Observers frequently perceive space behind an occluder as empty even without sufficient evidence, revealing a striking bias in spatial perception – the illusion of absence. This study examined two factors assumed to influence the illusion: occluder width and depth perception. Drawing on the generic view principle, we predicted that narrow occluders would strengthen the illusion. In addition, we expected that monocular viewing – by reducing depth cues – would weaken it.

To test these predictions, participants viewed real objects that were resting on a support concealed behind occluders of different widths and under binocular or monocular viewing conditions. Perceived levitation of the objects served as a proxy for the perception of empty space: if the support holding the object behind the occluder is perceived as non-existent, the object may be expected to appear to float. Our findings confirmed both hypotheses: the illusion weakened with wider occluders and under monocular viewing. However, there are fairly large individual differences in the data which warrant further study and explanation.

Beyond theoretical implications for the interplay between perception and knowledge in the context of amodal percepts, this work raises practical concerns for high-risk settings such as traffic safety. Upcoming research will explore how the illusion operates in dynamic environments under varying motion patterns, with the goal of identifying factors that further intensify or reduce the illusion.

This project was supported by the Research Council of Norway (Project No. 334817).