Binocular disparity as an explanation for the moon illusion

TL;DR

This paper proposes a perceptual model explaining the moon illusion by how the brain interprets binocular disparity and surface continuity, linking perceived size changes to sky distance cues and surface occlusion perceptions.

Contribution

It introduces a novel explanation for the moon illusion based on binocular disparity and surface perception, differing from previous theories.

Findings

The illusion strengthens as the apparent sky distance decreases.

Perception of surface occlusion influences size perception of the moon.

Distance cues affect the strength of the moon illusion.

Abstract

We present another explanation for the moon illusion, the phenomenon in which the moon looks larger near the horizon than near the zenith. In our model of the moon illusion, the sky is considered a spatially-contiguous and geometrically-smooth surface. When an object such as the moon breaks the contiguity of the surface, instead of perceiving the object as appearing through a hole in the surface, humans perceive an occlusion of the surface. Binocular vision dictates that the moon is distant, but this perception model contradicts our binocular vision, dictating that the moon is closer than the sky. To resolve the contradiction, the brain distorts the projections of the moon to increase the binocular disparity, which results in an increase in the perceived size of the moon. The degree of distortion depends upon the apparent distance to the sky, which is influenced by the surrounding objects and the condition of the sky. As the apparent distance to the sky decreases, the illusion becomes stronger. At the horizon, apparent distance to the sky is minimal, whereas at the zenith, few distance cues are present, causing difficulty with distance estimation and weakening the illusion.