4 Discussion
The present study focused on the effects of acute stress on pilots’ WM.
The results showed that the stress group scored significantly higher on
the S-AI after the TSST than the control group; compared to baseline,
the stress group had elevated cortisol concentrations after the TSST,
with no significant differences observed for the control group. This
suggests that the modified version of the TSST paradigm used in this
study was effective in inducing stress in pilots. The results of the WM
test showed that as the WM load increased, the participants’ reaction
time increased, and accuracy showed a gradual decrease. This is
consistent with the results of most previous studies, where the strain
on WM was elevated when the memory load in the N-back task was
increased(Itthipuripat et al., 2012). Studies that have used EEG
techniques found that the EEG components associated with WM updating
were impaired at high-load levels in participants under acute
stress(Gärtner et al., 2014), which may help explain the decreased
performance on the N-back task.
It was also found that there were no significant differences in reaction
times between the stress and control groups for the three memory load
conditions. In terms of accuracy results, there were no significant
differences between the two groups of pilots with the low/high memory
load, but there was a significant difference with the medium memory
load, with the accuracy being higher in the stress group than in the
control group. In other words, when the pilots were under acute stress,
their accuracy was significantly enhanced when they completed the medium
memory load task. The effect of acute stress on WM is moderated by WM
load. This is partially consistent with the results of previous related
studies. For example, Cornelisse (2011) et al. found that acute stress
could increase WM levels in a group of male participants under moderate
memory load conditions. Specifically, male participants had faster
reaction times in the moderate memory load condition after completing
the TSST. Although there was no significant increase in accuracy, the
performance of the stress group was better than that of the control
group. Duncko et al. (2009) similarly found that the stress group had
significantly faster reaction times in the high cognitive load condition
than the control group. When reviewing the controversial results
regarding the effects of acute stress on WM under different memory
loads, one researcher found that acute stress impaired WM under high
memory load conditions(Schoofs et al., 2008). Some researchers have also
found that stress impaired cognitive tasks only when memory load was
low(Yu et al., 2015). We speculate that three main causes are
responsible for this.
First, acute stress is induced in different ways. Previous studies have
found that different methods (psychological, physiological, and
pharmacological) can lead to different intensities and durations of
acute stress(Li and Ku, 2020), which may affect the response time and
accuracy of WM. In this study, we used the modified version of the TSST,
which is psychological. The induced acute stress levels were considered
moderate compared to the potential levels that could be induced by
physical and pharmacological interventions. Pessoa et al. (2009) found
that a moderate intensity of emotion enhanced cognitive task
performance, while a high intensity of emotion impaired task
performance(Pessoa, 2009). Therefore, we hypothesized that moderate
acute stress might produce similar results. It is worth noting that the
relationship between individual stress levels and job performance
follows an inverted U-shaped curve and is also influenced by task
difficulty(Broadhurst, 1957). In this study, moderate stress levels led
to better job performance when performing moderately difficult tasks.
Thus, the stress flight group had higher accuracy WM when completing the
moderately difficult task (1-back) than the control group.
Second, the participants were different. Some researchers have found
that sex, occupation, and mental state also influence the effects of
acute stress on WM(Cornelisse et al., 2011; Gärtner et al., 2014). The
participants of this study are pilots whose long-term training helped
them to have better WM. In addition, the study used a digital N-back
task, where pilots are more sensitive to digital information such as
radio frequency, altitude, and speed. Therefore, we assume that the
pilots in this study had a different difficulty level for the memory
load task than other subjects. This may explain the inconsistency with
previous studies.
Finally, the present study explored the behavioral results of WM.
However, some studies have found that there were no significant
differences in behavioral results, but there were significant
differences found in neurological results(Han et al., 2013; Yu et al.,
2015). For example, in a study by Zhang et al. (2015) examining whether
the effects of acute stress on WM were moderated by WM load, no
interaction was found between the group and WM load on reaction time.
However, EEG results revealed that the P3 component induced in the acute
stress group was found to be significantly greater than that of the
control group in the low WM load condition. We speculate that the
behavioral results may not yet reflect differences in WM because brain
neurological techniques such as EEG and MRI were not used in this study.
We acknowledge that our sample selection has some limitations, as it
does not represent all pilots or other occupational groups, and lacks
diversity in terms of gender, age, education, etc. This may affect the
generalizability and universality of our research findings. Future
studies can consider increasing the number of female or other background
participants to improve the quality and validity of the research.
In summary, we found that acute stress enhances the accuracy of WM for
pilots under moderate memory load. According to the Yerkes-Dodson law,
the effect of stress on hippocampal and prefrontal cortex-dependent
memory follows an inverted U-shaped curve(Broadhurst, 1957). According
to the Yerkes-Dodson law, moderate levels of stress promote memory
function, while high levels of stress impair memory function(Luethi and
Mathias, 2008). After the flight stress group completed the TSST task,
moderate stress levels instead promoted them to perform better on
moderately difficult WM tasks. The ability of pilots to correctly recall
SOPs and operate them accurately after an unexpected event is
particularly important for flight safety.