Partial modal completion under occlusion: what do modal and amodal percepts represent?

Area of Science:

• Visual perception
• Cognitive psychology
• Psychophysics

Background:

• The occlusion illusion describes the perception of partly occluded objects as less occluded than they are.
• Previous research (Palmer et al., 2007) proposed a partial-modal-completion hypothesis for this phenomenon.

Purpose of the Study:

• To confirm and extend findings on the occlusion illusion using a moving occluder.
• To investigate the relationship between sensory evidence and the illusion's strength.
• To explore the paradoxical nature of the percept in the occlusion illusion.

Main Methods:

• Subjects viewed a rotating disk with an open sector in front of a background.
• Participants reported the perceived angular extent of the occluder.
• Participants also reported the perceived visible extent of the background through the open sector.

Main Results:

• The perceived angular extent of the occluder was judged accurately.
• The perceived visible extent of the background was overestimated.
• The sum of perceived occluded and visible background extents exceeded 360°, creating an impossible figure.

Conclusions:

• The occlusion illusion is strongly related to the sensory evidence supporting occlusion.
• The paradox of the impossible figure can be resolved by reconsidering amodal and modal perception.
• Visual percepts are proposed to be modal when based on conclusive sensory evidence, and amodal otherwise.