The Ponzo Illusion in Virtual Environments: Correctly Applied Size Constancy?

Abstract

The perceived size of a stimulus can be greatly influenced by the surrounding depth cues. The effect size of the Ponzo Illusion was tested in a virtual environment with many depth cues (MC) and few depth cues (FC) conditions. The effect of size and the depth cues on the perceived proximal size of the stimulus was measured. Methods. A modified Wheatstone stereoscope was used to present the stimuli on a monitor positioned 80 cm from the subjects. The subjects used a 6AFC confidence rating scale to indicate whether the first or second of two sequentially presented stimuli had a greater proximal extent. The ‘many cues’ (MC) condition included texture, relative height, foreshortening, linear perspective, relative brightness, and relative size. The few cues condition consisted of linear perspective, relative height, and relative size. Stimulus size (proximal extent) was varied independently from all other depth cues. Results. Both depth cue context and proximal size were found to have a significant effect on the perceived proximal size for FC conditions. However, for MC conditions only the depth cues, and not proximal size, had an effect on perceived proximal size. The effect size for both depth cues and proximal size had a significant linear trend. But proximal size had a larger trend that was significantly related to the size constancy function.