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Chronic pain is a challenging and difficult to treat condition where traditional treatment methods often lead to frustration in both patients and therapists. The so-called rubber hand illusion, using visuo-tactile stimulation in order to introduce ownership over a virtual body experiment, resulted in a decrease in pain perception if a patient reports ownership over a fake body part. While the rubber hand illusion deals with the upper limbs, it is possible to extend the experiment to the full body during the so-called full-body-illusion. The focus of this work is the development of an easy-to-use mobile, virtual reality setup that can later be applied as a long-term treatment for patients with chronic pain. To induce the full-body illusion we use the so-called cardio-visual illusion, e.g. the patient’s heartbeat is displayed as a flashing silhouette synchronously on a virtual avatar in front of the patient in the virtual environment. The technical feasibility, usability, and acceptance as well as the effect on pain perception and ownership were investigated in this study.
The setup consists of two parts: a head-mounted display (HMD) to show the virtual environment and a device to measure the patient’s heartbeat. For the HMD, a setup with a smartphone (Samsung Galaxy S8+) was chosen for its mobility advantage. The heart rate was measured with smartwatches using photoplethysmography (PPG). Two different positions of the smartwatch were compared against a five-point ECG measurement regarding their accuracy.
Twenty healthy subjects participated in the study. The usability and eventual symptoms of cybersickness as well as ownership over the virtual body were investigated with questionnaires. Pain perception was assessed using state of the art algometric methods (e.g. Algopeg, Somedic algometer).
The usability scores were close to the maximum (i.e. the device is easy to use and well accepted), and those for sickness were close to the minimum (i.e. symptoms negligible). The verification of the heart rate measurement showed a high precision of the PPG measurement device, which was further improved by optimizing the filtering and peak detection algorithms. However, no significant effect for manipulation of body ownership and pain perception were found. We discuss the results and the limitations of our study in the context of the current literature on virtual-reality based interventions for chronic pain.