1 Introduction
Stress is a systemic, nonspecific response to a real or potentially threatening stimulus and can be divided into chronic and acute stress depending on its generation and duration[1]. Acute stress is a short period of psychophysiological and behavioral changes resulting from a sudden, violent stimulus. The hypothalamic‒pituitary‒adrenal axis (HPAA) is the slow response system in humans, and its activation results in the release of glucocorticoids (primarily cortisol in humans) into the bloodstream. Cognitive function is often impaired during acute stressors, resulting in reduced cognitive flexibility(Shields et al., 2016) and difficulty in memory extraction(Smeets, 2011).
The Trier Social Stress Test (TSST) has been employed to induce stress(Kirschbaum et al., 1993). It consists of a videotaped free speech and a subsequent mental arithmetic task in front of a nonresponsive audience (total duration: 15 min). In the nonstressful control condition, participants were instructed to read magazines and perform simple mental arithmetic tasks for a comparable period of 15 min. The TSST is a well-established laboratory paradigm that reliably elicits an increase in HPAA response(Dickerson and Kemeny, 2004; Kuhlmann et al., 2005).
Working memory (WM) refers to the structures and processes used to temporarily store and manipulate information(Cowan, 2017) that play an essential role in cognitive activity. The N-back task is the classical paradigm for measuring WM(Owen et al., 2010). In the N-back task, a series of stimuli are presented, and participants are asked to judge whether the stimulus presented in the present matches the one presented in N items before. It has been shown that the WM load can be varied by manipulating the value of N; the higher the N value is, the higher the load level. Therefore, changing the N value modulates the task’s effectiveness.
Researchers have explored the relationship between acute stress and WM, but it remains controversial. On the one hand, exposure to acute stress impairs WM(Eawa et al., 2019; Jiang and Rau, 2017; Schoofs et al., 2008; Schoofs et al., 2009). On the other hand, some researchers have found that acute stress enhances WM(Cornelisse et al., 2011; Duncko et al., 2009). A recent study showed that this paradoxical effect might be related to the load level of working memory(Yu et al., 2015). Working memory, as a higher-level cognitive ability, is a key executive function for handling flight requirements and acquiring and maintaining situation awareness(Cak et al., 2020).
In aviation activities, the ability of pilots to execute standard operating procedures (SOPs) can be affected by WM. However, when pilots encounter emergencies, such as engine failure, low fuel levels, and bird strikes, acute stress may impair their working memory and lead to human error. For example, the Little Rock air disaster in June 1999 in the U.S. was caused by the impaired WM of pilots under acute stress, which caused pilots to forget SOP procedures and open the spoilers.
As mentioned above, acute stress impairment and improvement effects on WM have been found. How are pilots’ WMs affected by acute stress, and does memory load moderate the relationship between the two? That question still needs to be answered. Therefore, this study used a modified TSST to induce acute stress in pilots and an N-back task to test pilots’ WM under different WM load conditions. Additionally, WM was compared with that of control pilots who did not experience acute stress. In this study, it was hypothesized that reaction time would increase and accuracy would decrease as the WM load increased, regardless of whether the pilots were under acute stress; at high levels of working memory load, the control group would react with higher accuracy and shorter reaction time than the stress group. To assess the effects of acute stress induction, State-trait Anxiety Inventory scale (S-AI) scores and salivary cortisol samples were collected at baseline and at various time delays. The aim of this study is to enhance pilots’ attention to cognitive abilities under acute stress, strengthen their management of WM under stress, and reduce the hazards associated with acute stressful events during flight missions.