The neuropsychophysiological correlates of compassion: A transcranial direct current stimulation (tDCS) study

Transcranial direct current stimulation (tDCS) has emerged as a promising tool to investigate and modulate emotional and social processes, including compassion. By targeting cortical regions implicated in empathy and prosocial behaviour (e.g., the left inferior frontal gyrus [lIFG] and the left supplementary motor area [lSMA]), tDCS may influence the distinct components of compassion: the sensitivity to suffering and the commitment to alleviate it. In this study we aimed to examine the effects of tDCS over the lIFG and lSMA on the neuropsychophysiological and behavioral correlates of compassion. In a randomized, double-blind, controlled study (N = 81), participants received 20 min of active (2 mA) or sham tDCS over the lIFG or lSMA while viewing a compassion-inducing video. We assessed self-reported compassion state (sensitivity and commitment) before and after stimulation, heart rate variability (HRV) at baseline, pre-, and post-stimulation, and brain activity at post-stimulation using high-density diffuse optical tomography (HD-DOT). Across the four groups mixed-effect models, no significant omnibus group effects or interactions were detected for self-reported sensitivity or engagement (all p > .05). However, compassionate images elicited significantly higher ratings compared to neutral (p < .001). Post-intervention, participants in all groups except those receiving active tDCS over the lIFG, presented an increased parasympathetic activity. There was a reduced neural activation of the anterior premotor cortex in the lIFG group when processing compassive eliciting images. Although a single tDCS session did not alter self-reported compassion, physiological and brain activity findings suggest distinct regional and temporal effects. These results underscore the complexity of modulating compassion using tDCS and the need for further research to clarify neural pathways and networks.