## 4 Discussion  
In the present study, we found significant changes in psychophysiological and brain activation following the VR exposure using a smartphone-assisted HMD. Firstly, the SSQ total score and its subscale scores (nausea, oculomotor symptoms, and disorientation) were significantly increased after the VR experience. Secondly, several brain regions showed a significant increase in activation. In particular, alpha1 activity showed a robust increase in the cuneus, while alpha2 activity increased in the cuneus and PCG. Thirdly, the change of PCG activation in alpha band showed significant correlation with the changes of SSQ total scores and its two subscale scores (oculomotor and disorientation). The increase in SSQ total and subscale scores clearly shows that participants experienced a severe level of cyber sickness during and after using VR in a smartphone-assisted HMD. Traditionally, researchers have attempted to explain motion sickness using two main theories: sensory conflict theory and postural instability theory. Sensory conflict theory suggests that the motion sickness occurs when the motion perceived by the visual, vestibular, and somatosensory systems differs from the expected experience [36]. On the other hand, postural instability theory implies that one's motion sickness occurs due to the user's inability to maintain a stable posture [37]. The cyber sickness experienced by participants in this study could be explained in the context of both theories. In smartphone-assisted head mount displays, two different images reach both eyes in a separate manner, at a separate time. Since these images are displayed on a close-up HMD screen, ophthalmological adjustment through accommodation and convergence may not occur successfully, causing eye strain and cyber sickness. Our results show a significant increase in brain regional activation (the cuneus of alpha1 and alpha2, and PCG of alpha2) after the VR experiment. These results suggest that alpha band power could be significantly related to the VR experience. The previous VR based driving simulation experiments have mainly reported changes in alpha band (8-13 Hz) [38, 39], when participants felt tired after VR experiences. The increase in alpha band power has been shown to be negatively related to cortical activity [40, 41, 42]. In addition, low band power (delta, theta, and alpha) increases in the occipital and parietal regions are correlated with the severity of motion sickness symptoms [38, 43, 44]. In sum,  
Figure 2: Scatterplots between PCG activation in alpha2 band and SSQ scores. The change in activation of PCG shows significantly negative correlations with the changes of SSQ in the oculomotor, disorientation, and total scores. increased alpha power in the cuneus and PCG could represent the reduced cortical activity on those regions after the VR experiment. The cuneus, which is part of the occipital lobe, has been related to visual processing [45]. It is known to receive visual information from the contralateral superior retina, and the processing of this information is modulated by other effects, such as attention, working memory, or reward expectation [46; 47]. In a previous study comparing navigation with video viewing, participants during the navigation showed higher activation in the parietal and occipital brain regions, including the cuneus [48]. Furthermore, the cuneus and post-central parietal lobe have been related to working memory and navigation tasks [46; 49]. Finally, considering that the activation changes in PCG were significantly correlated with the changes of SSQ total and subscale scores, the posterior cingulate cortex is a key node of the Default Mode Network (DMN)--a group of neural structures closely related to self-referencing and other self-related processing [50; 51; 52]. The immersion of 3D VR can induce a user to feel a sense of ownership of an avatar presented in a VR environment.