This section introduces the rest frame conflict measure studied below. A scale for the visual effect of a stimulus is defined in terms of its ability to influence the inertial perception of motion. This ``visual-inertial nulling'' technique draws its strength from the close connection between the visual and inertial motion perception systems in humans. Because these two systems are so closely related, it may be possible to measure subtle changes in the input to one system in terms of a change in perception arising from the other. See Section 2.3 for further details.