Most VR experiences involve another crucial component: interaction. Does the sensory stimulation depend on actions taken by the organism? If the answer is ``no'', then the VR system is called open-loop; otherwise, it is closed-loop. In the case of closed-loop VR, the organism has partial control over the sensory stimulation, which could vary as a result of body motions, including eyes, head, hands, or legs. Other possibilities include voice commands, heart rate, body temperature, and skin conductance (are you sweating?).