# Correlations between SSQ Scores and ECG Data during Virtual Reality Walking by Display Type
## 2 Research Methods
### Data Measurement and Analysis  
The subjective and objective indicators of cybersickness were measured under two conditions (screen-assisted walking and HMD-assisted walking). The Simulator Sickness Questionnaire (SSQ) survey was used as a subjective indicator, and the most commonly used evaluation method to measure subjective motion sickness in cybersickness [14], and there are many variations in the type or number of items. Therefore, although the total score can vary in each study, the SSQ score is considered highly reliable because it includes evaluation items that consider various circumstances leading to cybersickness, and it is not only affected by the visualization screen but also by control methods such as joysticks [15]. The SSQ consists of five main items, namely, eyestrain, overall discomfort, nausea, concentration, and headache, as well as 28 sub-items that belong to these items; a score of 1-7 is entered for each item. In this study, the SSQ questionnaires were completed by each participant directly after the baseline and walking phases in each of the two experimental conditions (screen and HMD). The SSQ test for each phase took about 3 min. ECGs were measured as the objective indicators, and the Trigno EKG sensor (Delsys, Natick, MA, USA) was used as the measurement equipment. For the experiment, two electrodes were used, with one attached under each side of the chest. The HR and HRV were extracted using HRV analysis (ANS Lab Tools). For the HRV, the RMSSD and LF/HF ratio were used to examine the activation of the sympathetic and parasympathetic nerves. The RMSSD is the root mean square of the differences in the RR interval and is calculated by using the time difference between consecutive heartbeats in milliseconds, squaring each value, finding the average of the result, and determining the square root of the total. It reflects the beat-to-beat variance in the HR and is used as a domain measurement to estimate the changes in the HRV [16]. A high value of the LF/HF ratio indicates that the sympathetic nerves are activated or that the activation of the parasympathetic nerves is  
Figure 3: Experimental design. Figure 2: Experimental environment under the two conditions: (**a**) screen-assisted walking; (**b**) HMD-assisted walking. blocked. In short, this value can help quantify the overall balance between the sympathetic and parasympathetic nervous systems. The HR, RMSSD, and LF/HF ratio measured in the experiments under the two conditions were analyzed based on variations in the values, which were determined by subtracting the baseline data from the walking interval data. To analyze the correlation between the occurrence of cybersickness and HR and HRV, the SSQ score of each subject was used to divide the subjects into two groups (motion sickness and no motion sickness) and perform an analysis. The SSQ scores after the baseline phase and those after the walking phase were compared; subjects with a score difference of less than 15 points were classified as the no-motion-sickness group, whereas those with a score difference of more than 15 points were classified as the motion-sickness group. Thus, six subjects were classified under the no-motion-sickness group (age 23.5 \(\pm\) 5.8, height 175.2 \(\pm\) 5.8 cm, weight 68.2 \(\pm\) 8.8 kg), and five subjects were classified under the motion-sickness group (age 24 \(\pm\) 1.4, height 178.0 \(\pm\) 5.3 cm, weight 82.4 \(\pm\) 11.1 kg) according to the differences in their SSQ scores, and an analysis of the HR and HRV according to the occurrence of motion sickness was performed. To examine the differences in SSQ scores, HR, RMSSD, and LF/HF ratio differences between the two conditions (screen and HMD) and the two groups (motion-sickness group, no-motion-sickness group), we conducted a two-way ANOVA using SPSS 25 (IBM, Armonk, NY, USA). Specifically, we first performed paired-sample \(t\)-tests to assess SSQ scores, HR, RMSSD, and LF/HF ratio differences within each group across the two conditions.