Your brain determines your mind, and your mind determines your brain; they are interactive, mutually causal; as your brain changes, so does your mind, and vice versa. While its tempting to think of our unexpressed thoughts as without substance or lasting significance, that's far from the case.
If you find yourself pondering certain thoughts repeatedly, the neural brain circuits that correspond to these thoughts will become increasingly sensitive, the synaptic structures will bond with greater stability and will become more active: You are neurally carving the thought circuits into your frontal lobe. Visualize a skier repeating the same route down a slope: the result will be a trail. The more the skier uses the trail, the deeper the trail becomes, and eventually the route will become so ingrained the skier will find it difficult to discard. Also, its easier to ski in trails then carve out new ones. It's the same process for your thoughts; the more you think a fearful, worst-case-scenario thought, the harder it is for you to put it aside; you've synaptically hardwired it.
Once ingrained, the brain doesn't like to do the hard work of wiring new synaptic connections; it prefers to fall back on what's been hardwired. This is why all humans have confirmation bias: our minds have a tendency to search for information in a way that confirms its ingrained thought patterns and beliefs; it requires a great deal of neural energy to carve out those new thought circuits.
Thoughts in the mind can create feedback loops in the brain that are difficult to abandon or override. Example: If a thought of financial insecurity activates your midbrain's fear center, the amygdala, that will invariably result in a release of the hormone cortisol. Cortisol, in turn, can trigger the hypothalamus, which will release both adrenaline and dopamine, creating both the motivation and drive to seek comfort foods to relieve your mind's agitated state. You might not note the connection between money concerns and eating peanut M&Ms, but the neural circuits created your behavior behind the scenes; such is the case of all tendencies and addictions. Evolution has set up your brain to suggest the fastest way to get rid of stress, regardless of the long term consequences. When the dopamine and adrenaline wears off, you're likely to feel a diminution of pleasure. As pleasure and reward depletes, you'll once again feel a greater sense of unease, which can result in stressful thoughts, repeating the circuit.
The more any impulse is acted upon—e.g. to eat while feeling stressed—the harder it becomes to inhibit the urge. It's only when you find a new way to resolve stress, rather than seeking immediate pleasure, that you'll be able to move away from an ingrained feedback loops.
And so its quite hard to give up maladaptive defense mechanisms, as our tendencies to avoid conflicts not only helped us to survive our childhoods, they're hardwired by adult life. Once instilled, our tendencies to avoid difficult conversations will sabotage our adult relationships, but we'll find the urges to run difficult to override.
Finally, note that when we have a negative experience—for example we feel abandoned by someone we love—the brain tends to note all the sensations that are present during the event; the brain has special memory systems devoted to any situation that feels threatening. All of the sensations, even the most benign, will create markers that can trigger anxiety to arise in the future. As these markers work subconsciously, anxiety can seem to arise without reason, though invariably it's because we've stumbled upon a trigger. Example: If ends a relationship with you is wearing a green scarf, and drinking Chai Tea, green scarves and Chai Tea may very well activate stress and distrust in the future, even though they probably had little to do with the relationship's demise.
Such "mistaken triggers" build up in the brain, meaning we're often activated for little or no reason. This is why, when people say "I'm learning to trust my instincts" I cringe, as instincts are quite often at the beck and call of the brain's oldest and least skillful processing regions. Sometimes they're right, often they're wrong: Is this the way we want to make important decisions? As we saw in the example of the green scarf above, the instinct to move towards, or away from, another can be easily corrupted by wildly mistaken triggers.
The more we understand how the mind and brain work in tandem, the more useful it is to conceive of the darma from a neuropsychological, causal perspective. For example, karma**: What I think today neurally wires my mind, making it more likely to repeat the same type of thought tomorrow. The way I use my brain today determines future tendencies. The more I act on impulses, the harder they are to override. As the Buddha noted in the opening of the Dhammapada, "If a person acts or speaks in a skillful way, happiness will follow them like a shadow."
So, how does this insight translate into my day to day life? A couple of ways:
• Thoughts have causal implications as the dharma notes repeatedly; they actively play a role in our future happiness. If I think actively negative thoughts about someone while pretending to present an open mind, I'm causing agitation for myself in the future, as judgement and criticism tend to result in far more stress and agitation than a mind that's receptive and unbiased.
• When feeling stress or mental unrest, unless I'm in genuine danger, the first impulse, which will invariably involve the midbrain's survival first circuits, are often not the best. While the brain pushes us to the same old survival first strategy, growth, not to mention free will, means expending the effort to seek alternative, skillful solutions, and to set about inscribing them into the brain.
Josh, dharmapunxNYC, 11/1/14
*By “mind” I mean both awareness and the information processed outside of consciousness, such as the underlying impulses and inclinations that influence one's thoughts, moods, feelings and perceptions. A mind cannot exist without a brain—no, your consciousness will continue on after the death of your body—but there are generally more processes at play: The mind and brain are parts of large networks, such as surrounding social structures and ecosystems.
**Karma can also have an interpersonal dimension as well: If I treat people kindly, I'll have less to fear, and less shame or self-justification, which means I'll be happier as well.