Stress has become a very common phenomenon — we are so used to it, we take it as a natural part of our everyday. Stress management techniques and tips go on hitting headlines in the media. Wearable technology is keeping up to offer new ways to detect if one’s being stressed.
Speaking scientifically though, stress can imply a variety of things. Among non-specialists, there is a tendency to identify stress with nervous tension or emotional excitement.
But stress is not limited to those. Basically, it is a universal physiological response to fairly strong stimuli — stressors — that manifests itself in different symptoms and phases, from activation of the physiological apparatus to its exhaustion. Stress can be caused by many reasons: sad events, hard work, relationships, illness, other changes in life, even positive ones, such as a wedding, traveling and the birth of a child.
Is it then possible to reduce all these and other stimuli to just one concept of stress? Would our bodily responses differ in each of the cases? Could we teach machines to accurately differentiate among them to predict if the stress we are going through is harmful? Let’s figure this out.
The history behind stress
The first scientist who investigated the physiological response of laboratory animals to stress was one of the leading physiologists in the US and most respected scientist of the 20th century Walter Bradford Cannon. Back in 1926, Cannon used the term stress to refer to external factors that disrupted homeostasis — the maintenance of steady states in the body and the physiological processes through which they are regulated, or simply speaking, physiological balance in the body.
Cannon found that in response to stress the organism resorts to the “flight-or-fight” reaction. When this happens, the sympathetic autonomic nervous system is activated and releases adrenaline to mobilize the organism. It controls blood circulation, blood sugar concentration and the ability of blood to clot to provide an emergency response to an action.
The second division of the autonomic nervous system, the opposite of the sympathetic — the parasympathetic — is responsible for the “rest-and-digest” or “feed-and-breed” reactions. The parasympathetic system stimulates activities that occur when the organism is at rest, especially after eating, including sexual arousal, salivation, lacrimation, digestion, and urination. The parasympathetic system returns the body to homeostasis after the “flight-or-fight” response has occurred.
Since then stress has been in focus of research and has begun to imply a number of things. The ambiguity in defining stress was first recognized by the Canadian physiologist Hans Selye. In a biological context, he defined the general adaptation syndrome as “the non-specific response of the body to any demand placed upon it”. Such avoidance of the term ‘’stress’’ was associated with its popularized use that referred to neuro-mental strain, while Selye’s theory also included physiological processes.
Selye is considered the father of stress research. He was the first to discover fundamental mechanisms of stress, in particular, the role of hormones in reactions to it. He also introduced the term chronic stress and draw the line between negative stress, distress, and positive stress, eustress. His idea was that stress was always “stressful”, regardless of whether one receives good or bad news, whether the impulse is positive or negative.
Upon perceiving a stressor, the body reacts with the “fight-or-flight” response and the sympathetic nervous system is stimulated as the body’s resources are mobilized to meet the threat or danger. At the second stage, the body resists and compensates as the parasympathetic nervous system attempts to return physiological functions to normal levels while the body focuses its resources against the stressor and remains on alert. If the stressor or stressors continue beyond the body’s capacity, the resources become exhausted and the body is susceptible to disease and death.
From the late 1960s, Selye’s concept of ‘stress’ has become widespread. Researchers have been trying to find the links between stress and disease of all kinds — it was generally thought intense stress can be harmful to the mind and body. (Read this Wikipedia entry for a short overview on the history of stress research.)
Since then, the classification of types of stress has been clarified. All in all, there is stress that is associated with normal body function and activates the resources of the body to perform different tasks, such as cognitive load and physical exercise. Then there is stress that is associated with psychological tension. The American Psychological Association (APA) differentiates among three different types of the latter — acute stress, episodic acute stress, and chronic stress.
Contemporary science, in particular, APA, binds prolonged or chronic stress with serious negative consequences for health. According to the newly published annual Global Emotions Report, on the global scale, 39% of people said they had experienced a lot of worries the day before and 35% reported being stressed. At the same time in the US, 55% of surveyed adults said they were stressed.
That is why companies and laboratories continue to introduce more accurate methods for the early detection and self-diagnosis of stress.
Automatic stress detection
‘’Bad’’ stress is tension to which the body cannot adapt and tension that cannot be released. The APA’s 2015 Stress In America Study found that among three leading sources of stress people indicated “money”, “family responsibility”, and “work”. Our ancestors had to fight or flee from danger, and we, on the other hand, cannot afford to escape from an important meeting. Locked in, stress becomes chronic and leads to disturbances in the body and mind.
Today we know exactly what are the bodily responses to stress. They include a large number of reactions following:
Acceleration of heart and lung action
Constriction of blood vessels in many parts of the body and dilation of blood vessels for muscles
Liberation of energy sources (particularly fat and glycogen) for muscular action
Inhibition of digestion, tear production and salivation
Partial loss of hearing and peripheral vision
Shaking, dilation of pupils, general effect on the sphincters of the body
The above mentioned physiological changes that occur during the “fight-or-flight” response are activated in order to give the body increased strength and speed in anticipation of fighting or running. In spite of this, the search for stress biomarkers is a difficult task for researchers and clinicians for several reasons. One of them is the confusion about what stress should be considered harmful. Another is that each person’s body functions and adapts to constantly changing conditions differently, in its own way.
Currently, there is no generally accepted standard for how to evaluate stress. Still, there are several methods for detecting stress:
Psychological tests (surveys)
Measuring the concentration of various substances in blood and saliva, such as cortisol or amylase
Heart rate detection
The change in the heart rate is most easily tracked under experimental conditions. Among various heart rate parameters, Heart Rate Variability (HRV) is one that can be used as an indicator of stress.
HRV characterizes the duration of heartbeat intervals, called R-R intervals (see the picture below). HRV reflects the ability of the heart to respond to various physiological and environmental stimuli: the greater the variability, the better we respond to stress. Low HRV transmits a monotonously regular heart rate and is associated with the impaired autonomic nervous system (ANS) function. This implies our body has ceased to adapt to each stressful situation and cannot return to its original homeostatic balance.
HRV can be measured via an electrocardiogram (ECG), which is a simple and widely available method in hospitals and laboratories. Or, with the recent technological development, it is now possible to measure HRV non-invasively and remotely. We have recently created an algorithm for remote heart rate detection via a webcam, the next step is to learn to identify stress with it.
Heart rate and stress
HRV has been associated with stress in many studies. In a number of them, HRV was measured for professions most exposed to stress.
For example, for professors, lectures had a negative effect on HRV (Filaire et al., 2010). Students with a strong academic load on average had lower HRV, with female students being more affected by academic load, because they had fallen in the category of high and severe stress more than males (Punita et al., 2016).
Of course, office workers and other employees are also in the risk groups. Orsila et al. (2015) conducted a long-term study on the employees of the electronics company, in which they tried to find how mental work correlated with stress by measuring the HRV parameters. They recorded employees’ heart rate using wrist monitors in the morning, at the beginning of the workday, during the workday, and before the sleep. It turned out HRV grew from night to morning, while stress decreased. During the day, the situation was exactly the opposite.
The method of measuring HRV was also used in the Midlife in the United States Study to evaluate daily stress. Sin et al. (2017) analyzed data from more than 900 participants who were interviewed about negative emotions they had been going through during the day and minor stressful events. The study examined 4 variables — stressor frequency (how often did stressful situations happen), stressor severity, affective reactivity (personal attitude to stress), and daily negative affect (the frequency of negative emotions).
Results showed that stressor frequency was unrelated to HRV. But individuals with more pronounced affective reactivity to stressors also had lower levels of the HRV parameters. This may mean that the better a person can cope with the daily stress and negative events in her life, the lower the risk of developing heart disease.
Studies show that not only work and study affect cardiovascular activity. Some researchers have been studying the effect of mental stress on short-term HRV. The Stroop Color and Word Test or arithmetic serial subtraction task are commonly used for that purpose.
The Stroop Color and Word Test was originally introduced by John Ridley Stroop. The test is based on the assumption that individuals can read words much faster than they can identify and name colors. The subjects have to name the color of the rectangles, the words of the congruent stimuli and the color of the incongruent stimuli, without reading the actual word itself. When presented with the incongruent stimuli, subjects fall into a stressful situation, which allows measuring a number of parameters related to stress.
Endukuru and Tripathi (2016) compared the mean blood pressure and HRV during the Stroop Color and Word Test in 50 healthy subjects. This study showed that HRV was sensitive to mental stress, and males were more prone to stress when compared to females.
You can try this task yourself and see how difficult it can be to name the color of the “color” word when there is a mismatch between the color and the word.
Task: Do not read the words below. Instead, say the color of each word in each column, in a loud voice as fast as you can.
The Stroop Color and Word Test to measure mental stress. Source: http://www.whatispsychology.biz/about-stroop-effect-definition
What is there for remote stress detection?
Stress detection is not an easy but achievable task in the nearest future. One of the methods to measure stress is via analyzing heart rate and in particular heart rate variability.
An ECG can be one way to do this. However, one of the drawbacks of this method is the need for electrocardiographic equipment. Well, a simple phone accelerometer can be used to register heartbeats, but even in this case, one would need to attach the phone to the body (stomach or chest) and remain motionless during the measurements, since this method is subject to movement artefacts.
A more flexible solution can be a system for remote monitoring of cardiac activity to track heart rate parameters via a camera pointed at a user’s face. The system does not require complete immobility and is easy to use. In addition, no special expensive equipment is needed — just a regular webcam and the “light shed on the pulse”.
Such stress detection systems will be in demand in a wide range of uses. These are employee well-being and health monitoring, driver state monitoring, self-assistance for elderly people, even smart homes and child care. Such systems would help us to take a break and let the body restore balance for new achievements with fresh forces and even more productively; they would allow for timely monitoring for those in risk groups; finally, they could help modernize healthcare and introduce an accurate and inexpensive alternative to costly medical equipment.