Having long considered that extraretinal information plays little or no role in spatial vision, the study of structure from motion (SfM) has confounded a moving observer perceiving a stationary object with a non-moving observer perceiving a rigid object undergoing equal and opposite motion. However, recently it has been shown that extraretinal information does play an important role in the extraction of structure from motion by enhancing motion cues for objects that are stationary in an allocentric, world-fixed reference frame (Nature 409 (2001) 85). Here, we test whether stationarity per se is a criterion in SfM by pitting it against rigidity. We have created stimuli that, for a moving observer, offer two interpretations: one that is rigid but non-stationary, another that is more stationary or less rigid. In two experiments, with subjects reporting either structure or motion, we show that stationary, non-rigid solutions are preferred over rigid, non-stationary solutions; and that when no perfectly stationary solutions is available, the visual system prefers the solution that is most stationary. These results demonstrate that allocentric criteria, derived from extra-retinal information, participate in reconstructing the visual scene. © 2001 Elsevier Science Ltd. All rights reserved.
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