Academic journal article Perception and Psychophysics

The Horizon Line, Linear Perspective, Interposition, and Background Brightness as Determinants of the Magnitude of the Pictorial Moon Illusion

Academic journal article Perception and Psychophysics

The Horizon Line, Linear Perspective, Interposition, and Background Brightness as Determinants of the Magnitude of the Pictorial Moon Illusion

Article excerpt

A total of 110 undergraduate students participated in a series of three experiments that explored the magnitude of the moon illusion in pictures. Experiment 1 examined the role of the number and salience of depth cues and background brightness. Experiment 2 examined the role of the horizon line, linear perspective, interposition, and background brightness. In Experiment 3, comparative distance judgments of the moon as a function of linear perspective, interposition, and the size of the standard moon were obtained. The magnitude of the moon illusion increased as a function of the number and salience of depth cues and changes in background brightness. Experiment 2 failed to support the role of the horizon line in affecting the illusion. Experiment 3 provided additional support for the illusory distance component of the moon illusion.

The moon illusion is the apparent overestimation of the size of the horizon moon when compared with the zenith moon and/or an apparent underestimation of the distance of the horizon moon when compared with the zenith moon. In reality, there is no significant difference in the distance from the earth to the moon as a function of the moon's position in the sky. As such, if perception is veridical, the observer should perceive the moon's size and distance to be constant.

Past research has revealed great magnitude differences between explorations of the natural moon illusion and drawings of the moon embedded in various contexts. However, images, similar to drawings that have been previously used to display optical illusions (e.g., the Ponzo illusion or the Müller-Lyer illusion), have been shown to be a viable means through which to examine this illusion (see, e.g., Coren & Aks, 1990; Redding, 2002). The present study incorporated the use of computer-generated drawings of the moon embedded in several different contexts. These drawings were adapted from Coren and Aks (1990; see Figure 1 below). It is important to note that the term elevated moon will be used to refer to a drawing of a moon in an elevated position in a two-dimensional plane, and the term zenith moon will be used to refer to the real moon viewed at optical infinity when it takes its position in the zenith of the sky.

In a series of experiments that sought to explore the magnitude of the moon illusion in drawings, Coren and Aks (1990) found that, in the presence of pictorial distance cues, the horizon moon was judged to be larger than the elevated moon overall. Participants selected a moon that they perceived to match in size to the moon in each experimental drawing from a series of comparison moons. Each drawing was rated on its overall impression of depth by several professors at the University of British Columbia. Participant overestimations of the horizon moon, when compared with the elevated moon, were found to increase as the number of depth cues in the drawings increased (see Coren & Aks, 1990, for a full review). Similar results were found in a follow-up experiment in which participants were asked to indicate the size of the horizon and elevated moons using the method of reproduction (i.e., reproducing the perceived diameter of the experimental moon by marking on a line). Results indicated that the horizon moon was perceived to be 11% larger than the elevated moon in the drawings rated highest in their overall impression of depth, and only 6% larger in the drawings that were rated the lowest in their overall impression of depth.

Overall, these results suggest that an increase in the number of distance cues in a picture is associated with a greater perceived size difference between a moon in the horizon position and a moon in an elevated position. In addition, apart from magnitude differences when compared with the exploration of the natural moon illusion, the role of depth cues in the magnitude of the illusion as reported by Coren and Aks (1990) is fairly consistent with research in field settings using the real moon, and in laboratory experiments using artificial moons (see, e. …

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