Effects of Soul Gate Breathing–HRV Closed-Loop Biofeedback on Acute Stress Recovery and Self-Regulation Skill Transfer in Adolescents: A Three-Arm Randomized Controlled fNIRS Study

This preregistered three-arm randomized controlled study will investigate whether real-time breathing–heart rate variability (HRV) closed-loop biofeedback embedded in Soul Gate, a serious game for adolescent stress regulation, facilitates acute stress recovery and the transfer of learned self-regulation skills. Adolescents aged 12–15 years will be randomized to one of three conditions: (1) real closed-loop biofeedback, in which game changes are temporally coupled to the participant's ongoing respiratory and cardiac signals; (2) sham biofeedback, in which participants receive standardized feedback trajectories that resemble real feedback but are not causally linked to their current physiological signals; or (3) a no-feedback breathing game that provides the same breathing practice, narrative, and game context without real-time physiological feedback. The primary confirmatory comparison will test whether the real-feedback group shows greater recovery in log-transformed root mean square of successive differences (lnRMSSD) during a subsequent no-feedback transfer phase than the sham-feedback group. Secondary and mechanistic outcomes will include respiratory rate and regularity, perceived agency/control, causal judgments regarding physiological feedback, state anxiety, subjective stress, and retention and use of the learned breathing strategy at a 7-day follow-up. Portable functional near-infrared spectroscopy (fNIRS) will be used as an exploratory measure of prefrontal hemodynamic changes across stress induction, game-based regulation, and no-feedback transfer. The study is designed to distinguish the specific contribution of temporally congruent physiological feedback from nonspecific feedback expectancy and the common effects of breathing practice and game engagement.