Can “prior knowledge” of isotropy be varied trial by trial? Yes, in slant from texture

When perceiving slant from texture, observers tend to presume that elements of texture are initially isotropic. Multiple studies confirm this presumption by showing that variation of aspect ratio of texture (i.e., deviation from isotropy) alters slant perception from texture. However, there is yet little literature on how this “prior knowledge” is made. The current study addressed this issue by attempting to convey “knowledge” of isotropy/anisotropy to observers. We introduced a set of two-folded surfaces, of which the upper parts were slanted and the lower parts frontal-parallel. Both parts were planar but connected smoothly with a curved surface. The whole surface was textured with Voronoi textures and the aspect ratio of texture were independently manipulated for the upper and the lower surfaces, in a range between 0.8 (compressed) to 1.0 (isotropic). The aspect ratio of texture on the connection part gradually changed to avoid any abrupt change of texture. In both experiments (N = 30 for Experiment 1; N = 47 for Experiment 2), observers viewed the two-folded surfaces and estimated 3D slants of the upper surfaces by aligning their hand with the orientations of the upper surfaces while they were asked to ignore the lower surfaces. The estimates from both experiments showed that decrease in aspect ratio of the upper surfaces led to the observers’ overestimation of surface slant, as previous studies have shown. Most interestingly, decrease in aspect ratio of the lower surfaces, which had no direct relations to the task, made the observers significantly underestimate slant of the upper surface, even when aspect ratios of the lower surfaces varied trial by trial. These findings indicate that isotropy is not fixed knowledge but more likely to be contextual information. Observers can flexibly choose whether to adhere to the presumption of isotropy, depending on their environmental understanding."