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Related Experiment Video

Updated: Jul 13, 2026

Applying Incongruent Visual-Tactile Stimuli during Object Transfer with Vibro-Tactile Feedback

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The occlusion illusion: partial modal completion or apparent distance?

Stephen E Palmer¹, Joseph L Brooks, Kevin S Lai

¹Department of Psychology, Tolman Hall, University of California, Berkeley, Berkeley, CA 94720-1650, USA. palmer@cogsci.berkeley.edu

|July 13, 2007

Summary

The occlusion illusion makes partly hidden objects appear larger. Research suggests this size illusion stems from the visual system completing the occluded shape, not the object seeming farther away.

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Area of Science:

Visual perception
Cognitive psychology
Psychophysics

Background:

The occlusion illusion describes how a visible part of an occluded object appears larger than an identical, fully visible object.
Two main hypotheses explain this: partial-modal-completion (visual system 'fills in' missing parts) and apparent-distance (occluded object perceived as farther).

Purpose of the Study:

To investigate the underlying causes of the occlusion illusion.
To differentiate between the partial-modal-completion and apparent-distance hypotheses.

Main Methods:

Psychophysical measurements of the occlusion illusion's magnitude across several experiments.
Experiment 4 specifically tested whether perceived size increase results from shape alteration or overall size change.

Main Results:

The magnitude of the occlusion illusion correlated with the strength of occlusion cues, supporting an occlusion-based explanation.
Results indicated that the perceived shape change, consistent with modal completion, was a stronger factor than perceived distance.

Conclusions:

The findings provide stronger support for the partial-modal-completion hypothesis.
The visual system's 'filling-in' of occluded contours significantly contributes to the occlusion illusion's perceived size exaggeration.