Surviving Combat: The Effects of Predator Stress on Brain and Body

Extreme stress significantly impairs memory, mood, and cognition in combat soldiers

Photo by the blowup on Unsplash

I deliberately avoid, to the greatest extent possible, political commentary in this newsletter, because there are plenty of newsletters and other sources filling this need. This week, however, I’m veering toward the political - my apologies for this. I’m going to focus on the intense, chronic, uncontrollable, predator stress of combat on the functioning of brain and body of the combat soldier.

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Predator stress, experienced in combat, can have especially severe and long-lasting effects on the brain and body, and understanding its effects is essential to dealing with it. I’ll first focus on stress, and then summarise some of what we know about the stress of combat.

First, let’s define a few terms.

Stress

Stress is a feeling of being overwhelmed or anxious caused by events or situations that are perceived as challenging or threatening. In the short term, stress can be helpful, but when chronic and prolonged, it negatively affects brain and body. Stress hormones like cortisol and catecholamines are released during stress, and have different effects on different areas of the brain, which can lead to problems with mental health and overall functioning (previous pieces on stress here).

More formally: (definition)  stress involves heightened excitability or arousal in the brain and body, a perception that events are aversive or noxious, and a lack of control over outcomes. Combat meets this criteria easily: sitting in a trench, being shelled, and just having to wait it out, with death possible at any moment.

Naturally, this is horribly stressful. Stress results in a physical and mental response preparing the body for action. But, in combat, often you can’t take action - you simply have to sit there, and wait it out, because doing something is just too dangerous - getting out of your trench and running away is much more dangerous.

So you sit, heart pounding, body and brain in overdrive … gears grinding away, just waiting, ruminating, anticipating.

Underlying fear and anxiety are two specific brain circuits, one to help us learn to be afraid, and another to allow brain different areas to affect fear and anxiety. But - these are not isolated: it is vital to think of the circuits involved in fear and anxiety as being part of larger brain and behaviour networks, involving memory, cognition, planning, perception, reflex thresholds, and more.

Here’s a graphic showing the various relationships from molecules (stress hormones) all the way to thinking, feeling, and acting. The stress response is controlled by the hypothalamic-pituitary-adrenal (HPA) axis, which is substantially regulated by the hippocampal formation. These relationships are very complex, but comprehensible: follow the block arrows. Stress gives rise to negative and positive feedback loops affecting all bodily functions.

consensus model of the interactions between chronic and severe stressors and their effect on neurocognitive functioning.

Physiology of stress

The physiology of the stress response has become reasonably well-understood over the past few decades. Lots of progress has been made in understanding how stress impacts on cognition - and especially on the brain systems that support learning and memory. The hippocampal formation is widely regarded as the substrate for the brain’s ‘cognitive map’ of the world, and is crucial for everyday memory and recollection; its synapses are highly plastic and its structure and functions are especially sensitive to stress.

Behavioural stress (e.g., from combat) and/or systemic stress (e.g., anoxia; infection; wounds) triggers the release of corticotropin-releasing hormone (CRH) from the hypothalamus into the portal circulation to the anterior pituitary, which releases adrenocorticotrophic releasing hormone (ACTH) into the bloodstream, causing cortisol (human) release from the adrenal cortices.

ACTH initiates ‘fight or flight or freeze’ responses, mobilises energy stores, decreases reflex thresholds, and increases respiratory rate, muscle tension and gastric motility. These effects, if short-lived, are generally positive; but harmful effects ensue for brain and body, however, from elevated and prolonged increases in cortisol levels (see footnote

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for a very short summary).

Predator stress

This is probably the most intense stress an animal or human can experience. Being the mouse toyed with by a cat; or a soldier in a trench aware of a drone overhead, one you can’t see, but which can see you, and which is going to drop munitions on you and your comrades.

Or being a Roman solider armed with shield, sword, hanging dagger, terrified while advancing, almost certainly to his death, on Saxon hordes throwing spears at him and his comrades who are also under severe combat stress. Death is everywhere, and may come at any time (another type of predator stress is torture: previous pieces on torture here).

Picture created by author using Stable Diffusion - text prompt is in alt-text description

Surviving Combat

To survive combat, you must be hypervigilant to the threat all around you, while simultaneously posing exactly that threat to someone in exactly the same hypervigilant state as you.

And to complicate matters: we possess a vicarious and automatic response to seeing another living being in distress. To experience distress, you merely need to see distress in others. Our brains possess a specialised network (the ‘pain matrix’) that automatically and reflexively responds to the distress, pain, and despair of another, and allows us feelings of empathy and sympathy for the suffering of another.

Combat Stress

What are the effects of extreme stressors on combat soldiers? Ideally, you need studies with a studies using a "pre-post" experimental design to measure changes in psychological and physiological variables from pre to post combat- and there are many such studies available.

Takeaway

Extreme stress significantly impairs memory, mood, and cognition in combat soldiers.

To ameliorate stress, combat soldiers need regular rotation out of combat in order to recover, and during prolonged combat, must find time to eat, sleep, drink liquids, and stay as comfortable as is possible. This is a tall order if you’re under attack. Here, rear-area combat logistics play an essential role: delivering food, water, and ammunition, and providing medical evacuation for the injured.

One of the leaders in this area is Charles Morgan and his colleagues, who have done many studies examining combat stress in soldiers (list of some at bottom

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). These studies use real-life stressors experienced by soldiers during combat training, and assess a wide range of psychological and physiological variables

The studies use a pre-post experimental design, where soldiers are initially assessed, then undergo simulated combat, and reassessed to determine any changes in performance attributed to the training. This design allows for the investigation of the effects of extreme stress on memory, cognition, and other psychological variables, as well as the effects on hormones.

Combined stressors degrade cognition, mood, and hormonal status in elite military soldiers on a combat training exercise

Morgan and colleagues have examined the effect of combining stressors such as sleep loss, heat, dehydration and undernutrition might have on cognitive performance, mood and hormonal status in an elite military unit, on a combat training exercise. They measured, in thirty-one male volunteers (who had about nine years active military service each) three times over five days the effect of the combination of simulated combat with sleep deprivation, heat stress, physical stress and a restricted diet on cognitive performance, mood, physical activity and hormonal status.

They measured reaction times, vigilance, memory and reasoning. They also administered a mood profile questionnaire, and measured sleep using an actigraph (an activity monitor). The soldiers all had free access to water so hydration was (initially) not a problem. They also measured body composition using a DEXA scanner (used clinically to measure bone density). Finally, they measured from saliva the stress hormone cortisol, testosterone and melatonin.

Over the test period of five days they found that simulated combat (and the severe stressors associated with it) caused a dramatic reduction in natural sleep rhythms. The soldiers slept for approximately three hours over this period of time, usually in short bursts of about twelve minutes or so. These periods were never sufficient to repay accumulated sleep debt.

They found the soldiers lost substantial amounts of weight – approximately four kilograms on average over the period of five days. The weight loss consisted mostly of water loss; it did not appear to be muscle mass or fat, despite the soldiers having free access to as much water as they required over the period of time.

The soldiers were severely impaired on all tests of cognition. Their mood also disimproved dramatically as did their hormonal status.

In summary “...even well trained leaders exhibit significant degradation in cognitive performance and mood when exposed to severe multi-factorial stress”.

Conclusion: Combining stressors (sleep loss, heat, dehydration, undernutrition - all common features of combat) degrades cognitive performance, mood, and hormonal status in elite military units during combat training exercises.

Other studies

One study conducted by Morgan and his team looked at the levels of neuropeptide Y (NPY) in soldiers undergoing military survival training. NPY is a naturally occurring compound secreted in response to stress to reduce anxiety. NPY levels increased in soldiers subjected to military-style interrogation, indicating a response to acute psychological stress.

Another study focused on levels of dehydroepiandrosterone (DHEA; also involved in the stress response). Morgan and his team measured DHEA levels in military personnel undergoing survival school training, along with administering the Clinician-Administered Dissociative States questionnaire, which measures the extent to which an individual is in touch with their environment or dissociated from it. DHEA levels were significantly lower in those who experienced higher levels of perceived stress and reported more dissociative symptoms.

Other studies conducted by Morgan and his team used a mock prisoner of war scenario to examine the effects of stress on memory and cognition. Soldiers subjected to the scenario had significantly impaired memory performance, including both episodic and semantic memory, as well as reduced cognitive flexibility and increased errors on a cognitive task.

Another study used combat simulation training to examine the effects of stress on memory and cognitive function. Soldiers who experienced the most stress during the training had significantly impaired memory performance, including both immediate and delayed recall, as well as reduced cognitive flexibility and increased errors on a cognitive task.

Conclusion: Extreme stress dramatically impairs memory, mood, and cognition in combat soldiers

Extreme stress can have serious consequences for soldiers, both during and after their time in combat, and highlight the importance of addressing and managing stress in military personnel.

The importance of sleep

Sleep is badly affected by combat conditions, and yet is an essential for normal function. With a very high degree of certainty, we know that prolonged sleep deprivation, including causes widespread changes in many organ systems. When sustained for sufficiently long periods of time, sleep deprivation results in death of experimental animals and humans.

Individuals suffering from chronic insomnia have lowered hippocampal volumes on average, and suffer deficits in memory.

Many experimental studies, in volunteer students, psychiatric patients and elite populations (such as soldiers), confirm sleep deprivation degrades memory and cognition, in direct proportion to the period of time since sleep deprivation was imposed.

Sleep pressure builds, and can’t be avoided. You must get sleep in order to function effectively (previous pieces on sleep here). Ensuring adequate sleep under combat is a major challenge.

I’ll do a piece soon on trauma after war, PTSD, and resilience.

1. We’re almost a year now into the horrific and barbaric war of imperial conquest being waged by Russia against Ukraine. There are many Substacks to keep up with what is going on. Here are a few:

Futura Doctrina

Polemology Positions

Timothy Snyder

Phillips’s Newsletter

TL;DRussia

The Other Hand

Chills

The Cosmopolitan Globalist

Comment is Freed

The UnPopulist

European Tomorrow

.

There are many other news sources as well, of course.

To briefly summarize a complex literature:

2. Prolonged behavioural and/or systemic stress

(i) inhibits long-term potentiation (LTP; the important biological model of synaptic plasticity and memory);

(ii) causes the hippocampal atrophy (i.e. a key region of the brain involved in memory reduces in size, leading to impairment in learning and memory;

(iv) contributes to brain ageing;

(v) causes many generalised behavioural changes and is implicated in many neuropsychiatric disorders;

(viii) depresses the immune system and slows wound healing.

3. Charles A. Morgan III and his colleagues have conducted one of the most important series of field and experimental studies of human brain functioning under extremes of stress (such as thermal, dietary, combat and predator, interrogative, and sleep- deprivation stress) in military personnel. Here's a selection of papers - some are discussed above.

Aikins, D. E., D. J. Martin, and C. A. Morgan III. (2010). “Decreased Respiratory Sinus Arrhythmia in Individuals with Deceptive Intent.” Psychophysiology 47 (4): 633–636. doi: 10.1111/j.1469-8986.2010.00976.x. Epub 2010 Mar 4.

Lieberman, H. R., G. P. Bathalon, C. M. Falco, C. A. Morgan III, P. J. Niro, and W. J. Tharion. (2005). “The Fog of War: Decrements in Cognitive Performance and Mood Associated with Combat-Like Stress.” Aviation, Space, and Environmental Medicine 76 (7 Suppl.): C7–C14.

Lieberman, H. R., G. P. Bathalon, C. M. Falco, F. M. Kramer, C. A. Morgan III, and P. Niro. (2005). “Severe Decrements in Cognition Function and Mood Induced by Sleep Loss, Heat, Dehydration, and Undernutrition during Simulated Combat.” Biological Psychiatry 57 (4): 422–429.

Morgan, C. A., III, A. Doran, G. Steffian, G. Hazlett, and S. M. Southwick. (2006). “Stress-Induced Deficits in Working Memory and Visuo-Constructive Abilities in Special Operations Soldiers.” Biological Psychiatry 60 (7): 722–729. Epub 2006 Aug 24.

Morgan, C. A., III, G. Hazlett, A. Doran, S. Garrett, G. Hoyt, P. Thomas, M. Baranoski, and S. M. Southwick. (2004). “Accuracy of Eyewitness Memory for Persons Encountered during Exposure to Highly Intense Stress.” International Journal of Law and Psychiatry 27 (3): 265–279. No abstract available.

Morgan, C. A., III, G. Hazlett, S. Wang, E. G. Richardson Jr., P. Schnurr, and S. M. Southwick. (2001). “Symptoms of Dissociation in Humans Experiencing Acute, Uncontrollable Stress: A Prospective Investigation.” American Journal of Psychiatry 158 (8): 1239–1247.

Morgan, C. A., III, A. Rasmusson, R. H. Pietrzak, V. Coric, and S. M. Southwick. (2009). “Relationships among Plasma Dehydroepiandrosterone and Dehydroepiandrosterone Sulfate, Cortisol, Symptoms of Dissociation, and Objective Performance in Humans Exposed to Underwater Navigation Stress.” Biological Psychiatry 66 (4): 334–340. doi: 10.1016/j.biopsych.2009.04.004. Epub 2009 Jun 5.

Morgan, C. A., III, S. Southwick, G. Hazlett, A. Rasmusson, G. Hoyt, Z. Zimolo, and D. Charney. (2004). “Relationships among Plasma Dehydroepiandrosterone Sulfate and Cortisol Levels, Symptoms of Dissociation, and Objective Performance in Humans Exposed to Acute Stress.” Archives of General Psychiatry 61 (8): 819–825.

Morgan, C. A., III, S. Southwick, G. Steffian, G. A. Hazlett, and E. F. Loftus. (2013). “Misinformation Can Influence Memory for Recently Experienced, Highly Stressful Events.” International Journal of  Law and Psychiatry 36 (1): 11–17. doi: 10.1016/j.ijlp.2012.11.002. Epub 2012 Dec 6.

Morgan, C. A., III, S. Wang, J. Mason, S. M. Southwick, P. Fox, G. Hazlett, D. S. Charney, and G. Greenfield. (2000). “Hormone Profiles in Humans Experiencing Military Survival Training.” Biological Psychiatry 47 (10): 891–901.

Morgan, C. A., III, S. Wang, A. Rasmusson, G. Hazlett, G. Anderson, and D. S. Charney. (2001). “Relationship among Plasma Cortisol, Catecholamines, Neuropeptide Y, and Human Performance during Exposure to Uncontrollable Stress.” Psychosomatic Medicine 63 (3): 412–422.

Morgan, C. A., III, S. Wang, S. M. Southwick, A. Rasmusson, G. Hazlett, R. L. Hauger, and D. S. Charney. (2000). “Plasma Neuropeptide-Y Concentrations in Humans Exposed to Military Survival Training.” Biological Psychiatry 47 (10): 902–909.

Taylor, M. K., and C. A. Morgan III. (2014). “Spontaneous and Deliberate Dissociative States in Military Personnel: Relationships to Objective Performance under Stress.” Military Medicine 179 (9): 955–958. doi: 10.7205/MILMED-D-14-00081.