2.2 Experimental design
2.2.1 Procedure
The experiment consisted of two parts: the Trier Social Stress Test (TSST) and the N-back task. First, the stress group participated in a modified TSST, and the procedure for the control group was the same as TSST but without stress induction. The second part was the N-back task, which tested WM.
In this study, State-Anxiety Inventory (S-AI) scores and salivary cortisol concentrations were selected as indicators of acute stress, and accuracy and reaction time were used as indicators of WM. Participants were informed not to exercise strenuously and to abstain from drinking alcohol 24 hours before the experiment and to abstain from eating and smoking 2 hours before the experiment. Three S-AI scores and three saliva samples were collected three times throughout the experiment: 5 min after the participant’s meditation (baseline), after the TSST, and after the N-back task. The study was a 2 (group: stress and control groups) × 3 (WM load: 0-back, 1-back, and 2-back) mixed experimental design, with ‘group’ as a between-subjects variable and ‘load level’ as a within-subjects variable.
2.2.2 TSST procedures
The TSST requires participants to complete several tasks, including a short preparation time, a videotaped free speech, and a mental arithmetic task(Kirschbaum C. et al., 1993). In this study, the modified TSST paradigm was appropriately adapted to the pilots’ characteristics, including preparation time (2 min), an interview (5 min), and a mental arithmetic task (5 min). During the preparation phase, participants could peruse the interview questions. The questions were selected from the oral proficiency interview of the international civil aviation organization (ICAO) and the oral assessment questions of the practical examination, which experts used to form six questions. In the interview phase, participants were randomly selected to answer the questions in front of the experts. In the mental arithmetic phase, participants were asked to report the results quickly and accurately (e.g., starting from 2031, perform the mental arithmetic task in decreasing order of 18). Before the TSST, the stress group was informed that they would be videotaped and scored. Two experts (a psychologist and flight instructor) were invited to record and evaluate the participant’s performance and posture. The experts wear uniforms and work permits, maintain a neutral expression throughout the process, and make specific reactions when necessary. Comparable to the stress group, the pilots of the control group participated in a similarly physically and mentally demanding task. In the preparation phase, participants perused material about civil aviation. In the interview phase, participants read the material at a normal speed. In the mental arithmetic task, participants only had to perform simple addition (e.g., mental arithmetic in successive increments of 5 starting from 3). Throughout the entire process, the experts did not wear formal attire, the tasks were not videotaped, and the pilots were not evaluated. It lacked the stress-inducing components of the TSST.
2.2.3 N-back tasks
After the TSST (stress induction vs. control situation), the WM performance of the participants was tested with an N-back task. Participants were conducted into a quiet and dimly lit room, approximately 60 cm from the display. The stimuli were presented on a 17-inch display with a resolution of 1024*768 pixels and a refresh rate of 60 Hz using the software E-prime 2.0. The experimental stimuli were all presented on a black background, and the stimulus was a white digit (e.g., 1~9). The practice block consisted of 3 blocks of 60 stimulus trials with feedback. It was ensured that participants understood the instructions before the formal experiment. The formal experiment consisted of 3 blocks of low (0-back), medium (1-back), and high WM load (2-back), with each block having a total of 180 trials. Of these, 1/3 of the trials were digits that required the participants to perform an “A” key response. The stimulus was presented for 500 ms, followed by a “+” fixation for 1500 ms, for a total response time range of 2000 ms. Participants pressed the “A” and “L” keys with the index finger of their left and right hands, respectively.
In the 0-back condition, participants were asked to determine whether the current stimulus was the same as the first stimulus at the beginning of the experiment and if it was the same, press key A; if not, press key L. In the 1-back condition, participants were asked to determine whether the current stimulus had appeared one position back in the sequence, and if it was the same, press key A, if not, press key L. In the 2-back condition, participants were asked to determine whether the current stimulus was the same as the second stimulus backward in time, and if it was the same, press key A, if not, press key L.