Stress, Arousal and Anxiety

Definitions of stress, arousal and anxiety

Stress

Stress is the physiological or psychological response to external triggers. For instance, you experience physiological stress when running to catch a train and you experience psychological stress when your manager, unexpectedly, reminds you of a due deadline.

Arousal

Both physiological- and psychological stress trigger physiological responses that prepare you to deal with task demands. For instance, to run fast or to think hard, your heart rate is raised to deal with the increased oxygene demand of your muscles and brain.

Like all mammals, humans are equipped with a dedicated nervous system that regulates such automated responses: the autonomous nervous system (ANS). The ANS prepares for impending efforts in a number of ways: 

• When aroused, the ANS slows down digestion to mobilize resources for the impending effort
• When aroused, the ANS raises heart rate and blood pressure to meet impending oxygene demands
• When aroused, the ANS initiates sweating to cool down the body in compensation of increased heat production when making an effort

As these physiological responses are initialized by autonomic arousal, the occurrence of these physiological responses is indicative of increasing autonomic arousal.

Anxiety

The American Psychological Association defines anxiety as a combination of worried thoughts, tense feelings and phyical responses like increased blood pressure. In their nature, these responses bear similarity to arousal, i.e. preparations for an adequate effortful response to an external trigger. However, anxiety is often manifested as a long-term state that does not prepare to deal with a specific treat or demand. Instead, anxiety is seen as a unprovoked trait or a long term effect of strong and longlasting emotional or physical stress experienced in the past. 

Measuring (physiological correlates of) anxiety and stress

The extent to which a person experiences stress can be assessed by a psychometric method: the person may for instance rate experienced stress on a numbered scale and describe the experience qualitatively. As physiological and psychological stress induce ANS arousal, the physiological correlates of ANS arousal may be used as stress indicators: 

• Heart rate: the (anticipated) increase of physical and mental effort raises ANS arousal, and thus causes a heart rate raise (or, consequently, an inter-beat interval decrease) and a reduced variability of the heart rate over time.
• Electrodermal responses: the (anticipated) increase of physical and mental effort raises ANS arousal, and thus causes increases of the rate of phasic electrodermal responses over time. 
  

NoldusHub can derive these physiological responses from ECG, PPG and electrodermal measures (phasic skin conductance responses). Using validated algorithms, NoldusHub calculates measures indicative of Arousal (EDA peak rates from conductance measures and heart rate from PPG or ECG measures).

Measure validity

Experimental design

Since physiological correlates of stress and anxiety can be induced by physiological stress and psychological stress, the validity of NoldusHub stress correlates depends on the experimental design of your experiment. Therefore, psychological tasks invoking stress should assure that task conditions do not invoke physiological stress as well (i.e. uncontrolled walking or other effort like standing up). It is the responsibility of the researcher to assure that Heart Rate responses and EDA responses can be exclusively attributed to effort on task. 

Individual variation in stress responses

Competence and fitness are factors that moderate the arousal response to a task demand. A very well-trained athlete will show a smaller relative increase of heart rate when doing a push up when compared to a person who hardly does any physical excercise. Likewise, a very competent and experienced pilot will show a lower arousal response when landing a jumbojet than a novice pilot landing the same plane. Also anxiety traits of subjects can confound the effects of stress conditions on physiological responses: anxious subjects will display higher responses levels, even at lower stress levels. Therefore, it is important to 1) apply different stress conditions to all subjects and use differential stress responses rather than absolute responses for effect size, or 2) assess basic anxiety levels of subjects and use these as a covariate in the statistical analysis of results.