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The first part of our study examined 165 German participants (113 women, mean age 29.17 ± 11.32 years). The second part of the study used fMRI and included 22 German participants (17 women, mean age 21.38 ± 2.89; thirteen subjects were randomly selected in the first study). None of the participants had a neurological or psychiatric history. The study was approved by the Ethics Committee of the Faculty of Medicine in Berlin (Germany) and adhered to the Declaration of Helsinki. All participants gave written informed consent to the study.

The datasets analyzed during the current study are available from the corresponding author upon reasonable request.

Procedure

All participants were asked to complete SPS questionnaires (HSPS-G2.9), personality (NEO-FFI, Costa21) and empathy (IRI, Davis22).

To measure SPS, we used a German version of HSPS that was developed by Aron et al.2.23 (see additional data). The HSPS scale has 27 items and is widely used to measure SPS.

Personality was measured based on the five-factor model. We used a German version of the NEO-FFI, a questionnaire established to measure the personality dimensions of the Big Five21.24. It includes 60 items to describe the human personality in five fundamental dimensions: neuroticism, extraversion, openness, conscientiousness and agreeableness. The neuroticism dimension is related to negative emotions such as anxiety and irritability. Extraversion is linked to sociability, assertiveness and talkativeness. Openness to experiences is manifested by aesthetic sensitivity and intellectual curiosity. Conscience describes disciplined and organized behavior. Agreeableness is described as a tendency towards altruism and politeness21.

Empathy was measured with the SPF, which is a German version of the IRI22.25. IRI is widely used and extensively validated (e.g.,26.27). It measures self-reported empathic behavior and includes 28 items with four subscales: perspective-taking, fantasy, empathic concern, and personal distress. Perspective taking reflects the propensity to cognitively imagine a situation from the other person’s point of view. Fantasy measures the participant’s ability to transpose into the feelings and actions of fictional characters in books, movies, or plays. The Empathetic Concern subscale refers to feelings of compassion, sympathy, and concern for others. Personal distress describes the tendency to feel distress or unease when witnessing distress in others22.

For the second part of our study, 22 people participated in an fMRI experiment. While we scanned their brain activity, participants received a passive touch from the hand of an experimenter, who was near the scanner. The experimenter touched the palm of the participant’s right hand (skin size approximately 4–6 cm) ten times in a caressing manner (touch condition), with a frequency of approximately one touch per second. The experimenter used his fingers (digits 2 to 5) to apply touch. The control condition was a time window (12 s) where we applied no contact.

Afterwards, we asked the participants to rate the strength of the touch felt (for two seconds) and how pleasant it was for them (2 s). Participants answered using a four-button key (Likert scale, 1 = not at all strong/pleasant, 4 = very strong/pleasant). These questions were included to test whether the perceived pleasantness or strength of touch was related to SPS.

There was a pause of 12 s after the participants’ response (= no-contact condition). In total, we applied 20 touch blocks (and twenty non-touch) in four passes.

FMRI data acquisition, image preprocessing and analysis

FMRI data were acquired with a Siemens Tim Trio 3T scanner (Siemens, Germany). BOLD responses were obtained using axial-oriented echoplanar T2-weighted images (TR = 2 s, TE = 35 ms, flip angle = 80 degrees, FOV = 224 mm, number of slices = 32, size of voxel = 3.125 × 3.125mm, slice thickness = 3.5mm). Before the functional series, high-resolution T1-weighted structural images were recorded for anatomical reference (MP-RAGE sequence, TR = 1650 ms, TE = 5 ms). Four participants were scanned with a system upgraded to a Magnetom 3 T Prisma Fit (analog procedures). Participants were allowed to take short breaks between runs. We placed foam cushions on the side of the subject’s head to minimize head movement.

Statistical parametric mapping software (SPM12, Wellcome Department of Imaging Neuroscience, University College London, London, UK) was used for data preprocessing and subsequent statistical analyses. Pre-processing steps included realignment to correct inter-scan motion (spatial realignment on averaged image), coregistration, normalization in standard anatomical space (MNI, Montreal Neurological Institute model), and smoothing with a Gaussian kernel of 8 mm.

We then calculated statistical parametric maps using multiple regressions with the hemodynamic response function modeled in SPM. First, we analyzed the data at the level of an individual subject (fixed-effects model, comparing touch blocks versus non-touch blocks). The resulting parameter estimates for each regressor at each voxel were subjected to second level analysis (random effects model). We report active regions at p28 or empathy in SPS14.

To test which personality measures explain SPS, we analyzed behavioral data by standard multiple linear regression analyzes (all Big Five personality measures and empathy entered into the model simultaneously). Additionally, the model included gender and age as predictors, as previous studies have discussed the influence of these variables on SPS.2.29.

Additionally, we calculated peak activations in the SI, bilateral SII, and bilateral anterior and posterior insula (along with other ROIs) and examined the relationship of these brain activations to the SPS using Pearson’s correlations (controlled for personality measures associated with SPS).