The Effects of Segmentation and Spatial Geometry on the Tilt Illusion

Abstract

In the tilt illusion, when the orientation of the center and surround gratings differ by a small angle, the center grating appears to tilt away from the surround orientation (repulsion); however, for a large difference in angle, the center appears to tilt towards the surround orientation (attraction). Orientation perception therefore depends strongly on the context. Changes in two-dimensional configuration or three-dimensional scene geometry may alter the segmentation features between the center and surround, and cause variations in central orientation perception. We first measured the effect of two sources of segmentation information, contrast, and stereo disparity differences, on the strength of the tilt illusion in human observers. We observed: when the center contrast was high, both low-contrast surround and stereo depth segmentation cues reduced the amplitude of both the repulsion and attraction effect, relative to a high-contrast, 2D surround; a higher surround contrast (70%) relative to the center (10%) decreased the repulsion effect but increased both the range and magnitude of the attraction effect. Next, we examined the effect of perceptual grouping behind a 3D occluder ring between the center and surround on perceived central orientation. We observed significantly stronger tilt repulsion effects across occlusion than the condition with a ring at the same plane as the center and surround. We also found that the tilt repulsion is selective for the peripheral location of the surround. These results suggest that scene organization plays a role in central orientation perception.