Physiological measurements

The alternative is to attach sensors to the user so that physiological measurements are automatically obtained before, during, and after the VR session. The data can be obtained continuously without interrupting the user or asking him to pay attention to symptoms. There may, however, be some discomfort or fear associated with the placement of sensors on the body. Researchers typically purchase a standard sensing system, such as the Biopac MP150, which contains a pack of sensors, records the data, and transmits them to a computer for analysis.

Some physiological measures that have been used for studying VR sickness are:

• Electrocardiogram (ECG): This sensor records the electrical activity of the heart by placing electrodes on the skin. Heart rate typically increases during a VR session.
• Electrogastrogram (EGG): This is similar to the ECG, but the electrodes are placed near the stomach so that gastrointestinal discomfort can be estimated.
• Electrooculogram (EOG): Electrodes are placed around the eyes so that eye movement can be estimated. Alternatively, a camera-based eye tracking system may be used (Section 9.4). Eye rotations and blinking rates can be determined.
• Photoplethysmogram (PPG): This provides additional data on heart movement and is obtained by using a pulse oximeter. Typically this device is clamped onto a fingertip and monitors the oxygen saturation of the blood.
• Galvanic skin response (GSR): This sensor measures electrical resistance across the surface of the skin. As a person sweats, the moisture of the skin surface increases conductivity. This offers a way to measure cold sweating.
• Respiratory effort: The breathing rate and amplitude are measured from a patch on the chest that responds to differential pressure or expansion. The rate of breathing may increase during the VR session.
• Skin pallor: This can be measured using a camera and image processing. In the simplest case, an IR LED and photodiode serves as an emitter-detector pair that measures skin reflectance.
• Head motion: A head tracking system is a rich source of movement data, which can help to estimate fatigue or postural instability with no additional cost, or distraction to the user.

A recent comparison of physiological measures and questionnaires appears in [60], and it is even concluded that one can determine whether a person is experiencing VR from the physiological data alone.