Reduction of Motion Complexity as an Objective Indicator of Cybersickness in Virtual Reality

Subjective measures, such as the Simulator Sickness Questionnaire (SSQ), Fast Motion Sickness Questionnaire (FMS), and discomfort scores, are widely used to assess cybersickness, but they often interrupt the user experience and are prone to bias. To overcome these limitations, researchers have also investigated objective indicators, though some approaches, such as using physiological data, can be cumbersome and impractical. Based on the loss of complexity hypothesis, which suggests that certain conditions, such as disease or aging, can produce a reduction of complexity in physiological system dynamics, we conducted an initial investigation of the relationship between movement complexity and cybersickness. We analyzed motion tracking collected from two previous cybersickness studies using the d95 score, a complexity metric derived using principal component analysis. The results revealed a systematic relationship between movement complexity and cybersickness across both experiments. Higher discomfort scores were associated with a reduction in complexity, thereby supporting the loss of complexity hypothesis. Furthermore, the 9-DOF complexity measure, which includes both physical head movement and virtual camera motion, was a more sensitive indicator than the 6-DOF measure computed from physical movements alone. These initial findings suggest that movement complexity may be a useful objective indicator for future cybersickness research.