The Relationship Between Virtual
Body Ownership and Temperature
Sensitivity
Joan Llobera, M.V. Sanchez -Vives, Mel Slater
Electronic Supplementary Information
Analysis of the Within Groups Experiment
Introduction
There were 40 participants in two groups of 20 each. The group CN first
experienced the consistent condition and then the inconsistent, and the group
NC the other way around. In the paper we only give results for the first trial of
each participant, thus considering the data as a between groups experiment.
Here we examine why the experiment could not be analysed as a within-groups
experiment.
Questionnaire Responses
Supplementary Figure 1 shows that the questionnaire responses strongly
support what is described in the main paper. Comparing the N and C conditions,
the C condition results in greater scores for body ownership (Q1), agency (Q2)
and ownership over the virtual arm that was moved (Q4), and the scores were
lower in C for the feeling that the virtual body was that of another person (Q3).
However, it can clearly be seen that the differences between N and C are
exaggerated for those in the CN group compared with those in the NC group.
Order certainly mattered.
Another way to see this is to carry out Wilcoxon rank-sum tests comparing the
questionnaire results across the two groups NC and CN. For each of the questions
Q1C,…,Q4C there are no significant differences between the NC and CN groups
(every P > 0.23). However, for Q1N,…,Q4N there are significant differences for
each question (P = 0.0003, 0.0022, 0.0017 and 0.0054 respectively).
The reason is quite clear why the results are different between the two trials.
Those who experienced the incongruent condition (N) in their first trial would
have experienced a virtual body co-located with their own (even if aspects of this
were incongruent with their own body posture and arm movements). As such
they gave fairly high questionnaire scores on the issue of body ownership. Then
when they came to the second trial experiencing the congruent condition (C),
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since they had already given relatively high questionnaire scores the first time
round, they could not increase those by much the second time around - i.e., there
was a ceiling effect.
Supplementary Table 1
Medians of Questionnaire Responses by Condition and Group
condition
N
C
N
C
P (Wilcoxon
rank sum test
- two sided
significance
levels)
Group NC (incongruent first)
Q1 (own)
Q2 (move)
Q3 (another)
4.5
4.2
4.45
7.4
8.1
2.55
Group CN (congruent first)
1.95
2.1
7.2
6.95
7.5
3.1
0.0003
0.0022
0.0017
Q4 (arm)
4.4
7.75
2.45
7.65
0.0054
Table 1 shows the median scores for the 4 questionnaire responses, comparing
groups NC (incongruent trial first) with CN (congruent trial first). See also
Supplementary Figure 1. It is very clear that there is a substantial difference
between the two groups. In particular it can be seen by comparing the scores for
the N groups that they are strongly significant different, with generally much
higher scores (Q1, Q2, Q4) for those who experienced the N condition in their
first trial (lower for Q3). This supports our contention that there is an
asymmetry between the conditions, and that analysis as a within groups study
would be inappropriate, since the meaning of the questionnaire responses is
different between the two groups NC and CN.
Hence experiencing the C condition first has a great impact on how the N
condition is experienced, tending to lower the scores on Q1, Q2 and Q4 and
increasing the scores on Q3.
Temperature Sensitivity Threshold
Supplementary Figure 2 shows the means and standard errors of log(tVR/tReal)
for the two conditions in the two groups. It is clear that there is a very great
difference between those who experienced the C condition first compared to
those who experienced the N condition first, and that the former give much
larger values than the latter.
Here we show furthermore that it is likely that the TST results in the second trial
were influenced by what happened in the first trial. The method is to use the
regression equation in Table 2, based on the first trial, and use these to predict
the results of the second trial. Then we compare the true results of the second
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trial with these fitted results. If there is a correlation then this is evidence that
the results of the first trial influenced the results of the second.
Supplementary Figure 1 - Box plots for the questionnaire responses. Q1N … Q4N
are the responses in the N condition, and Q1C…Q4C are the responses in the C
condition. NC and CN refer to the counterbalanced orders of the participant
trials. NC (n=20) had their first trial in the N condition and second trial in the C
condition, and vice versa for CN.
Supplementary Figure 2 - Means and Standard Errors of temperature threshold
differences comparing the results for the two groups and for the N and C
conditions.
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Here we are interested not in the difference log(tVR)-log(tReal) but rather we
want to see if we can predict the values of log(tVR) in the second trial from the
relationship found in the first trial. Regressing log(tVR) on condition, log(tReal)
and R = Q4*(1-Q1/10) only affects the coefficient of log(tReal) in the fit. The
regression equation for trial 1 is shown in the second column of Supplementary
Table 2.
Supplementary Table 2
Regression Analysis for log(tVR) (Trial 1)
n = 40, F(3,36) = 17.32, P < 0.00005, R2 = 0.59
Coefficient
Variable
-0.45
Constant
0.69
log(tReal)
0.31
Condition(N=0, C=1)
0.15
R Q4 1 Q1 /10 
S.E.
0.13
0.10
0.11
0.05
t
-3.40
6.97
2.78
2.94
P
0.002
0.000
0.009
0.006
Partial 2
0.57
0.18
0.19
Supplementary Figure 3 shows the fitted values (y) by applying the regression
equation of trial 1 (shown in Table 2) to the questionnaire responses of trial 2.
The overall correlation is significant (R2 = 0.28, P < 0.0005). ANCOVA of y on
condition with log(tVR) as the covariate also results in a highly significant fit (R2
= 0.66), with different slopes for the N and C conditions (P = 0.008, normality of
the residual errors is supported by Shapiro-Wilk test P = 0.66).
Conclusion
The results exhibit a clear order effect, so that a within groups analysis, or simply
combining all results into one group is not appropriate. It is clear that there was
a carry over from the results of the first trial to the second. We suspect that this
will almost always be the case in these types of body illusion experiments. The
norm is to do a within-groups experiment, and rely on counterbalancing to
eliminate possible order effects. This is unlikely to be valid, however, because of
the strong asymmetries between the first and second trials. We recommend that
such experiments at the least report on their order effects, and take these into
account in analysis.
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Supplementary Figure 3 - Scatter plot of fitted values (y) from the regression of
log(tVR) in trial 1 applied to the questionnaire results of trial 2. The horizontal
axis represents the actual log(tVR) trial 2 results.
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