## 3 Results
### Reality vs. Virtual Reality  
Virtual reality technology is gaining interest in many different areas such as rehabilitation, sport, education, or medicine [33]. Especially in a scientific context, VR offers new possibilities to examine and understand human perception and action. The in situ presentation of stimuli is the most realistic type of simulation to examine gaze behavior. However, the realistic game situations could not be standardized because none of the players involved were able to show the same movement for a second time. For this standardization, researchers used video presentations to show game situations or sports actions as a stimulus. The disadvantage of video presentations is the non-realistic representation regarding the depth of vision and restriction of the field of view. Visual VR (HMD) offers the opportunity to show such natural sports situations differently. The fast growth of VR simulations in academic research led to the validation of training applications for transferable skills in surgery or navigation [34]. Other research groups focused on the possibility of VR technology to learn general physical skills [35]. First, however, the question should be clarified to what extent VR can support valid gaze behavior studies based on the narrative review's identified study results. Ref. [36] experimented with gaze behavior and focused on the duel between a handball goalkeeper and a field player (thrower). The authors recorded the thrower's movements to create an avatar based on the movements (motion capturing). In this case, it was not the eye movements compared but the gestures to ward off the ball. The authors examined to what extent the VR presentation produced the same movements for the same throw as the real situation in the field. The results show that the gestures did not differ between real and virtual environments (Q4). The study did not show a real comparison between reality and VR with an HMD presentation and excluded gaze behavior. Nevertheless, the example shows a great advantage of VR for science: creating different scenarios with real-time interactions between the user and the VR, which cannot be created in reality [37]. In addition, the programmed procedures for an investigation ensure standardized and easy-to-control experimental conditions. The visual input predominates VR application compared to the acoustic and haptic information [38]. Integrating further measurement systems such as the eye tracker [39] or the EEG [40] offers new possibilities to get a more in-depth insight into visual information processing. For this purpose, mobile eye trackers were integrated into various existing VR devices such as HMD's or the Cave Automatic Virtual Environment (CAVE). However, this application requires a complex technical implementation. For this reason, some authors carried out studies to measure the accuracy and precision of the point of view made in the VR compared with those from the natural environment [39]. The results show no significant differences between the two forms of presentation mentioned and that the VR setting can be used to investigate gaze behavior in sport. Furthermore, in perception and anticipation, occlusion techniques showed that, through spatial depth information, athletes experienced a more realistic feeling in the virtual environment than in a 2-D video presentation. As a result, they were able to react better to enemy attacks in martial arts [41]. In addition to the facts mentioned above, the athletes' reactions and actions in the CAVE are more similar to those in reality than responses shown on a 2-D screen [42]. Moreover, [43] showed that additional VR training leads to an improvement in reaction behavior. These studies indicate that the use of VR in combination with the eye leads to new insights. Other gaze parameters such as saccades [44] and gaze fixations [45] can also be recorded in VR.