Filled space and filled time illusions vary similarly with analogous stimulus parameters.

Visual filled space (Oppel-Kundt) and auditory filled time illusions distort the perception of spatial width and temporal length, respectively. Although these two illusions occur in different physical domains, space and time, their construction principles are analogous, raising the question of whether they are related. In this study, visual and auditory stimuli were constructed in a comparable manner and presented in psychophysical experiments to investigate these distortions. The paired results revealed correlations in illusion strength change across various stimulus parameter changes. Illusion strength increased with the number of fillers, saturating at about six fillers. Relative illusion strength decreased with increasing interval length. Reversing stimulus order had little effect. Introducing gaps reduced illusion strength, but only for visual stimuli. These findings support the notion that related perceptual processes, possibly grounded in closely related neural structures, underlie both illusions.