Beta oscillations reveal ethnicity ingroup bias in sensorimotor resonance to pain of others

Abstract

People evaluate members of their own social group more favorably and empathize more strongly with their ingroup members. Using electroencephalography (EEG), we explored whether resonant responses of sensorimotor cortex to the pain of others are modulated by the ethnicity of these others. White participants watched video clips of ethnic ingroup and outgroup hands, being either penetrated by a needle syringe or touched by a cotton swab, while EEG was recorded. Time-frequency analysis was applied to Laplacian-transformed signals from the sensors overlying sensorimotor cortex in order to assess event-related desynchronization and synchronization (ERD/ERS) of sensorimotor mu (7-12 Hz) and beta (13-30 Hz) rhythms. When watching needle injections, beta ERD was significantly stronger for ingroup compared with outgroup hands. This ethnicity bias was restricted to painful actions, as beta ERD for ingroup and outgroup hands neither differed when observing no-pain videos, nor during presentation of the hands without any treatment. Such vicarious sensorimotor activation could play a role in social interaction by enhancing the understanding of the feelings and reactions of others and hence facilitating behavioral coordination among group members.

Keywords: empathy; ingroup favoritism; mirror neurons; mu rhythm; racial bias.

Fig. 1

Schematic display of the visual stimuli and their timing as used in the EEG experiment. A. Hands used as visual stimuli in the main and the control experiment. B. The trial sequence began with presenting a fixation cross (white on black background, duration varied between 1500 and 2000 ms), followed by a static display of a hand for a duration of 1500 ms. This static hand was followed by the video showing the action of hand treatment (i.e. motion of a needle syringe or a cotton swab, duration = 1500 ms). After the needle syringe or the cotton swab had reached their final position, a static display of this last frame of the video was shown for a duration of 1500 ms. This trial structure enabled us to assess unspecific responses to viewing pictures of hands of different ethnicity, as compared with the specific responses to painful or non-painful treatments of those hands.

Fig. 2

Upper panel: Schematic drawing of a head depicting the EEG electrode positions (small black dots). Highlighted are those sensors selected for analysis, overlying the sensorimotor cortex (green circles: left ROI, red circles: right ROI). Lower panel: Grand mean CSD ERD/ERS from all subjects of the main experiment (n = 36), averaged across all four experimental conditions, at sensor C3. Rectangles depict mu and beta windows for analysis of experimental effects (dashed: perception of hand prior to treatment, solid: perception of dynamic treatment action, dotted: perception of static treatment endpoint). Note that since epoch finished at time = 3000 ms, the last window in which EEG spectrum was calculated was centered at time = 2746 ms (for more details see Supplementary material).

Fig. 3

Mean beta (13–30 Hz) ERD/ERS for each experimental condition. Data are pooled from left and right ROIs. Negative values denote ERD, positive values denote ERS. Error bars represent the standard error of the mean.