Perception of slant by an active observer remains constant when looking at a rotating but not at a static plane

The relative rotation between a static planar surface, slanted around the vertical axis, and a laterally translating observer produces a continuous variation of the instantaneous gradient of the optic-flow (def). According to Wexler, Lamouret and Droulez (2001), non-visual information about head movements is used together with the stationarity assumption (i.e., 3D objects are static in an earth-fixed reference frame) to compensate for the optic-flow variation in order to achieve an allocentric representation of the surface which is not changing orientation during the execution of the head movement. An alternative theory postulates that non-visual information is ignored, and that only the instantaneous optic-flow determines perceived 3D-structure (Fantoni, Caudek, and Domini, 2010); accordingly, a static plane should be perceived as changing its orientation during head movement. Here, we contrasted these two theories by studying perceived slant for planes with 0° tilt viewed during lateral head movements. Two types of random-dot planes were used: static, with varying def, and rotating, with constant def. Constant def during head movements was achieved by appropriately coupling the surface rotation with the head translation. In this condition, perceived slant is expected to vary for the first theory, but nor for the second (given that def was constant). Results were consistent with the second theory. Perceived slant was dramatically affected by the instantaneous def variation when monocularly viewing the static, but not the rotating planes. Perceived slant of the static planes was 35% larger when the def at the end of the head movement was large relative to when it was small. No such an effect was found for rotating planes although their allocentric orientation varied during head movements. Our finding is inconsistent with the stationarity assumption. Active perception of plane orientation depends on the instantaneous def at the end of head motion, not on the average def.