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.