I recently read “The Introvert Advantage” by Marti Olsen Laney. The best chapter of the book by far was entitled “The Emerging Brainscape” which discusses the known physiological differences between introverts and extroverts. Though the book as a whole was too long for the amount of information it conveyed, this chapter was genuinely informative.
There are four key differences between the introvert and extrovert brain: the quantity of blood that flows to the brain, the path the blood takes through the brain, the chemicals needed to feel good and the type of nervous system most commonly activated.
Introverts have a greater blood flow to the brain than extroverts. Blood flows to parts of the body that are stimulated, suggesting that introverted individuals tend to be more easily stimulated than extroverted induviduals. Extroverts must compensate for this by appealing to the outside environment for stimulation through social contact, new experiences and physical activity, which is why they tend to be more engaged with the outside world.
What’s more, the path the blood takes within the introvert’s brain is longer and more complex than that of the extrovert. Perhaps unsurprisingly, blood in the introvert brain flows through areas that have a greater internal focus: like memory and planning. The extrovert blood pathway takes a more experiential route, focusing mostly on immediate sense experiences (excluding smell, for some reason).
Thirdly, introverts and extroverts require different chemicals to feel good. The extrovert engine needs dopamine to run, and lots of it. Unfortunately, the extrovert neural pathway is not very sensitive to dopamine and it must make more using adrenaline. Adrenaline is produced during action, which is why extroverts feel good the more active they are: they produce more adrenaline which in turn produces more dopamine.
Introverts, with their increased blood flow, are more sensitive to dopamine and can easily overindulge. So their dominant brain pathway avoids dopamine and instead uses the neurotransmitter acetylcholine (a-see-to-ko-leen). This chemical produces a good feeling when the person is engaged in thinking or feeling, so these activities are likely to be more rewarding than action for the introverted person.
Dopamine and acetylcholine activate different nervous systems. The sympathetic nervous system (“fight or flight”) is activated by dopamine and prompts action. The parasympathetic nervous system, on the other hand, is activated by acetylcholine and prompts withdrawal. The two clearly have parallels with the everyday behaviour of introverts and extroverts. Under stress, a person is more likely to revert to their dominant mode of functioning. In a “neutral” but stressful situation (rather than one that clearly requires a specific response, like being chased by a tiger) extroverts and likely to leap into nervous actions and introverts are likely to shut-down and withdraw.
The physiological quirks of introverts have a couple of interesting side-effects: due to their tendency to use a longer brain pathway, introverts have a slower response time than extroverts. It often takes them a while to process information before coming to a conclusion. Additionally, an introvert’s long-term memory is much better than their short term memory. They may not be able to remember where they just put their glasses down but their recall of experiences in the distant past or information learnt a long time ago is comparatively very good.
(The information summarised here can be found in Marti’s book.)