The moon illusion: a test of the vestibular hypothesis under monocular viewing conditions.

The results of earlier monocular experiments on the moon illusion have been either negative or confounded. To test the role of vestibular function, 24 subjects made forced-choice distance comparisons between stimuli mounted in translucent tubes. The stimulus tube for standard distance could be positioned in three viewing angles (45 degrees up, horizontal, and 45 degrees down). A comparison tube adjustable for distance was mounted horizontally. There was a greater perception of depth in the downward looking condition. The relatively weak effects are discussed in terms of a two-hypothesis explanation of the real-life moon illusion and the poor cues for depth perception in monocular viewing.