Smartphone Photoplethysmography as a Digital Stress Biomarker: A Laboratory Validation Study [Thesis]
Burgess, R. L., BAH, BHSc, BAH
2026
A THESIS PRESENTED TO THE DEPARTMENT OF PSYCHOLOGY IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE (B.A.). WITH HONOURS DEGREE
Abstract
Stress is increasingly understood as a process that contributes to a wide range of mental health outcomes. There is a need to establish whether smartphone-based facial biometric measures can validly detect stress in controlled laboratory settings. The present study examined whether a smartphone application using photoplethysmography-derived facial biometrics could detect experimentally induced stress in a sample of undergraduate students (N = 104; M age = 18.46). Participants were randomly assigned to a stress condition involving a serial subtraction task or a control condition involving rest. Stress responses were assessed using three indicators: self-reported stress (SRS), heart rate (HR), and a computed stress (CS) score derived from facial scans. Measures were collected across five time points. Results showed that the stress manipulation successfully increased SRS and HR in the experimental group compared to the control group. However, the facial biometric measure (CS) did not significantly differentiate between conditions. Overall, findings indicate that while subjective and physiological measures were sensitive to experimentally induced stress, the smartphone-based facial biometric system was not. These results highlight the need for further validation of digital biometric tools before their use in real-world stress monitoring.
Keywords: Stress, subjective stress (SRS), social stress, physiological stress (HR), photoplethysmography (PPG), digital biomarker, validation, students.
Acknowledgements
This section acknowledges those whom I owe my deepest gratitude. First, to my supervisors, Dr. Gary Goldfield and Dr. Shelly Brown; you created space where I could remain honest and focused, and provided the mentorship I needed throughout this process. Dr. Goldfield, your depth of knowledge and steady guidance shaped not only this work, but the way I learned to think through it, while providing form to ideas that might otherwise have remained uncertain. Dr. Brown, your belief in me, especially when I struggled to hold it myself, along with your patience, carried me through some of the hardest moments. Dr. Marcus Lopes, thank you for your constant presence; your guidance, detailed feedback, encouragement, and willingness to answer any question, no matter how small, made this process feel less isolating. Dr. Kevin Nunes, thank you for teaching me how to approach this work with rigor, discipline, and attention to detail. Thank you to Maude Boivin and my lab partners at HALO; your camaraderie was a constant source of motivation. To Katherine Coady, Bruce Hamm, and Melanie Chapman, who stood beside me with care when I needed it most, I cannot fully express my gratitude.
I wanted this acknowledgements section to be honest about the conditions under which this thesis was not only written, but insisted upon. Writing a thesis is demanding under ideal laboratory conditions; mine were not. During my time in this program, I experienced sexual violence; there were periods of fear, uncertainty, and real risks. I am deeply grateful to Dr. Robert Coplan, who reminded me that scholarship can be grounded in humanity, integrity, and the willingness to stand beside a student when it matters. I offer this thesis with awareness that, while I was studying stress biomarkers, I lived through sustained stress, at times both subject and witness to the conditions I was studying, carrying both the questions and their weight. This shaped the stakes, lens, and urgency of my work.
