It is not a sense but a multi-sensory system . There has to be a circulate adjustment process in order to perceive the world and also in order to behave in the world. I suggested that there were different levels but these levels were not elaborated in chapter 4 of the Perceptual Systems . The emphasis was on the activity of the vestibular subsystems and the maintaining of postural equilibrium. Nothing was said about the phenomenal accompaniments of orientation, especially at the higher levels.
All organisms seem to be capable of orienting to gravity, even plants.
They also orient to another invariant of nature, the fact that light comes from above . Animals also orient to the surface of support. All three constitute information, not just stimulation, i.e. multiple information for what might be called the up-down direction. Normally all three are coincident and redundant. The vestibular system, the ocular system, and the skin-pressure system work together to yield the maintaining of postural equilibrium.