A reversed structure-from-motion effect for simultaneously viewed stereo-surfaces.

A spatially flat stimulus is perceived as varying in depth if its velocity structure is consistent with that of a three-dimensional (3D) object. This is structure from motion (SFM). We asked if the converse effect also exists. A motion-from-structure effect would skew an object's perceived velocity structure to make it more consistent with the 3D structure provided by its depth cues. This proposed phenomenon should be opposite in sign from velocity constancy and could potentially interfere with it. Previous tests of velocity constancy compared stimuli presented at different times, not simultaneously. This explains why a reversal of SFM has not been previously reported, as it is expected to appear only for simultaneous presentations. We tested this prediction using random-dot stereograms to define two adjacent moving surfaces separated in stereoscopic depth. We found that subjects did not perceive velocity constancy with either simultaneous or sequential stimulus presentations. For sequential presentations, subjects matched retinal speeds, in agreement with previous work. However, for simultaneous presentations, the nearer surface was seen as moving faster when both surfaces were moving with the same retinal speed, an effect opposite in polarity from velocity constancy and a signature of the motion-from-structure phenomenon.