# Correlations between SSQ Scores and ECG Data during Virtual Reality Walking by Display Type
## 1 Introduction  
The demand for head-mounted displays (HMDs) for viewing virtual reality (VR) content has been increasing annually. In particular, VR technology has seen significant growth in the wake of COVID-19, owing to remote meetings becoming increasingly common. Moreover, the increase in the number of people who want to gain various experiences from the comfort of their homes through VR devices has led to a rapid increase in the number of VR users [1]. However, several side effects of VR have been reported, the most common being cybersickness, also known as simulator sickness or motion sickness, which occurs when using VR; it is accompanied by symptoms similar to those of actual motion sickness, such as eye fatigue, disorientation, and nausea [2]. Cybersickness is a condition characterized by symptoms such as nausea, dizziness, and headaches experienced by users in virtual reality (VR) environments, with the sensory conflict theory being reported as the primary cause. This refers to the discrepancy between visual information in the virtual environment and the motion and position senses from the inner ear, leading to motion sickness. Additionally, factors in VR environments such as parabolic motion representation, update latency, limitedfield of view, and individual differences among users have also been reported as causes of cybersickness [3]. Although studies have been conducted to identify and quantify the causes of cybersickness, its symptoms and degree vary among individuals, making research difficult. In most previous studies, a moving virtual environment was presented to stationary seated subjects, and the degree of motion sickness was measured. For subjective evaluations, the VR sickness questionnaire, which includes the simulator sickness questionnaire (SSQ), was used [4], and the motion sickness susceptibility questionnaire (MSSQ) was used to determine the individual differences in motion sickness due to various stimuli [5]. For objective evaluations, heart rate (HR) and heart rate variability (HRV) analyses through electrocardiograms (ECGs) were primarily used. In addition, electroencephalographs, which indirectly measure the electrical activity in the neurons of the brain [6; 7], breathing rate [8], and body temperature [9], were used for evaluating motion sickness. One of these biosignals, the HRV, indicates the changes in the HR intervals related to autonomic nervous system control, including the sympathetic and parasympathetic nervous systems [10]. Parasympathetic activity slows down the HR; conversely, sympathetic activity increases the HR. The dynamic balance between the parasympathetic and sympathetic activities causes continuous oscillation of the HR, which is called the HRV [11]. Physiological indicators such as the HRV parameters allow researchers to quickly ascertain the physical conditions of the study participants. HRV parameters can be used to examine the condition of a study participant based on their parasympathetic and sympathetic activities; therefore, the HRV parameters that are most highly correlated with cybersickness should be studied. Accordingly, questionnaires and various biosignal measurements are being used to examine these correlations with motion sickness; however, the majority of the existing cybersickness studies have been performed in a state of limited movement, and studies on motion sickness during regular movements such as walking are limited. In particular, the use of VR in walking rehabilitation is currently increasing; consequently, it has become necessary to examine cybersickness and control the factors affecting it [12; 13]. [PERSON] et al. (2011) examined the gait variables in 16 healthy seniors after walking on a treadmill while wearing or not wearing an HMD displaying a walking trail environment [12]. Increased variability in the gait variable and joint flexion were observed within 10 min of performing VR walking when compared with normal walking; however, after 15 min, the walking characteristics became similar to those of normal walking.