HUMAN BRAIN
(Anatomy)

THE LIMBIC SYSTEM

Functions

• sets the emotional tone of the mind
• filters external events through internal states (emotional coloring)
• tags events as internally important
• stores highly charged emotional memories
• modulates motivation
• controls appetite and sleep cycles
• promotes bonding
• directly processes the sense of smell
• modulates libido

Problems

• moodiness, irritability, clinical depression
• increased negative thinking
• perceive events in a negative way
• decreased motivation
• flood of negative emotions
• appetite and sleep problems
• decreased or increased sexual responsiveness
• social isolation

The deep limbic system lies near the center of the brain. Considering it's size -- about that of a walnut -- it is power-packed with functions, all of which are critical for human behavior and survival. From an evolutionary standpoint, this is an older part of the mammalian brain that enabled animals to experience and express emotions. It freed them from the stereotypical behavior and actions dictated by the brain stem, found in the older reptilian brain. The subsequent evolution of the surrounding cerebral cortex in higher animals, especially humans, gave the capacity for problem solving, planning, organization and rational thought. Yet, in order for these functions to occur one must have passion, emotion and desire to make it happen. The deep limbic system adds the emotional spice, if you will, in both positive and negative ways.

This part of the brain is involved in setting a person's emotional tone. When the deep limbic system is less active there is generally a positive, more hopeful state of mind. When it is heated up, or overactive, negativity can take over. This finding actually surprised us at first. We thought that excessive activity in the part of the brain that controlled emotion might correlate with enhanced feelings, not necessarily negative feelings. Yet, we noticed, again and again, when this area was overactive on SPECT it correlated with depression and negativity. It seems when the deep limbic system is inflamed, painful emotional shading results. New research on depression from other laboratories around the world has born this out. Due to this emotional shading, the deep limbic system provides the filter through which you interpret the events of the day. It tags or colors events depending on the emotional state of mind. When you are sad (with an overactive deep limbic system) you are likely to interpret neutral events through a negative lens. For example, if you have a neutral or even positive conversation with someone whose deep limbic structure is overactive or "negatively set" he or she is likely to interpret the conversation in a negative way. When this part of the brain is "cool" or functions properly, a neutral or positive interpretation of events is more likely to occur. Emotional tagging of events is critical to survival. The valence or charge we give to certain events in our lives drives us to action (such as approaching a desired mate) or causes avoidance behavior (withdrawing from someone who has hurt you in the past).

PMS, discussed in the last chapter, is a classic example of this emotional shading principle. As mentioned, in our study of PMS within 5-10 days before the onset of the menstrual cycle the deep limbic system becomes inflamed or more active with the drop in hormones. This deep limbic activation colors events in a more negative way. My best friend's wife has a fairly severe case of PMS. He tells me that during the first week of her cycle she looks at him with love and affection, and almost anything he does seems to be right. She is more loving, more affectionate and sees things in a positive light. Ten days before her period things are dramatically different. She doesn't want to be touched. She "has a different look" which he describes as a combination expression of scowl and "don't mess with me." Little he does is right. She emotionally colors most events in a negative way. Then a few days after her cycle starts, she's back to being more positive, loving and affectionate.

The deep limbic system, along with the deep temporal lobes has also been reported to store highly charged emotional memories, both positive and negative. If you have been traumatized by a dramatic event, such as being in a car accident or watching your house burn down, or if you have been abused by a parent or a spouse, the emotional component of the memory is stored in the deep limbic system of the brain. On the other hand, if you have won the lottery, graduated magna cum laude, or watched your child's birth, those emotional memories are stored here as well. The total experience of our emotional memories is responsible, in part, for the emotional tone of our mind. The more stable, positive experiences we have the more positive we are likely to feel. The more trauma in our lives the more emotionally set we become in a negative way. These emotional memories are intimately involved in the emotional tagging that occurs.

The deep limbic system also affects motivation and drive. It helps get you going in the morning and encourages you to move throughout the day. Overactivity in this area, in our experience, is associated with lowered motivation and drive, which is often seen in depression. The deep limbic system, especially the hypothalamus, controls the sleep and appetite cycles of the body. Healthy sleep and appetite is essential to maintaining a proper internal milieu. Both of these components are often a problem with limbic abnormalities.

The deep limbic structures are also intimately involved with bonding and social connectedness. When the deep limbic system of animals is damaged they do not properly bond with their young. In one study of rats, when the deep limbic structures were damaged mothers would drag their offspring around the cage as if they were inanimate objects. They would not feed and nurture the young as they would normally do. This system affects the bonding mechanism that enables you to connect with other people on a social level; your ability to do this successfully in turn influences your moods. Humans are not like polar bears, wandering the tundra alone eleven months out of the year. We are social animals. When we are bonded to people in a positive way we feel better about our lives and ourselves. This capacity to bond then plays a significant role in the tone and quality of our moods.

The deep limbic system directly processes the sense of smell. The olfactory system is the only one of the five sensory systems that goes from the sensory organ to directly where it is processed in the brain. The messages from all the other senses (sight, hearing, touch and taste) are sent to a "relay station," the thalamus, before they are sent to their final destination in different parts of the brain. Because your sense of smell goes directly to the deep limbic system it is easy to see why smells can have such a powerful impact on our feeling states. The multibillion-dollar perfume and deodorant industries count on this fact: beautiful smells evoke pleasant feelings and draw people toward you, whereas unpleasant smells cause people to withdraw. Expensive perfumes and colognes can make you beautiful, sexy and attractive to others, whereas a disagreeable body odor can make the other person want to rush to the far side of the room.

Bonding, smells, sexuality and the deep limbic system are intimately connected. Napoleon once wrote to Josephine to ask her not to bathe for 2 weeks before he came home from a battle. He wanted her scent to be powerful, because it turned him on sexually. Likely, positive, sexual smells cool the limbic system and make us more in the mood for love. Deep limbic overactivity, often associated with depression, frequently result in decreased sexual interest. For many years, I have hypothesized that decreased sexual activity is associated with increased deep limbic activity and more vulnerability to depression. Of course, my wife tells me that I would say that as an excuse to make love more often.

I studied this phenomenon in an adult male who had problems with depression and increased activity in his deep limbic systems on SPECT. I asked him to make passionate love with his wife. I then rescanned him within an hour. His limbic activity was significantly decreased. Orgasm has been described as a mini-seizure of the limbic system and tends to release or lessen deep limbic activity. Sexuality is good for the bonded human brain.

Whenever a person is sexually involved with another person, neurochemical changes occur in both their brains that encourage limbic, emotional bonding. Yet, limbic bonding is the reason casual sex doesn't really work for most people on a whole mind and body level. Two people may decide to have sex "just for the fun of it," yet something is occurring on another level they might not have decided on at all: Sex is enhancing an emotional bond between them whether they want it or not. One person, often the woman, is bound to form an attachment and will get hurt when the affair ends. The reason it is usually the woman is that the size of a female's limbic system, in comparison to the rest of her brain, is larger than it is for a man. Consequently, she is more likely to get limbically connected.

As mentioned above, current research has demonstrated that females, on average, have a larger deep limbic system than males. This gives females several advantages and disadvantages. Due to the larger deep limbic brain women are more in touch with their feelings, they are generally better able to express their feelings than men. They have an increased ability to bond and be connected to others (which is why women are the primary caretakers for children - there is no society on earth where men are primary caretakers for children). Females have a more acute sense of smell, which is likely to have developed from an evolutionary need for the mother to recognize her young. Having a larger deep limbic system leaves a female somewhat more susceptible to depression, especially at times of significant hormonal changes such as the onset of puberty, before menses, after the birth of a child and at menopause. Women attempt suicide three times more than men. Yet, men kill themselves three times more than women, in part, because they use more violent means of killing themselves (women tend to use overdoses with pills while men tend to either shoot or hang themselves) and men are generally less connected to others than are women. Disconnection from others increases the risk of completed suicides.

The deep limbic system, especially the hypothalamus at the base of the brain, is responsible for translating our emotional state into physical feelings of relaxation or tension. The front half of the hypothalamus sends calming signals to the body through the parasympathetic nervous system. The back half of the hypothalamus sends stimulating or fear signals to the body through the sympathetic nervous system. The back half of the hypothalamus, when stimulated, is responsible for the fight or flight response, a primitive state that gets us ready to fight or flee when we are threatened or scared. This "hard-wired response" happens immediately upon activation, such as seeing or experiencing an emotional or physical threat. In this response the heart beats faster, breathing rate and blood pressure increases, the hands and feet become cooler to shunt blood from the extremities to the big muscles (to fight or run away) and the pupils dilate (to see better). This "deep limbic" translation of emotion is powerful and immediate. It happens with overt physical threats and also with more covert emotional threats. This part of the brain is intimately connected with the prefrontal cortex and seems to acts as a switching station between running on emotion (the deep limbic system) and rational thought and problem solving with our cortex. When the limbic system is turned on - emotions tend to take over. When it is cooled down, more activation is possible in the cortex. Current research on depression indicates increased deep limbic system activity and shut down in the prefrontal cortex, especially on the left side.