Here's some rough neuroscience: the brain is composed of three regions, each corresponding to a stage of our species' evolution. Let's just say we have three brains: a reptilian brain, a mammalian brain, and a human brain. The reptilian brain generates basic instinctual impulses, the mammalian brain adds emotion and memory into the mix [1] , and the human brain gives us all that make us unique amongst mammals, of which the most commonly discussed feature is our capacity for abstraction and symbolic information processing.
While the entire brain is involved in the experience of fear, it's the limbic system, or the mammalian brain, that triggers a range of physiological and neurological changes when your being perceives a threat. These changes include activation of your sympathetic nervous system, which is hardware dedicated to saving your ass. It slows digestion to save energy and redirects blood to major muscle groups; it draws blood away from the surface of your skin to avoid blood loss in case of injury; it heightens your ability to recognize faces so you can distinguish friend from foe. This threat response system, housed in the mammalian brain, also draws blood away from your human brain so that you don't stop to think about what to do and get killed in the process.
One more time: the perception of physical threat inhibits your human brain.
What a sweet feature of late mammals (humans + primates)! We get all these awesome toys that dogs don't have, but when push comes to shove, we drop them and rely on our cat-like speed and reflexes to survive.
Unfortunately, our brains can interpret the most trivial of social slights as credible if minor death threats. And, yes, that minor death threat causes a minor activation of the threat response system, which will then inhibit your human brain just a bit. This is why its hard to think when we're significantly upset, why we say "I lost my head" or "I wasn't thinking" after acting out of anger and regretting the consequences. Indeed, we aren't thinking or perceiving with our full human capacity—we're reacting out of animal instinct.
Now, you might argue that this threat response is part of what makes us human, and you'd be right, and now I should apologize for the sensationalist title. But I believe the thing that makes us uniquely human is our ability to choose who we become, and that choice is most effectively exercised when our frontal lobes, the ones that earlier mammals are less fully endowed with, are all the way on.
You might also note that this threat response goes away following the removal of the threat from awareness, and our frontal lobes (part of the human brain) come back online. Yes, but, unfortunately, activating the threat response system makes it more sensitive—both in the moment of activation, and for all future situations. This is neuroplasticity in action: the brain strengthening active circuits, especially when there is a strong emotional queue. Fortunately, this applies in the positive direction, too, though at about 1/4 strength of the negative [2].
So, how do we keep the threat response quiet? Love, of course! Or the parasympathetic nervous system, which is the peaceful counterpart of the sympathetic, which runs the threat response. More on that next time. Until then, take a deep breath, exhale long and slow.
Note about the scientific claims made: I'm regurgitating neuroscience gleaned from Buddha's Brain: The Practical Neuroscience of Happiness, Love and Wisdom and Mindsight, amongst others.
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Consider this: prior to memory (and its trigger, emotion), an organism's behavioral algorithm was only ever updated by genetic mutation and natural selection. After the development of memory, an organism's behavioral algorithm began updating itself in every moment; each moment of experience comes with an emotional appraisal that encodes a memory which is used as a rough heuristic for responding to similar situations in the future—it allows for learning, or a kind of environmentally-directed behavioral change not achievable by simple genetic mutation (though let's give credit where credit is due—memory was the result of a genetic mutation). I think this represents a critical step in the evolution of evolution, on which more later.
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This is the neuroscience of trust: hard to build, easy to break.