The Mind-Body System
Here’s what the mind-body system looks like:
Your evolved psychology and conditioning are encoded on a physiological level (your brain circuitry), leading you to have certain emotional responses, thoughts, and behaviors.
These three components all influence each other bidirectionally. For example, you might get excited (emotion) at a friend’s suggestion of attending a concert together (thought). Your emotional response was likely the result of some conditioned memory (physiology) of having had a good time dancing and listening to music at a previous concert you attended. If you’ve never attended a concert before, then maybe your excitement is a result of expectations formed from hearing about concerts (thought) and this excitement causes your heart to beat faster (physiology) as a result, generating the feeling of enthusiasm (emotion). Your behavioral response is to accept your friend’s invitation to the concert. Your concert attendance leads to a new physiological state as a result of the experience you have, and therefore novel thoughts and emotions.
This Mind-Body System is far from simple, but for our purposes, what’s most important is that there are three different inputs under your control that result in a given behavior: physiology, emotions and thoughts.
You can control your thoughts and emotions by changing your physiology through exercise, breathing techniques or any other type of physical training. As a study on self-regulation concluded: “One's ability to self-regulate the quality of feeling and emotion of one's moment-to-moment experience is intimately tied to our physiology… Without establishing a new baseline reference, people are at risk of getting ‘stuck’ in familiar, yet unhealthy emotional and behavioral patterns and living their lives through the automatic filters of past familiar or traumatic experience.” To change your mind, to get “unstuck” from negative mental patterns, you can begin by changing your physiology.
Conversely, you can control your physiology by changing how you relate to your thoughts and emotions through meditation, cognitive behavioral therapy, or any other type of mental training.
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 McCraty, Rollin, and Maria A. Zayas. "Cardiac coherence, self-regulation, autonomic stability, and psychosocial well-being." Frontiers in psychology 5 (2014): 1090.