## 3 Results
### Study Results
#### 3.2.6 Subjective and Objective Tools for Psychophysiological Responses  
Diverse instruments were applied for measuring the self-reported stress experience. The present review focuses only on the subjective and objective assessment tools for stress among healthy individuals. However, all assessment instruments that were used in the included studies, are listed in Table 1. Among the 16 studies, 8 different stress-related tools were utilized. The most commonly used tool for recording subjective stress was the State Anxiety subscale of the State-Trait Anxiety Inventory (STAI) [66], a self-report questionnaire that evaluates the temporary anxiety that can change depending on the surrounding environment [31; 35; 47; 59; 57]. Although the study of [PERSON] et al. [50] utilized the Korean version of STAI, they did not specify if they used the full version of the questionnaire or the above-mentioned subscale. Another instrument that was utilized frequently, was the Perceived Stress Scale (PSS) [67], which measures the perception of stress [51; 54; 55]. The last two studies used, nevertheless, an adapted version of PSS so as to reduce response time and the interaction with the researchers [54; 55]. In their study, [PERSON] et al. [51] also applied the State-Trait Anxiety Depression Inventory-State (STADI-S) [68] and the Short Stress State Questionnaire (SSSQ) [69]. The first tool records the current state of anxiety and depression of a person, whereas SSSQ measures the level of worry and anxiety after a given task. In the study of [PERSON] et al. [52], the stress subscale of the Depression Anxiety Stress Scales (DASS-21) [70] was utilized to assess the presence of stress. Furthermore, [PERSON] and [PERSON] [35] evaluated one's perception of stress during the last three months through the Mesure du Stress Psychologique Questionnaire (MSP) [71]. Other instruments were the Visual Analogue Scale (VAS) for subjective tension [53] and Likert scales for assessing self-reported stress level [31; 58; 59]. However, there were studies that did not measure stress directly, but they asked participants to rate their emotional state at that time via a self-report questionnaire [46; 48; 49; 56].  
The change in physiological responses from one study phase to the next during the course of the experiment was also quantified by means of physiological indices. Eleven studies measured the cardiac activity of their participants [31; 35; 46; 47; 50; 51; 53; 56; 57; 58; 59]. Indeed, alternations in the cardiac component describe a complex interplay between the sympathetic and parasympathetic branches of the ANS, which is triggered from external or internal changing conditions [72]. Furthermore, skin conductance was evaluated in seven studies [46; 49; 51; 57; 58; 59]. This physiological parameter is controlled by the ANS [73], as it measures the electrodermal activity in the sweat glands and can be activated by exposure to a stressful event. Five studies evaluated participants' electrical activity in the brain [48; 53; 54; 55; 57], which is an objective indicator of physiological arousal in individuals [74]. Two studies also evaluated salivary markers of stress, namely cortisol and amylase [31; 56], which are associated with the function of ANS [56; 75]. Other physiological parameters for stress evaluation were electromyography, respiration, and skin temperature [50]. However, one study did not evaluate the physiological reactions of their participants [52].