Abstract
Identical letters can appear to differ in size and shape depending on whether they are interpreted as uppercase or lowercase. The effect is most dramatic with the letter p. Examination of subjects’ estimates of the magnitude of the effect for different stimuli suggests that two factors are involved in the illusion. One factor depends only on whether a letter is interpreted as uppercase or lowercase. Thisfactor can be manipulated by changes in the size or case ofthe surrounding letters. The second factor, which depends on the possibility of interpreting a vertical line alternatively as an ascender or a descender(as in p or y), seems to involve a change in the perceived size of the letters’ loops.
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The P& p illusion was brought to my attention by Harne Hess of the UNLV faculty, who first noticed the phenomenon in the process ofexamining a computer-generated matrix of responses to a true-false test in which a long series ofomitted responses were scored “P” for “pass.” Perhaps the phenomenon should be called the “Hess illusion” in his honor.
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Diener, D. The P & p illusion. Perception & Psychophysics 47, 65–67 (1990). https://doi.org/10.3758/BF03208165
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DOI: https://doi.org/10.3758/BF03208165