Notes that translate the heart’s breathing between feeling and language.
Chapter 2: The Neuroscience of Introversion
── The Structure Behind “The Three Inner Layers — Mapping the Quiet Depths Within”
I. The Confusion of Three “Inners”
In Chapter 2 of the main text, I wrote:
“Within the inner world, there are three layers.”
Introversion — The direction of energy
Introspection — The organization of thought
Mindful observation — The stillness of consciousness
These three all turn “inward,” so they’re easily confused.
But their brain mechanisms are completely different.
Why they’re confused
Generally, when we say “introverted person,” what do people imagine?
- Likes being alone (introversion)
- Thinks deeply (introspection)
- Meditative (mindful observation)
These three get lumped together under the single word “introverted.”
But neuroscientifically, these are separate systems.
II. The Neural Basis of Introversion
First, let’s look at Introversion.
Arousal Level Theory
Psychologist Hans Eysenck proposed the arousal level theory.
The human brain has an “arousal level.” This baseline differs from person to person.
Introverted people: High baseline
- Already aroused
- Sufficient with little external stimulation
- Too much stimulation causes over-arousal
Extroverted people: Low baseline
- Need to raise arousal level
- Seek external stimulation
- More comfortable with more stimulation
Therefore, introverted people prefer quiet environments.
This isn’t personality — it’s a characteristic of the nervous system.
Reticular Activating System (RAS)
What controls arousal level is the Reticular Activating System (RAS).
This is a neural network in the brainstem that filters external stimuli and regulates arousal level.
Introverted people have highly sensitive RAS.
Therefore, they become aroused with little stimulation.
Too much stimulation causes over-arousal.
That’s why “meeting people is exhausting.”
Dopamine and Acetylcholine
Neurotransmitter levels also differ.
Extroverted type: Dopamine-dominant
- Dopamine = reward system neurotransmitter
- Activated by socializing, action, new experiences
- Gains reward from the outer world
Introverted type: Acetylcholine-dominant
- Acetylcholine = neurotransmitter of introspection, memory, thought
- Activated by thinking alone, reading, creating
- Gains reward from the inner world
In other words, introverted people prefer being alone because being alone is a reward for the brain.
Stimulation Processing Pathways
As mentioned in the main text, introverted and extroverted types process stimuli through different brain pathways.
Extroverted type: Short pathway
Stimulus → Sensory cortex → Motor cortex → Response
- Fast
- Immediate response
- Shallow processing
Introverted type: Long pathway
Stimulus → Sensory cortex → Frontal lobe (thought) → Hippocampus (memory) → Motor cortex → Response
- Slow
- Contemplates before responding
- Deep processing
In this “long pathway,” introverted people compare with past memories, consider meaning, and process deeply.
Therefore, they can’t answer immediately. But they can understand deeply.
III. The Neural Basis of Introspection
Next, let’s look at Introspection.
Default Mode Network (DMN)
Introspection is related to the Default Mode Network (DMN).
DMN is a brain network that activates when doing nothing, when spacing out.
Main regions:
- Medial prefrontal cortex (mPFC)
- Posterior cingulate cortex (PCC)
- Precuneus
DMN is involved in:
- Self-referential thought (thinking about oneself)
- Theory of mind (inferring others’ minds)
- Episodic memory (recalling the past)
- Imagining the future
In other words, introspection itself.
Introverts and DMN
Research shows that introverted people have higher DMN activity.
In other words, even when spacing out, the brain is working actively.
Thinking about themselves. Reflecting on the past. Imagining the future.
This is constantly happening.
Therefore, introverted people, even when they appear to be “thinking about nothing,” are actually engaging in enormous amounts of thought.
The Risk of Rumination
But DMN overactivity has risks too.
Rumination.
Rumination is repeating the same thoughts over and over.
“Why did I say that?”
“I should have done it that way”
When falling into this thought loop, the risk of depression and anxiety increases.
What I called “thought loops” in the main text refers to this rumination.
Introspection and Self-Awareness
But introspection also has value.
Self-awareness.
Understanding one’s emotions, thoughts, and motivations.
This is essential for psychological maturity.
People good at introspection have deep self-understanding.
They can discern their own patterns. They know their own habits.
That’s the foundation for growth.
IV. The Neural Basis of Mindful Observation
Finally, let’s look at Mindful Observation (also called Inner Mindfulness).
Attention Control
Mindful observation is the ability to control attention.
Not swept away by thoughts, not swallowed by emotions, just observing.
This is related to the brain’s prefrontal cortex (PFC).
The prefrontal cortex governs executive function:
- Attention control
- Emotion regulation
- Impulse inhibition
Mindful observation training strengthens this prefrontal cortex.
Insula and Interoception
Another neural basis of mindful observation is the insula.
The insula processes interoception.
Interoception is the sensation of the body’s interior:
- Heartbeat
- Breathing
- Digestion
- Muscle tension
In mindful observation, attention is directed to these bodily sensations.
“Right now, my chest feels heavy”
“Right now, my shoulders are tense”
“Right now, my breathing is shallow”
That awareness activates the insula.
Calming the Amygdala
One effect of mindful observation is calming the amygdala.
The amygdala is the region that processes fear and anxiety.
When stressed, the amygdala becomes overactive.
But mindful observation training decreases amygdala activity.
Therefore, anxiety and fear decrease.
You become less likely to be swallowed by emotions.
The Difference Between Introspection and Mindful Observation
Let’s organize the difference between introspection and mindful observation.
Introspection: DMN-dominant
- Thinks
- Analyzes
- Finds meaning
- Linguistic
Mindful observation: Prefrontal cortex + insula dominant, DMN decreased
- Observes
- Accepts
- Notices
- Non-linguistic
Introspection uses thought. Mindful observation stops thought.
Introspection moves between past and future. Mindful observation stays in the here and now.
V. HSP and Sensory Processing Sensitivity
In the main text, I mentioned HSP.
Here, let’s look at its neuroscience.
Sensory Processing Sensitivity (SPS)
HSP (Highly Sensitive Person) is a neurophysiological trait called Sensory Processing Sensitivity (SPS).
Elaine Aron proposed four characteristics of SPS (DOES):
D – Depth of Processing
- Processes information deeply
- Notices details
O – Overstimulation
- Easily overwhelmed by stimulation
- Tires easily
E – Emotional Reactivity
- High empathy
- Responds emotionally easily
S – Sensing the Subtle
- Notices subtle stimuli like sound, light, smell
The HSP Brain
Brain imaging research shows that HSP individuals have higher activity in brain regions related to visual processing and attention.
Especially:
- Insula (interoception)
- Mirror neuron regions (empathy)
- Prefrontal cortex (deep processing)
In other words, HSP individuals are neurologically processing more information.
That is both a gift and a burden.
The Difference Between HSP and Introversion
HSP and introversion are often confused, but they’re different traits.
Introversion: Direction of energy
- Recharges internally
- Low optimal stimulation level
HSP: Depth of sensory processing
- Processes stimuli deeply
- High sensitivity
There are introverted HSPs and extroverted HSPs (about 30%).
But they often overlap.
VI. The Neuroscience of “Energy Sickness”
In the main text, I wrote about “energy sickness.”
The phenomenon where being near people with negative emotions causes headaches or fatigue.
This isn’t spiritual. It can be explained by neuroscience.
Mirror Neuron Resonance
Mirror neurons are nerve cells that activate as if you were experiencing it yourself when you see another person’s actions or emotions.
HSP individuals may have excessive mirror neuron activation.
Therefore, others’ emotions feel like their own emotions.
Autonomic Nervous System Synchronization
Furthermore, there’s a phenomenon of autonomic nervous system synchronization.
Humans unconsciously synchronize with others’ breathing, heartbeat, and muscle tension.
This is called physiological synchrony.
When the other person is tense, you become tense.
When the other person is anxious, you become anxious.
HSP individuals have strong synchronization.
Therefore, they receive others’ negative emotions in their body.
That manifests as headaches or pain in the back of the head.
Stress Hormone Influence
Furthermore, there’s the influence of stress hormones (cortisol).
When someone is in a stressed state, their body secretes stress hormones like cortisol.
HSP individuals detect those subtle changes (smell, facial expression, posture).
And their own body triggers a stress response.
That’s the true nature of “energy sickness.”
VII. Bridge to the Main Text
Chapter 2 of the main text spoke poetically:
“Within the inner world, there are three layers.”
This commentary decoded that structure.
Summary of Structure
Introversion: Arousal level and acetylcholine
- High RAS sensitivity
- Long stimulation processing pathway
- Being alone is rewarding
Introspection: Default Mode Network
- High DMN activity
- Self-referential thought
- Risk of rumination
Mindful observation: Prefrontal cortex and insula
- Attention control
- Interoception
- Amygdala calming
HSP: Sensory Processing Sensitivity
- Deep processing
- Mirror neuron overactivity
- Physiological synchrony
The three are separate systems. But they often overlap. Each has value.
Dialogue with the Main Text
The main text spoke to you:
“When these three are confused, the fog thickens.”
This commentary explained the reason.
Introversion, introspection, and mindful observation are different brain systems.
I proved that in the language of neuroscience.
But proof is unnecessary.
You already know yourself.
The value of being alone. The meaning of thinking deeply. The stillness of just observing.
You experience all of these.
To the Next Chapter
In the next chapter, we’ll look at the “outside.”
Why don’t we understand our outer selves? What happens when speaking in front of crowds?
What the main text called “blind spots of self-perception,” we’ll translate into the language of psychology.
Breathe in deeply, exhale slowly.
In that quiet rhythm, your brain — and your being — continue to live.
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