# The Onset Threshold of Cybersickness in Constant and Accelerating Optical Flow
## 1 Introduction  
Among the various fields adopting virtual reality (VR), education, training, and entertainment found its usefulness and actively embraced them. However, in a virtual environment (VE), cybersickness (CS) often causes nausea, dizziness, and discomfort. It is a serious problem that hinders the acceptance of VR because those who experience CS are reluctant to use the VE again; thus, the effectiveness of VR may get discarded. Since the exposure to moving visual scene caused CS such as nausea [1; 2; 3], discovering the impact of various optical flow on CS is vital for developing countermeasures for CS [4].  
Understanding the characteristics of optical flow can help to modify the dynamic field-of-view (FOV) [5], navigation speed [6; 7; 8], or VR locomotion techniques [9]. Previous studies suggested high velocity [10] and acceleration [11] of optical flow induce CS. Along with that, the rotational [12] and translational [13] motions in VR also increased the CS symptoms. However, no consensus has been yet reached regarding the relative importance of rotational (roll, pitch, or yaw) and translational (surge, lateral, or vertical) movements. Previous studies tried to find a dominant direction but failed to show a significant difference between those axes [12; 14; 15; 16; 17]. Since they measured the severity level of CS, there are chances that the symptom started in a different order, but the severity of symptoms was not different because the exposure time is either too long or too short.  
In this sense, this study tried to measure the threshold instead of severity to capture which motion activates CS for the first time. [PERSON] and [PERSON] (2020) found that rotation (roll and yaw) thresholds are lower than that of translation (forward and lateral) [18]. However, it is not decisive since they did notinclude vertical and pitch movements in the experiment. They expected that the vertical and pitch would follow a similar trend to the lateral and yaw movement, which is a fair assumption for the object moving on a surface, such as a ground vehicle. Nevertheless, we expected that the lateral and yaw threshold would differ from the vertical and pitch threshold for a flying object such as a quadcopter drone. Based on the rest frame theory [19], the more inconsistency between the perceived'vertical reference' from VR and sensed one from actual gravity cause severe CS [14]. Thus the upward vertical flow may distort the feeling of gravity more than the lateral flow, which leads to an earlier onset of CS. The subjective vertical theory [20; 21] explains that sensory conflict is related to the discrepancy between the sensed vertical from sensory systems and the subjective vertical expected from the past. The visual yaw rotation does not directly confound the perception of verticality [22], unlike pitch rotation. Therefore, we further examined the study for comparing lateral, vertical, yaw, and pitch movement as a continuing study of [PERSON] and [PERSON] (2020).  
This paper aims to find dominant movements causing the onset of CS by threshold detection. A human subject study investigated the difference in thresholds among rotational and translational optical flow moving in a constant velocity and with accelerations. The CS severity is also measured per each condition to detect a difference and compare it with the previous work. We used the same optical flow model developed by [PERSON] and [PERSON] (2020) [18] to ensure a fair comparison. This paper's organization is as follows: Section 2 presents the background of CS and related works about CS induced by scene movement along different axes. Section 3 details the method for the human subject experiment, followed by the experimental results about the detected threshold and severity level presented in Section 4. In Section 5, these results are further discussed. Finally, we conclude by summarizing the highlights of this study in Section 6.