# VR Sickness Evaluation Method Using Recurrence Period Density Entropy
## 4 Discussion  
In this experiment, we compared the RPDE values of baseline session and evaluation session states. In this experiment, we compared the RPDE values of the baseline session and the evaluation session. Although no direct correlation was found between RPDE values and SSQ scores, significant differences in RPDE change were observed between VR sickness. Generally, increased variability in heart rate (RR interval) is related with higher stress levels. The research of [PERSON] [18] confirmed a relation between RR intervals and stress levels. [PERSON] et al. [19] demonstrated that irregular heart rate variability correlated with increased stress levels. Additionally, according to [PERSON]'s research [20], during stressful states, the heart beats irregularly and rapidly, with a decrease in the RR interval and an increase in the heart rate. Similarly, [PERSON] [21] and [PERSON] [22] confirmed an increase in heart rate during VR sickness. Furthermore, in the experiment, it was observed that RPDE values indicating irregularity in the heart rate cycle increased during VR sickness, which aligns with the findings of previous research. Therefore, based on previous studies, it has been confirmed that higher RPDE change values may indicate a higher level of stress. Although high RPDE values may not directly indicate a state of stress, interpreting high RPDE change values could suggest a stressed state, similar to the characteristics of physiological indicators used to determine health status based on changes. A correlation analysis between the RPDE and SSQ scores was conducted to further investigate their association with VR sickness. However, the analysis revealed that the magnitude of RPDE was not directly correlated with the VR sickness. The results of the correlation analysis can be interpreted in two ways. First, the RPDE of the cardiac cycle is an indicator of individual characteristics. Generally, individual physiological indicators have either high or low values. Individual physiological indicators may have direct correlations with certain health conditions. However, when assessing health status based on changes in these indicators, a different perspective is often applied. Therefore, although high RPDE values may not directly indicate a state of stress, interpreting high RPDE change values could suggest a stressed state, similar to the characteristics of the physiological indicators used to determine health status based on changes. When interpreting these results, it is important to recognize that individuals who experience VR sickness can be divided into two groups: those who are sensitive to VR sickness and those who are not [23; 24]. Individuals who are less sensitive to VR sickness may find it difficult to perceive changes in any mitigation training or method because they  
Figure 4: Box plot of RPDE between sessions. experience less VR sickness. This is a limitation of the SSQ, as it is a subjective questionnaire that reflects subjectivity and operates on a 0-3 point scale, limiting its ability to capture subtle differences in sensations [25; 26]. However, when considering physiological data such as body or heart data, these subtle differences can be reflected. Therefore, changes in the VR sickness that may not be detected by subjective data could be reflected, as seen in the results of our study, by a decrease in RPDE values in situations where individuals experienced less VR sickness (evaluation sessions). In summary, according to previous studies, there is a direct correlation between RPDE and VR sickness. However, in this experiment, no clear correlation between SSQ and RPDE was observed. This is likely because the RPDE operates as an indicator of individual variability and assesses health status based on changes, whereas the SSQ provides subjective data, making it difficult to confirm specific indicators directly.