Anisotropy in motion sickness susceptibility during longitudinal and lateral motion while seated on a car seat

Motion sickness is critical in automated vehicle design, particularly as horizontal accelerations become more variable and pronounced. However, even within horizontal translation, whether sickness sensitivity differs between longitudinal and lateral directions remains unclear. To investigate this, we conducted a within-subjects experiment with 21 participants exposed to repeated sinusoidal accelerations while seated in a car seat without visual cues, restrained by seat belt and neck brace. Each participant experienced one combination of two acceleration levels and multiple frequencies (0.15, 0.20, 0.25, or 0.40 Hz for 3.0 m/s²; 0.15, 0.25, or 0.40 for 0.65 m/s²). Symptoms were assessed using the Motion Illness Symptoms Classification at 1-min intervals. Results revealed significantly greater symptom progression in longitudinal motion (adjusted-mean: 3.0, max: 4.75) than lateral motion (adjusted-mean: 1.67, max: 2.65), despite smaller head angular velocities. These findings provide empirical evidence for direction-specific sickness sensitivity and underscore the need to consider motion direction in vehicle design.