Cardiac Cycle Phase Modulates Pain Processing During Heartbeat-Enhanced Rubber Hand Illusion

Pain-a complex, highly subjective experience-is shaped by interoceptive signals, especially the systolic and diastolic phases of cardiac rhythmicity. While body ownership illusions (BOI, the perceptual attribution of artificial limbs to one's own body) are modulated by interoceptive signals, their influence on pain processing remains controversial, with conflicting findings in the literature. Critically, it remains unclear whether cardiac-phase-specific pain modulation occurs independently of BOI. To resolve this, we examined: (1) the effects of cardiac cycles, (2) the influence of BOI, and (3) their potential interactions on pain processing. In the present study, we used a virtual reality rubber hand illusion (VR-RHI) paradigm to induce BOI. In the control condition (object, OBJ condition), participants viewed a VR scenario with an inanimate object (cardboard) instead of a rubber hand, which does not induce BOI. Pain stimulation was administered under four experimental conditions: RHI-systole, RHI-diastole, OBJ-systole, and OBJ-diastole. We assessed pain perception-thresholds, intensity and unpleasantness ratings, and somatosensory evoked potentials (SEPs)-while delivering painful electrical stimuli timed to systolic or diastolic phases under BOI and control VR conditions. Results demonstrated that compared to the systolic phase, the diastolic phase was associated with significantly lower pain intensity and unpleasantness ratings, along with reduced SEP amplitudes. However, neither BOI nor its interaction with cardiac cycle exerted significant effects on these measures. Our findings suggest that while cardiac cycle modulates pain perception, this effect operates independently of BOI.