Toying with the moon illusion

Area of Science:

• Psychology
• Vision Science
• Perceptual Psychology

Background:

• The moon illusion, where the moon appears larger on the horizon than at the zenith, is a long-standing perceptual puzzle.
• Existing explanations often focus on the horizon moon appearing larger, but this study proposes an alternative perspective.

Purpose of the Study:

• To re-interpret the moon illusion by proposing the zenith moon appears small, not the horizon moon large.
• To explain the perceived size difference using principles of visual perception, distance cues, and accommodation.

Main Methods:

• The study is theoretical, proposing an interpretation based on established principles of visual perception.
• It analyzes the role of optical information, distance perception, and the phenomenon of empty field myopia.
• The size-distance-invariance hypothesis and accommodation responses are central to the proposed mechanism.

Main Results:

• The zenith moon appears small because the lack of distance cues leads to empty field myopia, with the moon being neurally processed as if at arm's length.
• The horizon moon, with available distance cues, elicits reduced accommodation, leading to its processing as more distant.
• Consistent with the size-distance-invariance hypothesis, this greater perceived distance results in the moon being judged as larger.

Conclusions:

• The moon illusion is best understood as the zenith moon appearing small due to under-accommodation and lack of distance cues.
• This perceptual phenomenon is a specific instance of a broader principle where distant objects appear small without accommodation stimuli.
• The proposed interpretation offers a unified explanation for the moon illusion and related perceptual effects like the toy illusion.