Two completely different motions of a subject relative to the earth can induce exactly the same stimuli to the vestibular, somatosensory and visual systems. When this happens, the subject may experience disorientation and misperception of self-motion. We have identified large classes of motions that are perceptually equivalent, i.e. indistinguishable by the subject, under three sets of conditions: no vision, with vision and earth-fixed visual surround, and with vision during possible movement of the visual surround. For each of these sets of conditions, we have developed a classification of all sustained motions according to their perceptual equivalences. The result is a complete list of the possible misperceptions of sustained motion due to equivalence of the forces and other direct stimuli to the sensors under the given conditions.

This research expands the range of possible experiments by including all components of linear and angular velocity and acceleration. Many of the predictions in this paper can be tested experimentally. In addition, the equivalence classes developed here predict perceptual phenomena in unusual motion environments that are difficult or impossible to investigate in the laboratory.