While evolutionary role of arousal system is apparent – being on alert and ready for fight or flight offers clear advantages in hostile environment in a jungle, – the role of mood is not as obvious. There are several evolutionary hypotheses about adaptive role of depression. It was suggested that our emotions prevent waste of time and effort when attempting a futile task. Like pain that teaches us to avoid hot surfaces and sharp objects, depression warns us about experiences that lead to disappointment and regrets.
This interesting idea about evolutionary role of depression was put forward by Randolph Ness, MD. In moving toward a goal, one needs to rely on previous experience to decide if the goal is worth perusing and precious resources of time and energy will not be squandered in fruitless efforts. He writes: “Failing to reach a goal is an especially potent elicitor of negative affect. There is considerable agreement that low mood helps disengage individuals from unproductive efforts.” http://facelab.org/debruine/Teaching/EvPsych/files/Nesse_2000.pdf
Role of mood (and anxiety) in decision making was extensively described in the literature: Descartes’ Error: Emotion, Reason; the Human Brain by Antonio Damasio; A Mind of Its Own: How Your Brain Distorts and Deceives by Cordelia Fine, to name just a few. Imagine how hard it would be to re-evaluate every situation from scratch, considering that many scenarios are repetitive with only few changing variables. Instead of weighing the data, one piece at a time, we employ emotions for decision-making shortcuts. Consider how many variables go into deciding which car to buy: model, cost, color, looks, safety record, financing, warranty, etc. It would take days, if not weeks, to sort out all combinations and possibilities. But here come emotions: “I like Toyotas and particularly Camry” and the rest is easy. All the subsequent arguments only serve to support an already made emotional decision.
Another hypothesis about evolutionary role of depression focuses on hierarchical relationship in social groups. Depression in lower ranking members is associated with social withdrawal and submission and might be protective. Alternatively, confrontational behavior against obviously superior (physically and otherwise) opponent can lead to injuries or death.
Mood is the state of feelings, high and low, joy and sadness. The distinction between mood and emotion is based on temporal (the duration) and situational differences. Mood refers to state of mind over longer period of time and is unfocused, while emotions are short-lived and usually in reaction to recent events.
Limbic system, a regulator of emotions, is a group of brain structures which include hippocampus, anterior thalamic nuclei, and limbic cortex (mostly cingulate cortex). In addition, other parts of the brain are sometimes considered in the limbic system: mammillary body, dentate gyrus, nucleus accumbens, and some others. While most researchers include amygdala (see previous posts) in the list, others separate it into a distinct arousal system. There is enough evidence to suggest that arousal and mood, while connected, operate semi- independently.
Various neurotransmitters (by themselves or in combination) contribute to mood regulation. Serotonin is the most targeted neurotransmitter in managing mood, particularly depression. However, as I have written before, it is not a chemical messenger itself but where (at which brain circuits) it is released that makes the difference. In addition, one has to consider up to 17 serotonin receptors with different, often opposite, properties. Others are norepinephrine, dopamine, glutamate, GABA, and several hormones.
It has been reported that two hemisphere are not equal in their processing of emotions, although precise relationship between the two is not entirely clear. One hypothesis suggests that the two hemispheres have a complementary specialization for control of different aspects of emotion.Left hemisphere primarily processes “positive” emotions and right hemisphere “negative” emotions. This concept is supported by studies showing that in a person exposed to negative stimuli the right hemisphere would activate and to positive – the left. Another observation was that left side brain injury are more likely to be associated with subsequent depression, while opposite side injury with mania.
Anterior Cingulate (AC) plays essential role in experiencing and managing of emotions. Ventral (frontal) AC offers primitive strategies in emotional regulation, while dorsal (in the back) AC is managing at a higher cognitive level (e.g. talking self out of sadness). With increase in maturity ventral AC decreases its influence on emotional regulation, while dorsal increases.
Reward/pleasure system – medial forebrain bundle (MFB) or pleasure bundle – starts at reticular formation and goes through ventral tegmental area (VTA) which releases dopamine and then proceeds to amygdala, prefrontal cortex. Nucleus accumbens (pleasure nucleus) play critical role in psychopathology of certain disorders (e.g. antisocial behavior, substances abuse, paraphilias, some psychosomatic disorders, etc.).
Reward system, also referred to as “pleasure pathway”, correlates with mood. It is active when mood is up and goes silent when the mood is down. The opposite is true about “disgust system” (ending at insula). Punishment circuits are acting opposite to reward system and include hypothalamus, the thalamus, and the central grey substance surrounding the aqueduct of Sylvius (the periventricular system, or PVS). They are also strong connections to the amygdala (anxiety, fear) and the hippocampus (long-term memory).
Jonathon Haidt in his book Happiness Hypothesis http://www.happinesshypothesis.com/happiness-hypothesis-ch1.pdf compares our rational and emotional mind with a rider on top of an elephant. The rider (reason, rational) believes that he controls and guides the elephant (emotions) to the direction he chooses. When the elephant goes its own way (more often than we think), the rider justifies elephant’s action with rational explanation. In reality, we do not control our emotions, only manage their intensity and our behavior when we experience strong feelings. Evolutionary, our drives (reptilian brain) and emotional limbic system developed much earlier than rational mind (neocortex or Executive Function Network) and they are still quite powerful in directing our behavior. Nevertheless, a mature intact brain has significant leverage in regulating emotions and dominating impulses.
Immature or impaired Executive Function Network due to trauma, neurodegeneration, or drugs has weak hold on emotions and let them go unhindered. With maturity and restoration of function one can make better decisions and plans, take into consideration various factors and give enough time to think and play possible scenarios.
Our mood colors our perception of present, past and the future. When happy, we arbitrarily select positive experiences from the past, excited and hopeful about our future. Elated, we might make decisions that we regret later because our enthusiasm at the moment warps our judgment. Cognitive distortions are even more prominent in depressed state. These distortions, described more than half a century ago by a “father of cognitive-behavioral therapy” Dr. Aaron Beck, include negative view of self, notion negative perception of ourselves from the others, and hopelessness (more about cognitive distortions later).
Michael Levin, M.D., Medical Director of EBPG, psychiatric group practice serving San Ramon, Danville, Dublin, Pleasanton, Walnut Creek, and other cities in Contra Costa and Alameda Counties, CA