the cortex (or 'neocortex', in evolutionary terms the most recent part of the brain to have developed) and older parts of the brain, such as the hypothalamus, [...] these are integrated and the 'conclusions' are fed back to the cortex and to the older, subcortical areas.”
The cortex controls and regulates the emotions set off by the limbic system. In cats, if we remove the cortex (10% of the brain), the cats behave more or less normally! Except, they fly into an uncontrollable animal rage whenever they experience pain10. If anything resembles the actions of a soul in the Human brain, it is the interaction between the cortex and the limbic system.
Neuroscientists have performed multiple studies of religiosity and the correlated neural responses in the brain. Related to this are the results of a series of experiments that show that electrically stimulating certain parts of the brain induces mystical experiences related to the culture of the individual.
“Most of the neurological phenomena associated with religious experience involve some form over-activation of the limbic system, and correspondingly intensified experiences. Conversely, Alzheimer's disease is associated with a deterioration of the limbic system and those afflicted tend to lose interest in religion, even those who have exhibited a lifelong interest.”