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.