EEG acquisition and analysis

Electroencephalography (EEG) data were recorded from a 63-electrodes scalp cap using the 10–20 system (Brain Products, Munich, Germany). The channel TP10 was used as the reference during recording. Two electrodes were used to measure the electrooculogram (EOG). EEG and EOG activity was amplified at 0.01 Hz ~ 100 Hz band-passes and sampled at 500 Hz. All electrode impedances were maintained below 5 kΩ. EEG data were pre-processed and analyzed using MATLAB R2011b (MathWorks, US) and EEGLAB toolbox⁴⁰. EEG data at each electrode were down-sampled to 250 Hz. The data were re-referenced to the common average. Then the signal passed through a 0.01–30 Hz band-pass filter. Time windows of 200 ms before and 1000 ms after the onset of the picture were segmented from EEG and the whole epoch was baseline-corrected by the 200 ms time interval prior to target onset. EOG artifacts were corrected using an independent component analysis (ICA)⁴¹. Epochs with amplitude values exceeding ± 50 µV at any electrode were excluded from the average.

Further statistical analysis was conducted in IBM SPSS Statistics 22 (IBM Corp., Armonk, NY, USA. Previous studies using similar stimuli suggested that the early components N1, P2, N2 and later component P3 and LPP were particularly related to observing other’s pain¹¹. Analyses were conducted over the peak of the amplitude of N1, P2 and N2 component (peak to baseline) and the mean amplitudes of P3 and LPP component. According to the topographical distribution of the grand-averaged ERP and the literature^42,43, five sets of electrodes from the frontal (FC3, FCz, FC4), central (C3, Cz, C4), centra-parietal (CP3, CPz, CP4), parietal (P3, Pz, P4) and parieto–occipital (PO3, POz, PO4) regions were chosen. Mean amplitudes were obtained from waveform averaged for all selected electrodes within each region. Repeated measures ANOVA (2 (Mood: Happy/Sad) × 2 (Picture: Painful/Non-Painful) × 5 Regions (frontal, central, centra-parietal, parietal, parieto-occipital) were performed for each component within its most pronounced time windows (N1 (100–160 ms), P2 (160–220 ms), N2 (200–300 ms), P3 (300–400 ms) and LPP (450–650 ms)). Degrees of freedom for F-ratios were corrected according to the Greenhouse–Geisser method. Statistical differences were considered significant at p < 0.05; post-hoc comparisons were Bonferroni-corrected at p < 0.05.