Zhong and Braunstein (VSS 2003) reported that the speed of a frontal plane in the background had a significant effect on the perceived magnitude of a dihedral angle. The effect could be divided into three phases, based on the relation between the background speed and the speed of the angle's front edge. We proposed that these phases were related to variations in perceived rigidity between the angle and background. In the present study we directly address the question of whether the relative speeds of an object and background influence perceived rigidity. In the first experiment the stimulus displays were perspective projections of a horizontal concave dihedral angle shown against a frontal plane, both undergoing horizontal translation. The projected speeds were 2 /sec for the dihedral edge and 2.25 /sec for the front edge. The background speeds were 1, 2, 2.25, 2.5, 4 or 5 /sec. The subject's task was to rate rigidity between the frontal plane and the dihedral angle on a scale of 0 to 10, with 0 indicating that two or more separate objects were perceived and 10 indicating that a single rigid object was perceived. We found a significant relation between rigidity ratings and background speed. The rigidity ratings were high (>7) when the background speed was close to the angle's front edge speed and dropped to around 1 when the speed difference exceeded 1.25 /sec. In the second experiment this effect was examined with displays of orthographic projections of a concave dihedral angle rotating about a vertical axis against a frontal plane translating in phase. The projected center speeds of the dihedral edge and front edge and the background speeds were the same. The results were similar to those in the first experiment. Our conclusion is that background speed affects perceived rigidity in a manner consistent with its effect on perceived object shape, for both translating and rotating objects shown against moving backgrounds.
Supported by NIH Grant EY-12437.