Emotions: the key to rational thought?
For years, being emotional was often looked down upon. It was thought stupid people were emotional, and rational people were intelligent. To criticize an opponent, one might disparage their emotional character, saying they were too weak to make intelligible choices. Turns out, having emotional responses is key to making any rational choice.
Before we investigate this claim which researchers have started to propose, we need to understand the way that emotion is processed in the brain. If you think about it, emotion is a bit confusing. In the most straightforward scenario, you have input which enters your brain, your brain processes this input, and provides an output, in a cyclic pattern that continues for all of your life. But how are these different mediums converted?
Let’s take the sense of smell for example. There are alternative theories for what the actual mechanism of olfaction is, but it all starts with a simple proposition, put forth in the 1st century by the Roman philosopher, Lucretius, and more recently shown by the researchers Linda B. Buck and Richard Axel. The basic story line is that different shaped odor molecules stimulate the olfaction organ. 
The first step of translation from the odor molecules is their passage through the nasal cavity, and dissolution in the mucous layer. There, the molecules bind to receptors on the dendrites of the olfaction sensory neurons.
What Buck and Axel did, which gained them the Nobel Prize in 2004, was to pair specific odor molecules with their particular receptor proteins. Each of the odor receptors recognizes a specific molecular marker, which is present in a class of odor molecules. The odor receptors act as a lock and key; the chemical smell binds to the receptor, which initiates intracellular action.  There are three major competing theories at the moment for how the receptors actually work. The shape theory says that the receptors recognize a certain facet of the molecule, which instigates the signal. The odotope, or weak-shape, theory states that the receptors detect only small pieces of the molecules, and put together a full olfaction from the combined activation (similar to how light works in photoreceptors in the retina). The third theory is an alternative theory, proposed by the scientist Luca Turin, called the vibration theory. His theory suggests that the receptors sense the frequencies of vibrations of the odor molecules in the infrared range. 
The binding of the receptor influences a second messenger system, which opens ion channels which depolarize the cells. These neurons are somewhat distinctive in that they don’t use protein kinase A, but rather couple directly couple to ion channels. This is also similar to the mechanism used in photoreceptor cells, pointing to a possible co-evolved, adapted mechanism.
These messages are then projected into the brain through the olfactory nerve, or the cranial nerve 1. The axons project to the olfactory bulb, where there are small structures called glomeruli. Mitral cells (olfactory neurons) in this region form synapses in the glomeruli, which then sends information about the smell received to other areas of the brain. From the olfactory bulb, signals leave in the lateral olfaction tract. This tract synapses in 5 major regions of the brain; the anterior olfactory nucleus, the olfactory tubercle, the amygdala, the piriform cortex, and the entorhinal cortex. These all have roles in processing the odor. The piriform cortex has a role in projecting to a region of the brain which mediates the conscious perception of odor. Another interesting pathway to consider is the entorhinal cortex projections. The entorhinal cortex projects to the amygdala, and is involved in the emotion responses to odor. This same region also projects to the hippocampus, and is involved in motivation and memory. Because of this connection, there is an association between odors and emotional memory. 
So smelling a smell can cause an emotion. However, it’s not as simple as that. One might ask, how did the original memory become what it was? Associative? The brain is constantly working and processing, it can create emotion from no input at all. Or, we can have input that sometimes causes emotion, and other times doesn’t. This system has no way of explaining why this is. In part, this is because we’ve only discussed a limited pathway in the brain, but it is also due to the chaos which underlies this system. In a very simplified example, the permeability of a neuron to sodium may vary at one moment, which may be the moment an input occurs, leading to firing of an action potential when previously this input would not have created an emotional response. Unpredictability and human thought are powerful tools, and the way we mold our mind to be receptive of input can change the effect that input has. Previous experiences can mold the brain so that new experiences are different for themselves then for others.
Take the famous example of Proust and his madeleine. To most people, eating a madeleine would simply be a culinary experience, a nice French cookie to finish off a gastronomical feast. To Proust however, the taste of a Madeleine dipped in tea recalled memories from his childhood. To explain his experience he says, “But when from a long-distant past nothing subsists, after the people are dead, after the things are broken and scattered, taste and smell alone, more fragile but more enduring, more unsubstantial, more persistent, more faithful, remain poised a long time, like souls, remembering, waiting, hoping, amid the ruins of all the rest; and bear unflinchingly, in the tiny and almost impalpable drop of their essence, the vast structure of recollection.”
The connection between sensory input, especially taste, are evidently linked to emotion. Personal experience can attest to this. We’ve already outlined the pathway the smell of sense takes in the brain, but what is the emotional pathway?
This is a difficult question to address, especially due to the fact that a simple “emotional pathway” doesn’t really exist. However, we can discuss a part of the brain, which regulates the emotional memory of the brain, the amygdala.  The amygdala is an almond shaped part if the brain that consists of several different nuclei. The amygdala receives input from several systems, including the olfaction and other sensory systems, the hippocampus, and the brain stem. It sends output to many systems, such as the to the hypothalamus, which activates the sympathetic nervous system (aka the fight or flight response), to the reticular nucleus, which increases reflexes, to the trigeminal nerve, which causes facial expressions of fear, and to the ventral tegmental area and the locus coeruleus for activation of dopamine and norepinephrine. 
An important function of the amygdala is its role in long term potentiation. This is the way neural pathways are strengthened, and the way learning is thought to develop. It has been observed in a few brain structures, including the hippocampus and the amygdala.  Learning is an important feature of the amygdala, and when drugs are injected into the amygdala, subjects tend to remember more, whereas when drugs that inhibit neural firing in the amygdala, remembrance is not observed.  This brings up in interesting correlation between memory and emotions, which could influence “intelligent” choices or behaviors. 
So emotion in the brain is modulated in the amygdala, sensory input can alter this emotion, and it can cause a behavioral output. But what does this have to do with the original claim that emotion is necessary for rational thought?
If one looks at it logically, it makes sense. How can a person make a decision without having knowledge about how it will affect other people, and its implications? Logic has an emotional capacity. It has an intrinsic human quality we cannot run away from; it is not as people once thought, that universal rationality existed, separate from our own limitations and constructions as humans. We tend to think of logic as stone cold and hard. In reality, there is some balancing that has to be done. A person who has emotions that are out of control (as judged by the norm) will also have logic that is out of control, but the two are not as dichotomous as once thought. 
Studies have been done which show that patients who have an injured amygdala have no emotions, but also make horrible choices.  This is a far cry from the expected results that some people might have had about no emotions meaning people aren’t “clouded” in judgment, and can make purely rational choices. It shows that understanding human emotion, and being able to process human emotions is integral to rational decision making. According to the neuroscientist Antonio Damasio, “What makes you and me 'rational' is not suppressing our emotions, but tempering them in a positive way.” 
What we can gain from this is to understand the complexity of the human brain and the role it plays in synthesizing outside stimuli, emotion, reasoning, judgment, and memory. We also can see how much room there is for further research into these areas, before we can really get a good idea of what is going on. The question I’m left with is wondering how much of these processes we have control over. Our brain shapes us, and we shape out brain by our thought processes, but how much of it can we choose and how much of it is simply our brain acting on the genetic and environmental cues it is given?
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