Thinking Brain versus Survival Brain

The Science Behind Stress: Part 1 – What is stress?


I had to split my blog into two parts to approach stress scientifically. The first part is about the neurological processes we all experience under stress as humans. In part 2, I try to offer solutions that have a beneficial effect on stress reduction.

This blog covers some of the definitions and background information needed to understand the stress-related processes in our bodies. It contains many technical terms, but I try to present them visually. I got this information mainly from the excellent work of Elizabeth A. Stanley 'Widen the Window’.

The mind-body and two brains

Let’s start with the first definition, the ‘mind-body’. By which I mean our entire human organism: the brain, the nervous system, neurotransmitters, the immune system, the endocrine system, and the body, organs, skeleton, muscles, fascia, skin, and fluids.
By using this definition, I indicate how complex our whole body is. We need to consider all aspects to understand our body and its processes better.
It is common to identify the brain by its physical appearance. Without going into too much detail, we can split the physical appearance into three main physical parts:

The brainstem serves as a relay station, passing messages back and forth between various body parts and the cerebral cortex. Many simple or primitive functions that are essential for survival are located here.
The Cerebellum fine tunes motor activity or movement, e.g. the fine movements of fingers as they perform surgery or paint a picture. It helps one maintain posture, sense of balance or equilibrium by controlling the tone of muscles and the position of limbs. The Cerebellum is critical in performing rapid and repetitive actions such as playing a video game.
The Cerebrum is the largest part of the brain. The Cerebrum initiates and coordinates movement and regulates temperature. Other areas of the Cerebrum enable speech, judgment, thinking and reasoning, problem-solving, emotions and learning. Other functions are related to vision, hearing, touch and other senses.


Brain region by physical appearance
Figure 1: Brain region by physical appearance

On the other hand, our brain is designed to function as a cohesive whole, with each region processing information. Although these regions have overlapping circuitry, we can differentiate them by their respective functions.

1) The thinking brain, evolutionarily the newest region, is the neocortex. The thinking brain engages in top-down processing – our mostly voluntary and conscious cognitive responses to our experiences (ethical choices, reasoning, abstraction, and analytical capabilities). The thinking brain has an explicit learning and memory system to support these functions.
2) The survival brain comprises the evolutionary older limbic system, brain stem, and Cerebellum. These brain regions engage in bottom-up processing – our involuntary emotional and physiological responses, including emotions, relationships, stress arousal, habits, physical sensations, vocalisations. An essential function is neuroception, an unconscious process of rapidly scanning the internal and external environment for opportunities/safety/pleasure and threats/danger/pain. To support the neuroception, the survival brain has an implicit learning and memory system – fast, automatic, and unconscious, bypassing the thinking brain.

Together, the thinking brain and survival brain comprise what many people call ‘the mind.’

Brain regions by function
Figure 2: Brain regions by function

Important: Awareness does not belong to the thinking brain or the survival brain. That’s why we can pay attention to thoughts, emotions, physical sensations, and the body’s position, temperature, pressure, pain.
Tai Chi training helps us learn how to direct and sustain our attention – so that we become aware of all these different body-mind experiences.

Two main nervous systems

The nervous system is commonly divided into the central nervous system (CNS) and the peripheral autonomic nervous system (ANS). The central nervous system comprises the brain, its cranial nerves and the spinal cord. The peripheral nervous system is composed of the spinal nerves that branch from the spinal cord and the autonomous nervous system (divided into the sympathetic and parasympathetic nervous systems):

  1. Central Nervous System (CNS)
  2. Peripheral Autonomous Nervous System (ANS)

2.1 Parasympathetic Nervous System (PSNS)

                       2.1.1 Ventral PSNS

                       2.1.2 Dorsal PSNS

2.2 Sympathetic Nervous System (SNS)

High-level Nervous System
Figure 3: High-level Nervous System overview (Illustration @ VictorMine)

It is essential to know that the ANS is responsible for stress arousal and recovery and plays a significant role in our patterns of engagement and interaction with others.

Relationship between the thinking brain and the survival brain

The relationship between stress arousal and performance -including thinking brain functions- is the Yerkes-Dodson Curve inverted U-shaped curve.

Yerkes Dodson Stress Performance Curve
Figure 4: Yerkes Dodson Stress Performance Curve (Illustration © Vaeenma)

In 1908, psychologists Robert M. Yerkes and John D. Dodson were the first to posit the Inverted U-shaped relationship between perceived stress levels and performance. Performance on a task improves as someone approaches moderate stress arousal but steadily decreases past this point until it drops off completely.
Thus optimal performance, conscious learning, and effective decision-making are most likely to occur at moderate stress levels. With this in mind, our neurobiological window of tolerance to stress arousal is how we can adjust our stress levels upwards or downwards to remain within the optimal performance zone of moderate arousal.
The optimum window lies between the Baseline and Threshold, as depicted below.

Peak performance
Figure 5: Peak performance

Not everybody has the same threshold, and the level will also vary for each person daily.
Depending on the situation, each person will reach their limit at a certain point.

Baseline & Threshold

Widening the window

Raising the threshold is the solution to perform better under the same stressful conditions. We call this widening the window.
The wider the window, the more likely we can maintain accurate neuroception and effective integration of thinking brain and survival brain processes, even during the level of high-stress arousal and emotional intensity.

Before widening the window
Figure 7: Before widening the window
After widening the window
Figure 8: After widening the window

Three pathways narrow our window. The first pathway to narrowing our window is chronic stress and developmental trauma during childhood. The second pathway is shock trauma when we experience too much stress activation or comes up too fast. And the third pathway is a challenge that goes on too long or comes up too often – leading to chronic stress. I will focus on the latter.

Flipping the Yerkes-Dodson Curve

To continue visualising the neurological processes, I will flip the axes of the Yerkes-Dodson Curve. Showing the same result but will facilitate the visualisation of the following chapters.


Yerkes-Dodson curve
Figure 9a: Yerkes-Dodson curve


Flipped Yerkes-Dodson curve

The body during stress

The Stress Equation explains how the mind-body system produces stress activation/stress arousal. Whenever we experience a (1) Stressor, an internal or external event that (2) the survival brain perceives as threatening or challenging, then our mind-body system turns on (3) Stress arousal, which is physiological activation in the body and mind.



Perception of threat



(internal or external event)


(survival brain’s neuroception)


(activation in the mind-body)

Whenever the first two components of the equation arise together, there is NO way around experiencing stress arousal. It’s just not possible. Remember, however, that neuroception is a survival brain job.

Two hormonal waves

Stress activation is all about shifting energy from long-term to immediate needs. When perceiving a threat, our mind-body system is designed to focus on danger and mobilise energy to react FAST. Receiving upsetting mails of getting stuck in traffic is different from being chased by a sable-tooth tiger. Nevertheless, we still mobilise the same response that cave people used to survive.
When the survival brain neurocepts threat, it sends messages to the endocrine system to release hormones needed for immediate survival and inhibit hormones used for long-term needs. These hormonal changes controlled by the HPA axis occurs in two waves.

HPA axis activation proceeds from the hypothalamus to the pituitary gland to the adrenal glands. The hypothalamic-pituitary-adrenal axis, or HPA axis as it is commonly called, describes the interaction between the hypothalamus, pituitary gland, and adrenal glands. The hypothalamus and pituitary gland are located just above the brainstem, while the adrenal glands are found on top of the kidneys.

HPA Axis
Figure 10: HPA Axis Illustration @ Brian M Sweis

First, once the survival brain perceives a threat, it directs the endocrine system to release adrenaline. Within seconds, adrenaline increases our heart rate to quickly pump blood to the organs and big muscles in our limbs so that we can move fast. In the lungs, adrenaline dilates our bronchial tubes to increase our breathing rate so we can take more oxygen. Adrenaline also mobilises the body to release glucose, so we’ll have a ready source of energy.
At the same time, blood flow shifts away from the digestive system, which we may experience as nausea or butterflies in the stomach. Adrenaline also constricts the blood vessels supplying our skin. That’s why our skin may feel cold and clammy, and our palms may get sweaty.
Everything about stress activation is initially aimed at transporting oxygen and glucose since we need energy and brain focus right now.
After the first wave, the survival brain and HPA axis controlling stress hormones work together to adjust our stress level to correspond with the particular stressor. Now the HPA axis provides a finetuning second wave of stress activation. The HPA axis will amplify stress activation if we still perceive a threat. Thus, whether we perceive ourselves as having agency in the situation plays a critical role in our second wave of stress activation.
The most crucial energy-mobilisation hormone is cortisol. Cortisol has two jobs during the second wave. First, it replenishes energy stores that got depleted during the first wave’s adrenaline rush. Second, cortisol boosts immune functioning.
The HPA axis also activates other hormones that prioritise immediate needs:

- Endorphins that blunt our perception of pain,
- Vasopressin regulates the cardiovascular system,

Conversely, the HPA axis inhibits hormones related to long-term needs, like

- Growth hormone
- Sex hormones (estrogen, progesterone, testosterone)
- Insulin to inhibit energy storage

Later, after the stressor has passed, it activates hormones to facilitate recovery. During both the second wave and recovery, the HPA axis works with the immune system and the autonomic nervous system (ANS)

Three lines of defence – Three nervous branches

We have three hierarchical levels of ‘defence in depth’, with each defensive strategy supported by a distinct neural circuit between the brain and the ANS.

Our hierarchical defence's three stages developed evolutionarily, with each new defensive strategy building on evolutionarily older ones.

Ideally, this means we’ll rely first on the most recently developed defence, our ‘social engagement system’. You try to negotiate with the assailant, look around to locate safety, or call for help.

If this doesn't work, we can fall back to two evolutionarily older defences.

The second defence is called ‘fight-or-flight’. If you were going to run from a sable-tooth tiger, for example, you want this response to save your life. When we have a fight response, we can have anger, rage, irritation, and frustration. If we have a flight response, we can have anxiety, worry, fear, and panic. Physiologically, our blood pressure, heart rate, and adrenaline increase and decrease digestion, pain threshold, and immune responses.

The third defence is called ‘freeze’ when you stop struggling with the assailant and are disconnected from what is happening. This means that we are entirely shut down. We can feel hopeless and feel like there’s no way out. We tend to feel depressed, conserve energy, dissociate, feel overwhelmed, and feel like we can’t move forward. Physiologically, our fuel storage and insulin activity increases and our pain thresholds increase.

The survival brain’s neuroception process automatically determines which of these defences is activated at any particular time. This leads to the well-known Polyvagal chart. You will notice that the parasympathetic nervous system (PSNS) is divided into two branches because it uses two forks of the vagus nerves. The ventral PSNS branch is located along the front side of the body, while the dorsal PSNS is located on the backside of the body.

Polyvagal Chart

Simplifying the Polyvagal chart and changing the time axis back to the performance axis as in the Flipped Yerkes-Dodson curve gives me the following picture:

Three Stress stages in Defensive Mode
Figure 12: Three Stress stages in Defensive Mode

However, this is only half of the complete story. Whenever the survival brain neurocepts safety, we’re inside our window of tolerance. Inside the window, we can access all the nervous system branches in ‘well-being mode’. This gives the following graphical view:

Branches of ANS and their functions
Picture 13: Branches of ANS and their functions

When viewing the same info but with split up between the three different ANS branches, it leads to the following overview:

Branches of ANS and their functions
Picture 14: Branches of ANS and their functions

The survival brain during stress

Just as stress arousal involves our nervous system and body, it also affects our brain, especially learning and memory. Taking an evolutionary view, this makes sense: remembering and learning from stressful events is vital for survival.
To support neuroception, the survival brain needs a learning and memory system that’s fast. Thus, its learning system is reflexive, unconscious, and involuntary – bypassing the thinking brain completely.
The survival brain’s learning system is called implicit learning, and it occurs predominantly in the amygdala. The learning and memory functions occur unconsciously at any level of stress arousal. Moreover, the greater the stress arousal, the more the survival brain learns and remembers.
At moderate stress levels, the amygdala works with the hippocampus to create explicit memory (part of the thinking brain), with the amygdala providing the emotional component of the memory.

The parts of the brain involved in memory
Figure 15: The parts of the brain involved in memory (Illustration by Levent Efe)

The thinking brain during stress

The thinking brain also performs functions for our survival by analysing, planning, deliberating and making decisions.
It engages in ‘top-down processing’ – our mostly voluntary and conscious cognitive responses to our experiences.
Parallel to the survival brain’s neuroception, the thinking brain is responsible for executive functioning, predominantly in the prefrontal cortex.
Our intelligence and other individual differences influence explicit memory, but our stress arousal levels also profoundly affect it.
Mild to moderated stress arousal enhances explicit memory and conscious learning in the short term. However, executive functioning and explicit memory functions may be impaired or damaged with prolonged or high-stress levels.


Stress activation/stress arousal is the internal response that our mind-body system creates whenever we have an experience that the survival brain perceives as threatening or challenging.
The process of perturbing our inner equilibrium and returning to our regulated baseline is called allostasis. Allostasis allows us to mobilise the appropriate energy and focus for coping well before, during, and after the threat or challenge. However, with chronic or prolonged stress, our mind-body system doesn’t fully recover after a stressful experience – instead, it remains in an active state.
Allostasis allows us to vary internal conditions, including heart rate, breathing rate, temperature, and blood sugar levels, so we can galvanise the appropriate amount of energy and focus for coping with the crisis.


Without adequate recovery after chronic stress, the mind-body system remains activated and doesn’t return to its regulated equilibrium. When the allostatic load accumulates, we can experience a range of physical, emotional, cognitive, spiritual, or behavioural symptoms.

Allostasis load accumulation
Figure 17: Allostasis load accumulation

Allostasis happens through communication between (1) the brain; (2) the autonomic nervous system; (3) the immune system; and (4) the endocrine system, especially the HPA axis, which controls the stress hormones. Thus, the allostatic load can manifest as imbalances or malfunctioning in any of these four systems.

In other words, dysregulation can affect all aspects of our mind-body system – and thus manifest as a range of cognitive, emotional, physiological, spiritual, and behavioural symptoms.

So it is time to see what solutions we can offer you. Read more in my next blog

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