The face size illusion is specific to human faces.

Area of Science:

• Cognitive Psychology
• Neuroscience
• Visual Perception

Background:

• The human visual system possesses specialized mechanisms for upright face perception, leading to the face inversion effect.
• A recently identified face size illusion shows upright faces perceived as smaller than identical inverted faces, an effect previously thought specific to faces.

Purpose of the Study:

• To investigate whether the face size illusion is specific to faces generally or exclusively to human faces.
• To determine if non-human primate and feline faces elicit the same size perception bias as human faces.

Main Methods:

• Participants judged the relative physical size of simultaneously presented upright and inverted faces.
• The experiment utilized human, monkey, and cat faces across different blocks.

Main Results:

• Human faces exhibited the expected illusion: upright faces perceived as smaller than inverted ones.
• Monkey and cat faces did not show this illusion; instead, a bias in the opposite direction was observed.

Conclusions:

• The face size illusion is highly specific, not only to faces but critically to human faces.
• These findings underscore the specialized nature of human face processing in the visual system.