The role of perspective effects and accelerations in perceived three-dimensional structure-from-motion

It has been suggested that perceived three-dimensional (3D) structure-from- motion can be accounted for by a 2-frame orthographic approximation of the flow field. This study investigated the extent to which higher order cues ( perspective and acceleration) are used in addition to first-order flow. Par ticipants matched the 3D dihedral angle of a hinged plane (probe) defined b y multiple-depth cues to one defined by motion only, for stimulus sizes of 8 and 33 degrees, using perspective and orthographic projection. The result s show that perspective effects can be important even for relatively small stimuli (8 degrees) and that accelerations contribute to perceived shape. I n all conditions, large biases were found. These are well accounted for by a model in which all relevant flow measurements (first-order, perspective, and acceleration) are used together with estimates of the noise in each. Th e model has no built-in bias toward particular 3D shapes. Instead the visua l system may act as an optimal estimator of 3D stricture-from-motion.