Difference thresholds in 3D space depend on perceived size, not depth.

People typically perceive objects to be the same size and shape despite changes in viewing distance, a phenomenon known as object constancy. Recent work suggests that perceptual processing of object shape and size may be enhanced in near space, consistent with similar reports from attentional tasks. These studies, however, have some methodological limitations, as the viewing distance was manipulated using illusory 2D depth cues, for which perceptual processes may differ significantly from those in real three-dimensional (3D) environments. To address this, a series of psychophysical experiments were conducted in which participants' visual resolution for object length was tested within and outside their peripersonal space. In Experiments 1 and 2, difference thresholds (Just Noticeable Differences, JND) were measured using the Method of Adjustment, while Experiment 3 employed the Method of Constant Stimuli. Experiment 4 tested whether the findings can be extended to 3D stimuli. Results from Experiments 1 and 2 showed that perceived size (PSE) varied with distance. Critically, however, across all experiments, JNDs were modulated by the size of objects and were insensitive to differences in retinal size or viewing distance. These findings suggest that in natural 3D environments, perceptual resolution is constrained by high-level visual representations of object size. More broadly, the study points to potential differences in how perceptual mechanisms operate in 3D versus 2D contexts, consistent with recent findings in the perception and action literature.