Recent studies suggest that the active observer combines optic flow information with extra-retinal signals resulting from head motion. Such a combination allows, in principle, a correct discrimination of the presence or absence of surface rotation. In Experiments 1 and 2, observers were asked to perform such discrimination task while performing a lateral head shift. In Experiment 3, observers were shown the optic flow generated by their own movement with respect to a stationary planar slanted surface and were asked to classify perceived surface rotation as being small or large. We found that the perception of surface motion was systematically biased. We found that, in active, as well as in passive vision, perceived surface rotation was affected by the deformation component of the first-order optic flow, regardless of the actual surface rotation. We also found that the addition of a null disparity field increased the likelihood of perceiving surface rotation in active, but not in passive vision. Both these results suggest that vestibular information, provided by active vision, is not sufficient for veridical 3D shape and motion recovery from the optic flow.
