Keshavarz B, Hecht H. Axis rotation and visually induced motion sickness: the role of combined roll, pitch, and yaw motion. Aviat Space Environ Med 2011; 82:1023–9.Background: Motion sickness (MS) is a well-known phenomenon in aviation and in virtual environments such as simulators or computer games. The severity of MS is thought to be due to the amount of sensory conflict, which should increase with the complexity of the simulated motion. The present study focused on the direction and complexity of simulated body rotations in the genesis and severity of visually induced MS. Methods: Three simulated rollercoaster rides including translational movement in the fore-aft axis and additional rotational motion either in pitch only, along the pitch and roll axes, or in pitch, roll, and yaw were generated. We presented video clips of 15 min on a large projection screen to a total number of 61 volunteers, who were randomly assigned to 1 of the 3 rotational motion groups. MS was measured using the Fast Motion Sickness Scale (20-point verbal rating scale) and the Simulator Sickness Questionnaire. Aftereffects were captured up to 5 h after the experiment was finished. Results: Analyses indicated lowest MS scores in the pitch-only condition (1.95). Dual- (4.33) or triple-axis (5.30) combinations revealed significantly higher MS scores than the single-axis condition, but surprisingly did not differ from each other. MS started to subside rapidly after about 1 h past stimulus presentation. Discussion: We conclude that the complexity of visual motion does not increase MS linearly. Instead, we propose that MS reached a plateau in the dual-axis condition and adding a third rotational axis did not further surpass the severity of MS reached.