JUDGMENTS OF HEADING

To study the contribution of vision to the perception of ego-motion, o ne often dissociates the retinal flow from the corresponding extra-ret inal information on eye, head and body movement. This puts the observe r in a conflict concerning the experienced ego-motion. When the retina l flow of a translating and rotating eye is shown to a stationary eye, observers often perceive ego-motion on a curved path. In contrast, wh en they receive the same retinal flow with a rotating eye subjects cor rectly perceive the simulated rectilinear ego-motion. Thus, different visual representations of ego-motion gain precedence when using the co nflict stimulus and when using conditions in which the visual and extr a-retinal information accord. Because the flow-pattern can be decompos ed in many different ways, the brain could represent the same flow-pat tern as a rotation about an axis through the eye plus rectilinear ego- motion or a rotation about an axis outside the eye (corresponding to c ircular ego-motion) plus motion towards the axis of rotation. The circ ular motion path percept minimizes the conflict with extra-retinal eye movement information if the axis of rotation is placed at the fixatio n point. However, in simulated eye rotation displays subjects also per ceive illusory motion in depth of the stationary fixation point. This illusory motion is argued to reflect the ego-centric decomposition. Er rors are small when subjects judge their heading on the basis of this illusory motion. For the same displays much larger errors are made, ho wever, when subjects judge heading from the entire motion pattern, whi ch often results in perceived ego-motion on a curved path. This indica tes that subjects can choose between two different representations of ego-motion resulting in different perceived heading. Copyright (C) 199 6 Elsevier Science Ltd.