# The Onset Threshold of Cybersickness in Constant and Accelerating Optical Flow
## 5 Discussion
### Dominant Axis Evoking Cybersickness  
For the translational optical flow, the threshold for vertical movement is significantly lower than the lateral one. LS (LA) had an average threshold of approximately 0.60 m/s (0.33 m/s\({}^{2}\)), whereas VS (VA) had a threshold of 0.40 m/s (0.31 m/s\({}^{2}\)). For rotational optical flow, the pitch threshold tended to be lower than the yaw, though the difference (\(p=0.098\)) was just suggestive. It supports the previous study that the most annoying rotation axes follow the pitch, roll, and yaw axis order [36]. Our results proposed that YS (YA) had an average threshold of 17.88 deg/s (14.50 deg/s\({}^{2}\)) and PS (PA) had a threshold of 14.61 deg/s (11.06 deg/s\({}^{2}\)).  
Optical flow induced the visual-vestibular conflict for stationary participants staring at the red anchor dot-point. Their nervous system picks the point as a rest frame to maintain their spatial representations; thus, the observer selects to be stationary with the anchor point. Soon after the exposure, the optical flow inconsistent with the rest frame stimulated peripheral vision, and the nervous system cannot keep the single rest frame [19]. In this case, the upward vertical flow causes a greater discrepancy than the lateral in orientation sensation due to the direction of actual gravity [14]. In the pitch movement, there is a mismatch between the sensed vertical integrated from all sensory systems  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & **Lateral** & **Vertical** & **Yaw** & **Pitch** \\ \hline \multirow{2}{*}{Total score} & Steady & 19.52 (18.21) & 22.21 (19.97) & 20.10 (19.77) & 24.43 (19.45) \\  & Accelerating & 18.88 (19.21) & 14.90 (13.18) & 17.82 (20.21) & 18.93 (15.68) \\ \hline \multirow{2}{*}{Nausea} & Steady & 10.43 (13.78) & 14.16 (14.52) & 11.93 (15.30) & 17.44 (16.26) \\  & Accelerating & 10.88 (15.75) & 7.45 (10.12) & 9.84 (13.49) & 12.97 (12.18) \\ \hline \multirow{2}{*}{Oculomotor} & Steady & 20.73 (17.93) & 20.25 (18.62) & 18.36 (17.60) & 20.37 (17.02) \\  & Accelerating & 18.00 (18.84) & 15.87 (14.72) & 16.94 (19.65) & 16.23 (15.97) \\ \hline \multirow{2}{*}{Disorientation} & Steady & 19.45 (25.22) & 24.44 (30.76) & 23.49 (29.23) & 27.84 (28.28) \\  & Accelerating & 21.71 (24.70) & 15.01 (17.42) & 20.45 (27.28) & 21.53 (21.61) \\ \hline \hline \end{tabular}
\end{table}
Table 5: Mean (standard deviation) of the SSQ scores for the severity level of cybersickness.  
and the subjective vertical expected from previous experience [21]. The visual system detects the pitch movement, but the vestibular system perceives uprights. However, for the yaw movement, both visual and vestibular sense an upright body position, while the visual scene is rotated for the on-vertical axis. If the participant could fix one's head perfectly, there are no conflict stimuli based on subjective vertical theory. Moreover, the sickness severity for yaw rotation is typically slight, and occurs with a long delay [22]. These can be explanations for the suggestive difference for thresholds between pitch and yaw movement. Nevertheless, the participants reported symptoms for the yaw axis. As noted by [PERSON] et al. (2008), CS might be caused by a minor confusion of the center of rotation or misrecognition that they are watching lateral movement [22].