Although the degree of required realism may vary based on the tasks, one requirement remains invariant: The health and safety of the users. Unlike simpler media such as radio or television, VR has the power to overwhelm the senses and the brain, leading to fatigue or sickness. This phenomenon has been studied under the heading of simulator sickness for decades; in this book we will refer to adverse symptoms from VR usage as VR sickness. Sometimes the discomfort is due to problems in the VR hardware and low-level software; however, in many cases, it is caused by a careless developer who misunderstands or disregards the side effects of the experience on the user. This is one reason why human physiology and perceptual psychology are large components of this book. To engineer comfortable VR experiences, one must understand how these factor in. In many cases, fatigue arises because the brain appears to work harder to integrate the unusual stimuli being presented to the senses. In some cases, inconsistencies with prior expectations, and outputs from other senses, even lead to dizziness and nausea.
Another factor that leads to fatigue is an interface that requires large amounts of muscular effort. For example, it might be tempting move objects around in a sandbox game by moving your arms around in space. This quickly leads to fatigue and an avoidable phenomenon called gorilla arms, in which people feel that the weight of their extended arms is unbearable. For example, by following the principle of the computer mouse, it may be possible to execute large, effective motions in the virtual world by small, comfortable motions of a controller. Over long periods of time, the brain will associate the motions well enough for it to seem realistic while also greatly reducing fatigue. This will be revisited in Section .
Steven M LaValle