Judgements of heading.

To study the contribution of vision to the perception of ego-motion, one often dissociates the retinal flow from the corresponding extra-retinal information on eye, head and body movement. This puts the observer in a conflict concerning the experienced ego-motion. When the retinal flow of a translating and rotating eye is shown to a stationary eye, observes often perceive ego-motion on a curved path. In contrast, when they receive the same retinal flow with a rotating eye subjects correctly perceive the simulated rectilinear ego-motion. Thus, different visual representations of ego-motion gain precedence when using the conflict stimulus and when using conditions in which the visual and extra-retinal information accord. Because the flow-pattern can be decomposed in many different ways, the brain could represent the same flow-pattern as a rotation about an axis through the eye plus rectilinear ego-motion or a rotation about an axis outside the eye (corresponding to circular ego-motion) plus motion towards the axis of rotation. The circular motion path percept minimizes the conflict with extra-retinal eye movement information if the axis of rotation is placed at the fixation point. However, in simulated eye rotation displays subjects also perceive illusory motion in depth of the stationary fixation point. This illusory motion is argue to reflect the ego-centric decomposition. Errors are small when subjects judge their heading on the basis of this illusory motion. For the same display much larger errors are made, however, when subjects judge heading from the entire motion pattern, which often results in perceived ego-motion on a curved path. This indicates that subjects can choose between tow different representations of ego-motion resulting in different perceived heading.