Abstract
The present study investigated a size illusion composed of two horizontal lines that were vertically separated and parallel to each other. When the two lines were of equal length, the upper line was consistently perceived to be a little longer than the lower line, therefore it was termed as horizontal parallel lines (HPL) illusion. We investigated the effect of color and luminance contrast on the HPL illusion by manipulating the color and luminance of the two lines. Results indicated the following: (1) differences in color between the two lines reduced the illusion; (2) differences in luminance between the two lines reduced the illusion; (3) Effect 1 was greater than Effect 2; (4) the illusory effect could not be affected as long as both of the lines were of the same color or luminance. The results suggest that the color or luminance contrast may contribute to the overall decrease in the illusory effect for lines with different colors/luminances, but generally the illusion decreases as the two lines are less similar to each other. These findings indicate that the similarity or ‘sameness’ effect dominates the effect of color/luminance contrast on the size illusion over the effect resulted from contrast difference or depth perception.
Highlights
Perceptual illusions refer to distorted visual perception of an object or a figure that mismatches reality
While research on the Muller–Lyer illusion shows that applying different colors to the whole stimuli could result in differences in the magnitude of the illusion [11,12], no such effect was found for the horizontal parallel lines (HPL) illusion [29]
The purpose of this experiment was to replicate Zhao et al.’s findings and to test whether the HPL illusion occurred with the upper and the lower lines being of the same color
Summary
Perceptual illusions refer to distorted visual perception of an object or a figure that mismatches reality. In addition to the Ponzo illusion, there are other well-known size illusions, like Muller–Lyer illusion, moon illusion, etc. Explanations for these size illusions often involve in a compensation mechanism of size constancy for distance variation. The phenomenon of size constancy demonstrates that the brain could account for an object’s angular size variation with viewing distance, since the apparent size of the object remains roughly constant regardless of distance. If an object’s angular size does not show the expected variation with the perceived distance, the brain infers that its physical size must be varying its appearant size is compensated based on the perceived distance, resulting in size illusions like the Ponzo illusion.
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