Mr. Basil Sajeev,
St. Xavier's College (Autonomous),
Mumbai, Maharashtra, India
We often hear this “You should speak more often or socialize more”, society puts a line between extroversion and introversion and considers it to be a personal choice that one can make, but it is actually not. It has been a trend in society to keep labeling people on the basis of their personal traits. Introverts and extroverts are basically two personality traits that are opposite in nature. An introvert is a person who wishes to turn inward and shows interest in his/her own imaginations and feelings, on the other hand, an extrovert’s focus is on people around and the external world.
The existence of both these categories of people can be seen in every walk of life. For instance, take the scenario of a college party. There is one guy who walks into the room like he owns it, the center of attraction of everybody, so playful and whom everybody aspires. Whereas on the other end of the room is the nerdy guy who is barely noticed, and leaves the party even before the first champagne bottle is popped.
One of the major differences between introverts and extroverts lies in the activity of dopamine reward networks. Dopamine is a neurotransmitter released in the brain which provides a person the stimulation to achieve external rewards and goals. This does not mean that introverts are having fewer dopamine levels, as mentioned earlier it is the difference in the activity of the network that matters. Extroverts are having higher activity in their dopamine reward network as compared to introverts.
As dopamine is for extroverts, we can associate another neurotransmitter to introverts and that is acetylcholine. The acetylcholine activity can be also linked with the reward of pleasure but in another way, it helps us to feel good when we try to become inward. This could explain more about why introverts tend to focus and think more deeply and intensely. 
Randy Buckner of Harvard University in his research found out that the gray matter in the prefrontal cortex in introverts is larger and thicker as compared to extroverts. Based on the finding Bucker reached a conclusion that this difference could be a reason an introvert to think much before he before making a decision while an extrovert would end up in a decision without any considerations, which for sure have its own positive and negative effects.
Another difference between introverts and extroverts could be attributed to the side of the nervous system they prefer to use more. The sympathetic side of the nervous system is linked with triggering immediate responses, while the parasympathetic side is linked with “rest and digest mode”. One could associate the sympathetic side to be the urge to increase the speed of a car, while the parasympathetic to be the urge to apply brakes. During the action of the sympathetic system our body gears up for action, while engaging the parasympathetic system the relaxation of muscles takes place, energy will be stored and food will be metabolized. Both introverts and extroverts would of course use the sympathetic and parasympathetic system, the difference lies in which side they would engage more. From the mode of action, we can understand that introverts tend to engage a parasympathetic system more as compared to the sympathetic system.
The analysis of neuronal activity that determines the human personality traits provides much more insights into the functioning of the brain. Task-related neuroimaging /brain imaging studies have shown that a wide range of brain-like dorsolateral prefrontal complex (dI-PFC), anterior cingulate cortex (ACC), middle temporal gyrus (MTG) is associated with extraversion. According to Eysenck’s theory of extraversion, introverts show an increasing level of basal activity as compared to extroverts and thus produce more tonic cortical arousal than extroverts. This hypothesis further proposes that extroverts are under-aroused and can be bored easily. Neuroimaging done using electroencephalography (EEG) has also shown the reduced basal activity in extroverts.
Development in the field of neuroimaging has led us to scientifically establish some differences between extroverts and introverts. Research conducted by Fisher, Wik & Frederickson (1997), on regional cerebral blood flow has led to the discovery that the amount of dopamine release and reward is higher in extroverts than introverts. Furthermore, an introvert displays an increased brain activity in regions for learning, listening, and consciousness whereas an extrovert has brain activity in subcortical, neostriatal, and dopaminergic regions. The functional magnetic resonance imaging techniques done by Lei and his colleagues indicated the association of extraversion activating the regions in the anterior cingulate cortex, dorsolateral cortex, middle temporal cortex, middle temporal gyrus, and amygdala. Further studies have suggested that introversion can be associated with the increased flow of blood in the frontal lobes and in the anterior thalamus.
On a concluding note, extroverts and introverts can be considered to be the opposite ends of the spectrum and therefore people should not be labeled on the basis of their personality traits. The concept of ambiverts, someone who has the characteristics of both introverts and extroverts should also be considered while understanding personality traits.  It is not possible for a person to be a complete introvert or extrovert, early experiences, mood, goals, context and social conditions also play important roles in molding a person.
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What introverts like to do.
(Image retrieved from https://introvertdear.com/news/extroverted-introvert-illustrated/)
Extrovert vs Introvert