Subjects

Thirty-five adults (mean age = 22 years, SD = 2.6; 19 men) participated in experiment 1 and thirty-two different adults (mean age = 24 years, SD = 3.4; 15 men) participated in experiment 2. We based our sample size calculation on the previous results obtained in our group regarding the effect of social contact through visual modality on bodily self-awareness6,23. The minimum effect size f in these studies was 0.42. Based on this value, we computed a sample size of 24 for a power of 0.8, at alpha 0.05 (with a medium value of non sphericity of 0.75) using the software G*Power 346. We then included thirty-five participants in Experiment 1 and thirty-two in Experiment 2 with the aim of accounting for potential exclusion of participants due to technical errors or outlier values.

For both experiments, all participants had normal or corrected-to-normal vision and no history of neurological or psychiatric illness. All participants were right-handed, French speakers, and naïve to the goal of the experiments. For experiment 1, all participants had normal audition and for experiment 2, the participants did not have any chronic skin disease. Informed written consent was obtained from every participant in accordance with the 1964 Declaration of Helsinki and its later amendments. All procedures were approved by the local ethics committee (CPP Ile de France III, approval n°C07-28). All the methods used in our experiments were performed in accordance with the guidelines and regulations of the local ethics committee. Each participant received a payment of 20 Euros for his/her participation. Four participants in experiment 1 were excluded due either to technical error (N = 3), or because the ‘Other Name’ was not unfamiliar (N = 1). In experiment 2, two participants were excluded due to health issue (eczema on the forearm for one participant, and temporary fainting during the experiment for the other participant). After additional outlier exclusion (see below), the final data sample included 28 participants (mean age = 22 years, SD = 2.4; 14 men) for the first experiment, and 25 participants (mean age = 24 years, SD = 4.6; 11 men) for the second experiment.

Stimuli

Emotional stimuli

The emotional stimuli displayed in both experiments were the same forty-eight emotional pictures (24 positive and 24 negative) as in Baltazar et al.6 and Hazem et al.23. They were selected from the International Affective Picture Systems47 to induce emotional experience. The full procedure of stimulus selection is described in Baltazar et al.6.

Context stimuli, Experiment 1

Thirty-two audio recordings of first names (female voice in half of the cases, male in the other half) were created to generate the ‘Own Name’ and ‘Other Name’ contexts. The uttered name was the name of the participant in half of the audio recordings (‘Own Name’ context) and another unfamiliar name in the other half (‘Other Name’ context), for both the male and the female voices. They were uttered with a calling intonation to ensure a communicative intent. The’Own Name’ and the ‘Other Name’ were matched in terms of gender, duration (ranging from 500 to 600 ms), frequency (medium frequency of occurrence of 15,000 to 30,000 times in France between 1980 and 2000: see https://dataaddict.fr/prenoms/) and phonetic properties (same number of syllables, with no more than three syllables). The loudness of the ‘Own Name’ and the ‘Other Name’ was normalized with the AUDACITY 2.0.6 freeware. Moreover, prior to the experiment, each subject provided the first names of their five closest friends and of their relatives (parents, siblings, romantic partner, and children), in order not to select any of these names for the ‘Other Name’ condition. At the end of the experiment, we asked the participant whether the ‘Other Name’ was familiar to him/her and we eliminated from the analysis the only participant for whom it was the case. The white noise for the Low-level control condition was generated with AUDACITY 2.0.6, with a duration of 450, 500, 550, 600, or 650 ms.

Context stimuli, Experiment 2

Tactile stimulations in both Human and Brush touch conditions were gentle strokes delivered with a pressure of approximately 8 Pa and a velocity of about 4 cm/s, aimed at inducing optimal activation of the CT afferents neural pathway, thought to be the channel of social touch48. All stimulations were proximo-distal in orientation. In the Human Touch condition, the stimulations were performed by the male experimenter MB using his joined index and middle fingers. In the Brush Touch condition, the stimulations were performed with a makeup goat hair brush (3 cm diameter). Both types of stimulation were delivered on 2 different skins regions of 8 cm length on the participant’s left forearm, previously delineated with a washable marker. The stimulations were alternating between the two skin regions to minimise the rapid habituation of CT fibers, which require a 30 seconds inter-trial period to return to their baseline level49.

Before the experiment, the stimulations were practiced by the experimenter and video-recorded to verify stroking velocity and pressure. During the experiment, the experimenter was visually guided by a cursor moving on a screen in order to achieve the correct speed and length of the touch. Moreover, the stroking velocity was evaluated post-experiment from the video-recordings. For this, we randomly extracted 100 video recordings of the touches performed during the experiment (50 human touch and 50 brush touch videos). The speed of the touches was computed and we performed a repeated-measures ANOVA on the measured speed with Context (Human Touch/Brush Touch) as within-subject factor. This showed that there was no significant difference between the Human and the Brush Touch conditions (Mean = 4.22 ± 0.10 cm/s; F 1,98 = 1.63, P = 0.20).

Experimental design

The participants were seated at a distance of 70 cm from a 22-inch computer screen with a resolution of 1280 × 1024 pixels. Each trial was initiated by the presentation of a fixation cross of 3-by-3 degrees of visual angle. After a random duration of 0.8 to 1.5 s, it was followed by the presentation of the context stimulus.

Experiment 1

The auditory context stimulus was delivered binaurally through headphones, at about 75 dB SPL maximal intensity. This context stimulus was the name of the participant (Own Name condition), another unfamiliar name (Other Name condition)—uttered by a female (in half of the cases) or a male voice (in the other half), or a white noise (Noise condition). Each condition was presented in one-third of the trials.

Experiment 2

The context stimulus was the slow gentle touch done with the hand of the experimenter (Human Touch condition), the slow gentle touch done by the experimenter using a brush (Brush Touch condition), or no-touch (No-touch condition). The participant’s touched arm was placed behind a curtain so that the experimenter and the type of touch performed were not visible.

In both experiments, we used 16 trials per context condition (for a total of 48 trials). The order of the conditions was pseudo-randomized so that any given experimental condition was not repeated more than two times in a row. In each trial, the context stimulus was followed by the emotional picture, so that the delay between the context stimulus onset and the emotional picture onset was 1.5 s in Experiment 1 and 2.5 s in Experiment 2. The emotional picture was presented for 6 s. It covered 39-by-24 degrees of visual angle, in the centre of the screen. It was immediately followed by a continuous vertical scale ranging from 0 to 100, which was displayed until the participant provided his/her response. The participants were instructed to focus on the internal bodily changes that the emotional picture caused in them during its presentation and to evaluate on this basis the intensity of their emotional reaction from 0 (not at all intense) to 100 (very intense) by moving a vertical cursor along the scale. It was specified that the bottom of the scale corresponded to feeling completely relaxed, quiet, calm, and without any particular bodily change induced by the picture, whereas the top of the scale corresponded to feeling very stimulated, excited or nervous, with intense bodily reaction to the picture. In addition, for a quarter of the trials, a new screen appeared after the participant’s response, displaying the following words, one alongside the other: “Propre Prénom”, “Autre Prénom”, and “Bruit” (Own Name, Other Name, and Noise) for the first experiment, or “Toucher Humain”, “Toucher Pinceau”, and “Pas de Toucher” (Human Touch, Brush Touch, and No-touch) for the second experiment. The participant had to indicate which type of context stimulus had been presented before the emotional picture. This constituted a secondary memory task to ensure that participants paid attention to the context stimuli. Between trials, the screen remained black for a duration that varied randomly between 12 and 17 s in order to allow skin conductance to return to baseline level. The association between the context image and the emotional stimulus was counterbalanced across participants so that every emotional picture (N = 48) was seen once during the experiment with always 8 positive and 8 negative pictures associated with each context condition.

Physiological recordings and analysis

Physiological responses were recorded using ADInstruments© (ML870/Powerlab 8/30) acquisition system.

Skin conductance response (SCR)

Two Ag–AgCl electrodes filled with isotonic NaCl unibase electrolyte were attached to the palmar surface of the middle phalanges of the index and middle fingers of the non-dominant hand. The raw SCR signals were recorded at a sampling rate of 2 kHz, amplified and low-pass filtered online at 10 Hz. Then, the SCR was offline downsampled at 2 Hz. Trials containing artefacts or noisy baseline (i.e. mean of activity during the second preceding the context onset) were manually rejected (less than 8% of the trials). The SCR in response to the emotional picture was defined as the maximum change from the baseline level occurring between 1 and 6 s after the emotional picture onset. The SCR amplitudes were then log-transformed [Log (μS + 1)] to normalize the data25.

Correlations between SCR and subjective ratings of emotional response intensity

The two experiments were analysed independently from each other. For each experiment, we computed the correlation (Pearson’s r coefficient) between the SCR evoked by the emotional pictures and the ratings provided by the participants, in each context condition separately and for each participant. Correlation r values are typically not normally distributed50,51. Therefore, we performed Fisher’s r-to-z transformation in order to normalize the correlation values. Shapiro-Wilk statistical tests confirmed that the Fisher z scores distribution did not significantly differ from a normal distribution, for each condition in both experiments (all P > 0.20). We then screened the Fisher’s z scores for extreme values. Three participants in the first experiment and five participants in the second experiment had outlier values (Fisher’s z scores differing by more than two standard deviations from the group mean in one or more experimental conditions); they were excluded from all subsequent analyses.

Statistical analyses

Statistical analysis of the SCR-rating correlation data

For each experiment, a one-way repeated-measures ANOVA with Context (Experiment 1: Own Name/Other Name/Noise and Experiment 2: Human Touch/Brush Touch/No-touch) as within-subject factor was run on the SCR-rating correlations transformed into Fisher’s z scores. We used the Greenhouse-Geisser correction to correct for deviations from the assumption of sphericity (the ε GG correction factor and the corrected P or P corr are reported). Effect sizes (eta-squared, η2) are reported together with F and p values of the main effects. Planned comparisons between the conditions taken two-by-two were further performed using bilateral, paired Student t-tests when main effects were observed.

Control analyses

Main effects of context on arousal and ratings

For each experiment, additional one-way repeated-measures ANOVAs with Context as within-subject factor were carried out on SCRs and on subjective ratings, respectively.

Secondary memory task

For each experiment, one-way repeated-measures ANOVAs with Context as within-subject factor were run on the RTs and on the %CR obtained in the secondary memory task.

Control analysis specific to the second experiment

A repeated-measures ANOVA with Context (Human Touch/Brush Touch/No-touch) as within-subject factor and participant’s gender as between-subject factor (Female/Male) was run on the Fisher’s z scores of SCR-rating correlations.

Analysis across sensory modalities: Experiment 1, 2, and Baltazar et al.6

A repeated-measures ANOVA with Experiment (Experiment 1/Experiment 2/Experiment from Baltazar et al.6) as between-subject factor and Condition (Social Contact condition: Own Name – Human Touch - Eye Contact/Non-social control condition: Other Name - Brush Touch - Averted Eyes/Low-level control condition: Noise - No-touch – Fixation cross) as within-subject factor was run on the Fisher’s z scores of SCR-rating correlations.

Data availability

The datasets generated and/or analysed during the current study are available from the corresponding author on request.