Signals from multiple senses and proprioception are being processed and combined with our experiences by our neural structures throughout our lives. In ordinary life, without VR or drugs, our brains interpret these combinations of inputs in coherent, consistent, and familiar ways. Any attempt to interfere with these operations is likely to cause a mismatch among the data from our senses. The brain may react in a variety of ways. It could be the case that we are not consciously aware of the conflict, but we may become fatigued or develop a headache. Even worse, we could develop symptoms of dizziness or nausea. In other cases, the brain might react by making us so consciously aware of the conflict that we immediately understand that the experience is artificial. This would correspond to a case in which the VR experience is failing to convince people that they are present in a virtual world. To make an effective and comfortable VR experience, trials with human subjects are essential to understand how the brain reacts. It is practically impossible to predict what would happen in an unknown scenario, unless it is almost identical to other well-studied scenarios.
One of the most important examples of bad sensory conflict in the context of VR is vection, which is the illusion of self motion. The conflict arises when your vision sense reports to your brain that you are accelerating, but your balance sense reports that you are motionless. As people walk down the street, their balance and vision senses are in harmony. You might have experienced vection before, even without VR. If you are stuck in traffic or stopped at a train station, you might have felt as if you are moving backwards while seeing a vehicle in your periphery that is moving forward. In the 1890s, Amariah Lake constructed an amusement park ride that consisted of a swing that remains at rest while the entire room surrounding the swing rocks back-and-forth (Figure 2.20). In VR, vection is caused by the locomotion operation described in Section 2.2. For example, if you accelerate yourself forward using a controller, rather than moving forward in the real world, then you perceive acceleration with your eyes, but not your vestibular organ. For strategies to alleviate this problem, see Section 10.2.
Steven M LaValle 2020-01-06