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Humans have what is called a triune brain, or a three-part brain. The midbrain or reptilian brain is the oldest part of our brain, where our survival instinct lives; the limbic or mammalian brain is where our emotions live; and finally the neocortex is our thinking brain. Adult humans with a fully developed neocortex are typically operating from the top down, from the neocortex down to the midbrain. This basically means we (normally) don’t bang each other on the sidewalk or resort to fistfights to settle disagreements at work because our moral, rational, thinking brain—the neocortex—asserts control over our base survival instincts. The neocortex, and specifically the prefrontal cortex, is where our judgment, personality, willpower, inhibition, social skills, morality, decision making, planning, and loads of other functions live. If the brain is a car, the survival response (midbrain) is the gas, and the prefrontal cortex is the brake. In alcohol addiction, the top-down control gets flipped, and the survival, animal instinct overrides the rational, thinking brain. This is due to two different causes. First, the prefrontal cortex loses its strength and volume; it’s like a muscle, and the chemical component of alcohol (it’s a neurotoxin, as in it attacks gray matter or the regions of the brain involved in sensory perception, memory, emotions, speech, decision making, and self-control), along with the consistent deferral to the survival instincts, weakens its function. So the part of our brain that is responsible for inhibiting actions (willpower), making decisions, moderating social behavior, constructing our personality, upholding our ethics, and planning our future goes offline. At the same time, the midbrain—which thinks only about the next fifteen seconds, not tomorrow or next year—
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Holly Whitaker (Quit Like a Woman: The Radical Choice to Not Drink in a Culture Obsessed with Alcohol)