# Impact of Latency on QoE, Performance, and Collaboration in Interactive Multi-User Virtual Reality
## 2 Related Work
### Single-User Experiences  
[PERSON] et al. [18] provided a systematic evaluation of different levels of delay across a variety of perceptual and motor tasks during full-body action inside a Cave Automatic Virtual Environment (CAVE). To this end, participants were presented with their virtual mirror image, which responded to their actions with delays ranging from 45 to 350 ms. The impact of these delays on motor performance, sense of agency, sense of body ownership, and simultaneity of perception was measured. In addition, interaction effects between these variables were analyzed to identify possible dependencies. The results showed that motor performance and simultaneity perception were affected by latency above 75 ms. In addition, it was observed that the sense of agency and body ownership never broke down completely despite significant delays. However, they started declining at a latency higher than 125 ms,and deteriorated for a latency greater than 300 ms. Interestingly, participants perceptually inferred the presence of delays more from their motor error in the task than from the actual level of delay. Whether or not participants notice a delay in a virtual environment might, therefore, depend on the motor task and their performance rather than on the actual delay. Although not directly related to network distortions, this study, therefore, provides interesting insights in subjective delay perception in virtual environments. [PERSON] et al. [19] explored the effect of increased end-to-end latency in immersive single-user VR applications. To this end, three tasks were developed: (1) a searching task in which the user had to find circular targets (platforms), spawned on the ground in one of three corridors; (2) a reaching task where a user needed to touch a cube appearing in a virtual scene; and (3) an embodiment task in which users had to observe themselves in a virtual mirror. The three tasks were used to study the influence of latency on cybersickness, user performance (completion time and error rate), and the sense of body ownership, respectively. The results showed that an end-to-end latency above 63 ms induced significant cybersickness symptoms. In addition, user performance decreased with increasing delay, and with end-to-end latency above 69 ms, the users needed significantly longer to complete the task. The results also showed that end-to-end latency affected body ownership significantly later, namely, not until 101 ms.  
[PERSON] et al. [20] presented a VR simulator of a forestry crane used for loading logs onto a truck. In their study, the authors focused specifically on the effects of latency on the subjective experience, with regards both to delays in the crane control interface as well as the lag in the visual scene rendered in the head-mounted display (HMD). To this end, a subjective study was performed in which the delay to the display update and to the joystick signals was controlled. The results showed significant effects on comfort quality and immersion quality for higher display delay (30 ms), but a very small impact of joystick delay. Furthermore, the display delay exerted a strong influence on the occurrence of cybersickness, causing test subjects to decide not to continue with the complete experiments. This was shown to be connected especially to the longer added display delays (\(\geq\)20 ms). In a follow-up study, similar results were observed, as a strong effect of latency in the display update and a significant negative effect of 800 ms added delay on latency in the hand controller were found. The results on cybersickness showed significantly higher scores after the experiment compared to before the experiment, although a majority of the participants reported experiencing only minor symptoms. Some test subjects ceased the test before finishing due to their symptoms, particularly due to the added latency in the display update. [PERSON] et al. [21] sought to understand the impact of network delay on a user's QoE whilst interacting with a VR application.