Evolution of the limbic system – bringing out the animal in us all
The Papez circuit is just part of the limbic system, which also includes the amygdala and the front parts of the brain. The limbic system can be loosely divided into three sections, each with its own function.
First is the amygdala and the front part of the hippocampus. They are primarily involved in self-preservation, particularly feeding, the search for food and the fighting and defensive actions required to eat and obtain a meal. In most animals the olfactory system is a major input into this part of the limbic system, as is the hypothalamus, which signals hunger and satiety. In particular, the amygdala is responsible for learnt fear responses, preparing the body for fight or flight. The fear response from the amygdala is based on recognizing a fearful situation, which may be quite complex. This means the input has already been through most of the brain, to be interpreted and assessed for threat, before it arrives at the amygdala.
Second are the collections of neurons just in front of the thalamus, (the septal nuclei), the inner part of the cingulate gyrus and the tail end of the hippocampus. This part of the limbic system deals with reproduction and is involved in sexual function and behaviours, encouraging sociability and mating. The olfactory system and hypothalamus also feed strongly into this section of the limbic system.
Third are the outer parts of the cingulate gyrus and its connections with the outer layers of the brain. The front parts of the thalamus feed into this section, whereas the olfactory system and hypothalamus do not. This part of the limbic system is highly developed in mammals, but does not exist at all in reptiles. It is responsible for the parental care and family groups characteristic of mammals. There are connections between this part of the limbic system and the pain centres, suggesting that the pain of family separation is hard-wired into us.
Some of the emotional circuits – how Vulcans miss out
The amygdala receives simple threat signals, such as the sound of an explosion, direct from the thalamus. No complicated processing is required to recognize these likely threats, and so they can go directly to the amygdala. If the threat signal is more complicated, for example, that represented by a particular face or social situation, the signal has to come from an association centre in the outer cortex, because it has to have been recognized as a possible threat. The outputs of the amygdala feed directly into the hypothalamus and brainstem, where they activate the appropriate responses, such as increased heart rate, sweating and breathing. There is also an output to the cingulate gyrus, which processes the emotional significance of the signal. Without this circuit, we would be unable to distinguish the emotional difference between a child with a toy gun and an adult with a real one. During surgery, it is possible to stimulate the amygdala with an electrical probe, which results in people describing anything from mild anxiety to anger, terror and a sense of “someone behind me”.
In evolutionary terms, the more social the species, the larger the amygdala. Humans with damage to the amygdala lose the ability to interact appropriately with others. Some people develop hypersexuality and, just as an infant does, become fixated on putting things in their mouth. In general,
amygdala damage leads to placidity, muted emotions and an inability to recognize aggression.
The amygdala also has a critical role in memory. In a right-hander, the right amygdala is used for subconscious emotional learning while the left is used for conscious emotional learning.
The temporal pole
The temporal pole is the tip of the temporal lobe and receives inputs from most other areas of the limbic system. A right-handed person uses the left temporal pole to remember the name that goes with a face. The full role of the right temporal pole is not clear but we can use it to recognize a sad face.
The cingulate gyrus
While the amygdala recognizes threats and controls temper, the cingulate gyrus assesses the emotional significance of the experience, for example, to register the emotion behind facial expressions; therefore it is very important for social interaction. People with damage to the cingulate gyrus cannot feel the emotional component of pain and it no longer makes them angry or tearful. This loss of emotion also means that they are no longer driven to pursue activities that they used to enjoy.
The front part of the cingulate gyrus is responsible for “gut feelings”. It helps direct our attention towards a potential solution when we are faced with a number of choices, for example in social or novel situations and especially when the possible outcome cannot be known.
The orbitofrontal cortex
The orbitofrontal cortex is just behind the eyes. It plays an important role in matching up the inputs for smell, taste and sight and it is likely to be where we experience “love at first sight”. It is extremely important for social interaction and controls responses to social situations, producing changes in heart rate and blood pressure, breathing responses, facial flushing, pupil size, and the “butterflies”. Learning to react to and control these autonomic functions is a great part of adolescence.
This part of the cortex acts as our conscience, helping us to censor, self-monitor and incorporate experience into decisions about behaviour. We use it to guess what mental state others are in, allowing us to reason out social problems – we see things from their point of view. One could argue that it most defines what it is to be a social animal and is the part that seems to be under functioning in forms of autism such as Asperger’s syndrome.