Electronic Supplementary Material
Familiarising participants with the experiments 1 and 2
Before starting the experiment, and after instructions were given, participants were shown other
video clips to make the scenario more realistic. These videos showed two professional actors playing
the roles of the clinician and the volunteer (the volunteer was the same actress who was used for the
stimuli of facial pain expression). One video clip was supposedly showing the set-up of the clinical
trial and was shown to participants prior the experiment. Other video clips were shown prior each
new condition to show the progress of the clinical trial. The use of these videos enabled us to obtain
well-controlled experimental conditions while strengthening the ecological and realistic aspects of
the experimental context.
In the practice trials, the sequence was identical to the experimental task with the exception that no
video was shown to preserve the participants’ belief that the clinical trial would be run at the same
time. Participants had to select one pain intensity, apply the electrical shock and watch a black
screen displaying the word "video".
Stimuli preparation and validation for experiments 1 and 2
We recorded video clips of a professional actress making facial expressions of pain with
different intensities (low, moderate, intense, very intense). We obtained 265 two second
video-clips, each starting with a neutral facial expression for 500 ms, which then evolved to a
painful expression for 1500 ms. All video clips displayed brow lowering, orbit tightening, and
either cursing or pressing of the lips, or mouth opening or stretching, depending on the
intensity of the stimulation. These movements have consistently been attributed to the facial
expression of pain [1]. To select the video clips for all four pain intensities, 29 naïve judges
1
(17 women, aged 42.4 ± 19.2) were shown all the videos and were asked to score them on a
four-point scale of pain intensity. We kept the best five categorized videos for each of the four
pain intensities (over 69% of the judges classified the videos with the same intensity level).
Debriefing for experiments 1 and 2
The analyses of the debriefing ensured us that participants were indeed well involved in the
task. The debriefing first revealed that 65% of the participants of the two experiments strongly
believed in the realism of the experimental context (22.5 % had absolutely no doubt about the
authenticity of the scenario and 42.5 % had only a slight doubt), 25% had some doubts, and
only 10% reported not having believed in the scenario. Second, analyzing participants’
emotion scores showed that their moral emotion (embarrassment, guilt, shame and
indignation) were higher in the Decide and Execute condition than in the 'Observe' condition
(experiments 1 and 2: (F(2/1,19/18) ≥ 13.24, p < 0.001, partial η2 ≥ 0.46). Finally, in the
'Decide and Execute' condition, participants choose more frequently the lower intensities
(Table S1), although they were asked to select all pain intensities an equal number of time
(experiments 1 and 2: (F(3,18/16) ≥ 4.73, p ≤ 0.015, partial η2 ≥ 0.27). This demonstrates
their unwillingness to inflict the painful treatment, which was further evidence that they
successfully put themselves in a position in which they were inflicting pain to another
volunteer.
2
Table S1. Frequency of choice (mean %) for each intensity current in both experiments.
Different superscript letter within each column indicate significant differences (Tukey test
p<0.04).
Experiment 1:
Experiment 2:
Responsibility
Sense of Agency
Frequency
Intensity
Frequency (%)
S.E.M.
S.E.M.
(%)
1
28.81ab
1.09
29.65 a
1.19
2
29.52 a
1.31
23.40 b
1.13
3
23.69b
0.86
24.19 b
0.83
4
17.98c
1.62
22.76 b
0.99
Physiological measurements for experiments 1 and 2
We recorded data using an ANT (Advanced Neuro Technology Company) amplifier
(maximum sampling rate frequency 2048 Hz). Bipolar Ag/AgCl miniature electrodes (four
mm in diameter) filled with electrode paste and connected to the dedicated auxiliary inputs
were used. Before the electrodes were attached, the skin was cleaned with alcohol. The
ground was placed on the back of the left hand. All types of signal were digitally filtered
offline to remove any influence of 50-Hz power line interference (49–51 Hz band-reject filter)
using EEGLab filtering functions [2].
3
Electromyogram
EMG signal was filtered to remove low-frequency artifacts (20 Hz high-pass filter). Data
were then visually inspected but no remaining artifacts were observed. The filtered signal was
full-rectified and smoothed using the moving average method.
Heart Rate Variability
Interbeat Intervals (IBI) were computed using the algorithm developed in ECGLab [3].
Afterwards, we used Kubios HRV 2.0 [4] both to evaluate Standard Deviation (STD) of the
IBIs and to perform spectral analysis of HRV. Low frequency bands (LF) ranged from 0.04 to
0.15 Hz and High Frequency bands (HF) ranged from 0.15 to 0.4 Hz. STD and the ratio
LF/HF were computed throughout each experimental session, therefore independently of pain
intensity.
Discrepancy in the intensity levels (experiment 2)
The discrepancy consisted of a difference of up to two levels in intensity between the chosen
and the observed pain intensity, either higher or lower, (e.g. for a chosen intensity level of 1,
participants could observe a facial pain expression corresponding to an intensity level of 2 or
3). The distributions of the different discrepancy levels were programmed to obtain around 50
% of congruent trials and 50% of incongruent trials (with an equal distribution across the
different levels of discrepancy), despite the subject’s free choice. The trial stopped as soon as
the necessary number of combinations was reached or after seven minutes if it was not
reached
4
Sense of agency judgments (experiment 2)
In order to check the validity of our paradigm, we first tested whether in the sense of agency
condition, the degree of discrepancy between the selected intensity level and the observed
pain intensity modulated participants’ sense of agency. To this goal, we compared the
percentages of self-attribution responses across the five degrees of discrepancy (level 0: no
discrepancy; level +1: the intensity level observed was one level higher than the intensity
chosen; level -1: one level lower; and level -2/+2: two levels lower/ higher).
We showed that the degree of discrepancy modulated participants’ sense of agency (Friedman
ANOVA Chi² (N = 19, dl = 4) = 66.78 p ≤ 0.001). Participants gave self –attribution
responses in 93 % of the no discrepancy trials, this proportion decreased when participants
observed a pain intensity that differed of one level from the selected intensity level (26 % and
47.9 % for one level lower and one level higher respectively). When the observed pain
intensity differed of two intensity levels from the selected intensity, the proportion of selfattribution responses dropped to 1.7% and 1.3 % (for lower and higher intensities
respectively).
References
1. Ekman, P. & Rosenberg, E. L. 1997 What the face reveals: Basic and applied studies of
spontaneous expression using the Facial Action Coding System (FACS). Oxford University
Press.
2. Delorme, A. & Makeig, S. 2004 EEGLAB: an open source toolbox for analysis of singletrial EEG dynamics including independent component analysis. J Neurosci Methods 134,
9–21. (doi:10.1016/j.jneumeth.2003.10.009)
5
3. De Carvalho, J. L. A., da Rocha, A. F., de Oliveira Nascimento, F. A., Neto, J. S. &
Junqueira Jr, L. F. 2002 Development of a Matlab software for analysis of heart rate
variability. pp. 1488–1491. IEEE.
4. Tarvainen, M. P., Niskanen, J.-P., Lipponen, J., Ranta-aho, P. & Karjalainen, P. 2009
Kubios HRV—A Software for Advanced Heart Rate Variability Analysis. pp. 1022–1025.
Springer.
6