2.3. Acute stress induction
To induce acute stress, the Montreal Imaging Stress Task (MIST) was employed. This task is a validated paradigm to increase negative affect and levels of cortisol (Dedovic et al., 2009). Participants were asked to complete the mental arithmetic task shown on the screen. This task consists of two sessions: a training session with 15 trials and an experimental session with 75 trials. This study involved the random selection of math problems of varying difficulty: the easiest problems only had three one-digit integers and used addition or subtraction only (e.g., 2+9-7); the medium difficulty problems had three integers, two of which were two-digit numbers, and used multiplication (e.g., 3*12-29); the hardest problems had four integers, three of which were two-digit numbers, and used multiplication and division operations (e.g., 14*16/4-52).
In the stress condition, the difficulty and time limit of the mental arithmetic tasks exceeded the participant’s psychological ability, and the negative feedback from the computer further increased their stress. A participant’s reaction time and correct response rate were continuously recorded throughout the experiment. If a participant answered a series of three consecutive problems correctly, the time limit reduced to 10% less than the participant’s average response time; conversely, the time limit increased by 10%. Every time a participant completed the arithmetic test, the appropriate feedback (“correct” or “incorrect”), response time and cumulative correct response rate were presented. If no response was submitted within the allotted time, the feedback ”timeout” was shown. At the same time, participants received negative feedback from the investigator on their performance compared to a fictitious user who exhibited high-performing behavior. In the control condition, participants were instructed to complete the task as quickly and accurately as possible, but the investigator also makes it clear that this was a control condition, therefore performance was not being evaluated. Similarly difficult mental arithmetic tasks were completed by the participants in the control condition, but there were no time restrictions, aural cues, or disapproving comments from the software or investigator.
In both the stress and control conditions, a 500 ms fixation cross appeared on the screen at the beginning of each trial and was followed by a mental arithmetic test. After the participant entered the answer or reached the time limit, the corresponding feedback appeared on the screen for 2000 ms. The pupil diameter was continuously sampled at 1000 Hz during the experimental phase of the MIST/control task.