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Survival Stress Research and Post-Event Memory

Research has indicated that a survival stress event not only affects an officer’s perceptual processing of the event as it is taking place, but also the officer’s post-event memory. Combatants such as street police officers have known for years that there are problems with trying to write up their reports about a survival stress encounter before going home. Not giving the officer the time to calm down, sleep, and reflect upon what happened is perhaps a disservice to the officer and often falls short of legal and departmental expectations.

Threatening events cause the Sympathetic Nervous System (SNS) to uncontrollably and automatically engage, leading to various psycho-physiological effects. These include increases in heart rate, adrenaline, and the stress hormones cortisol, epinephrine, and norepinephrine. Combat motor skill performance is affected, exclusionary tunnel vision results, and there is a sensory overload combined with a fixation on some aspect of the incident.

While it is important for the officer to remember everything, the greater the stress, the greater the potential for memory problems to take place. Bruce Siddle and Dave Grossman called this phenomenon Critical Incident Amnesia in 1998 as part of their research on post deadly force memory distortions. However, since their original 1998 paper, there has been little “hard” research exploring stress and memory beyond post incident interviews that were based on the officer’s memory. Siddle decided expand upon his and Grossman’s research with a controlled study that examined how survival stress (Sympathetic Nervous System) impacted memory.

Siddle is a former police officer and a proponent of combat firearms training. He has conducted use-of-force training for criminal justice agencies, the U.S. military, and foreign governmental agencies. He is also the founder of PPCT Management Systems, a research-based, use-of-force training organization that has also funded or coordinated more than 30 studies and grants. The research mentioned in this article examines how stress impacts learning, perceptual processing, decision making, post-event memory, and combat motor skill performance.


Intuitively, professional instructors have known for hundreds of years that training stress—what sports psychologists refer to as “performance stress”—is different from the stress of actual combat. According to Siddle, the subconscious mind can distinguish between simulated stress and real-life survival stress. But only in the past 10 years has technology been able to track and record the difference.

To create a test with measurable, reproducible results, Siddle / PPCT Management Systems conducted a controlled scenario study using a number of different types of stressors to attack as many senses as possible in order to trigger the Sympathetic Nervous System. Traditional training simply does not trigger the survival circuits unless the training is structured very carefully and with an understanding of how the SNS can be “startled” or “sensory overload” activated. This is why there is such a diversity of training techniques within the law enforcement community.

PPCT Managements Systems conducted its controlled training environment research based upon the concept that traditional training methodologies usually make demands within a timeframe of performance. An example is a shooting drill where the officer must engage three targets, shooting each target twice within six seconds. In the next drill the time is reduced to four seconds; the third drill reduces the time to two seconds.

Another training methodology might use combat-oriented video games, where the level of difficulty is succeeded by the engagement of additional threats in shrinking response times. In such exercises, the officer’s stress level will increase in proportion to the increased demands for more accurate and faster response times.

This conscious awareness to perform activates proportional-level stimulation of the SNS, which in turn increases the heart rate and the release of stress hormones. Put in perspective, the average working heart rate is about 70 beats per minute, but as demands increase with reduced response times, the heart rate and release of stress hormones will increase somewhat proportionally. Low to moderate stress typically elevates the heart rate to ranges between 115-145 bpm, while high stress many induce heart rates in excess of 175 bpm. Escalation of the heart rate takes place over a specific period of seconds to minutes, based upon the speed and accuracy needed to respond.

However, a training event usually has no consequences of failure beyond a deflated ego. Scientists have charted survival events where the imminent deadly force threat triggers the Thalamus-Amygdala circuit within the brain—referred to as the “fight or flight” circuit. This is a significant variable.

Scientists know that the brain—through the Amygdala—is preprogrammed to recognize encounters that are life-threatening. The Amygdala is one of four basal ganglia in each cerebral hemisphere associated with arousal, controlling automated responses associated with fear, emotional responses, and hormonal secretions.

This fight or flight reaction results in a cascade of sympathetic nervous system and stress hormone activation that prepares the body for survival by increasing arterial pressure and blood flow to large muscle mass. Gross motor skills and strength capabilities are enhanced, while the vasoconstriction of minor blood vessels at the end of appendages, pupil dilation, and cessation of the digestion process also result.

The SNS enhances man’s ability to become fast, quick, and strong, but at a cost to high level analytical processing and the ability to execute fine complex and precision motor skills. It is within this state where visual distortion, auditory exclusion, cognitive impairment, bizarre behavior and the loss of precision and accuracy skills in combat can be expected. Unless traditional training is structured very carefully, it simply does not trigger the survival circuits.

The Scenario

Using a PRISim™ system, the research scenario began with a man working on the engine of a car. When the officer approached, the man said he had a little car trouble. He then turned, reached under the hood, produced a shotgun, and fired two shots. The non-assailant in the car’s front seat lay down out of sight. As the first assailant ran off or was “killed,” a second assailant, hidden with camouflage, stepped out of the woods and fired a semi-automatic pistol.

At this point, the research evaluator—presumably tracking performance—blasted a hidden air horn from 3 feet behind the officer as he engaged the second assailant. Finally, a third assailant rose out of the rear driver-side passenger seat and began shooting at the officer.

In their scenario, researchers used several different types of stressors: they forced the officers to key and respond verbally to an experienced dispatcher during the entire encounter; chose a scenario that had three assailants who came from different angles, with the last two attacking from previously unseen locations; timed the nylon balls fired from PRISim’s laser-aimed cannon to match the assailant’s engagement (the high-speed rounds stung on impact and created a threat at some subconscious level); and had the evaluator blow the air horn.

Researchers tracked physiological and cognitive changes as they occurred by monitoring heart rate and changes in stress hormones through before and after blood tests. Recall was checked using real-time video and a post-event survey that each officer filled out. The survey tested the officers’ memories of the specifics of the assailants, the assailant’s weapons, shots fired by assailants and the officer, and accuracy.


The way the officers remembered their stress-incident performance and their actual performance differed. The growing consensus in the medial community and the conclusions offered by the literature Siddle and Grossman reviewed concerning post incident memory distortions in the case of acute trauma—in this case a deadly force event—indicated that high levels of cortisol were produced, which somehow interrupted memory formation.

Scientists have demonstrated that the stress hormone cortisol disrupts and binds to the receptors in the hippocampus—one of the structures in the brain tasked with memory consolidation. When this happens, the hippocampus’ ability to form explicit memories is interrupted. In Siddle’s video-taped PRISim scenario research, blood was drawn to capture pre- and post-event stress hormone levels. This study confirmed what the medical community already knew.

In testing the officers’ performance and memory through a post-event survey, the officers were quizzed about the scenario, the number of assailants, vital data about the assailants (location, headgear, pants, shoes, shirt, weapon), the assailant’s actions, the officers’ own performance (number of shots fired, accuracy, reaction time), as well as overall perceptional skills such as whether they experienced tunnel vision, auditory exclusions, color vision, time distortion, etc.

It was found that overall memory of the stress event was much lower than predicted. Only 19% of the officers properly identified the make of the vehicle, 42% properly identified the model, and 58% identified the color, even though the car was in the first part of the scenario when the officers were not under any stress.

The officers’ overall memory of the vital data of the first assailant was 64%, 38%, for the second assailant, 44% for the third assailant, and 59% for the non-assailant. When all officers were included, even those who did not remember the third assailant and the non-assailant, the figures dropped to 30% for the third assailant and 33% for the non-assailant.

Only 13% of the officers correctly identified the number of rounds they fired during the scenario, and only one officer out of 49 heard the air horn blast. In addition, some officers identified the color of the pants and shoes that the third assailant and the non-assailant were wearing, even though they both remained in the car and their shoes and pants were not visible.


The lessons are numerous. The study was a controlled environment where the officer knew he was reasonably safe; therefore the level of stress hormone production was probably insignificant to the amount produced by actual combat. Memory is obviously fragile with moderate but spontaneous SNS activation, therefore it can be expected that larger memory problems will occur with a real deadly force encounter. Administrators and legal defense teams need to be educated on the realities of this data. Officers need to be educated that after a deadly force encounter, their statements may be a “best guess” at best, as they tend to fill in the blanks in their memory with how they think they must have acted, etc.

Siddle and Grossman’s research found that in the most traumatic and stressful situations, there is usually a sensory overload during which law enforcement officers will experience memory loss; the greater the stress, the greater the potential for memory problems.

Usually during a survival stress event encounter, the combination of sensory overload and fixation concerning a particular aspect of the incident are often exclusionary to everything else. Immediately after the incident, there will be post-incident amnesia, with most of the observed information forgotten.

The best memory recovery will take place after a healthy night's sleep. Seventy-two hours after the event, the final memory will occur, but it will be contaminated or partially reconstructed due to the individual integrating information from all other sources.

Jim Weiss is a retired lieutenant from the Brook Park, OH Police Department and a frequent contributor to LAW and ORDER. He can be reached at
Mickey Davis is a Florida-based writer and author. She can be reached at

Published in Tactical Response, May/Jun 2009

Rating : 7.0

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