As mentioned in Section 2.3, the balance sense (or vestibular sense) provides information to the brain about how the head is oriented or how it is moving in general. This is accomplished through vestibular organs that measure both linear and angular accelerations of the head. These organs, together with their associated neural pathways, will be referred to as the vestibular system. This system plays a crucial role for bodily functions that involve motion, from ordinary activity such as walking or running, to activities that require substantial talent and training, such as gymnastics or ballet dancing. Recall from Section 5.3 that it also enables eye motions that counteract head movements via the VOR.
The vestibular system is important to VR because it is usually neglected, which leads to a mismatch of perceptual cues (recall this problem from Section 6.4). In current VR systems, there is no engineered device that renders vestibular signals to a display that precisely stimulates the vestibular organs to values as desired. Some possibilities may exist in the future with galvanic vestibular stimulation, which provides electrical stimulation to the organ [81,80]; however, it may take many years before such techniques are sufficiently accurate, comfortable, and generally approved for safe use by the masses. Another possibility is to stimulate the vestibular system through low-frequency vibrations, which at the very least provides some distraction.