arises in determining the kind of object it represents and in how best to classify
it, given the mixed set of information available.
One of the most fundamental properties of normal human perception is the
tendency to transform ambiguity and uncertainty about the environment into a
clear interpretation that you can act upon with confidence. In a world filled
with variability and change, your perceptual system must meet the challenges
of discovering invariance and stability.
Illusions Ambiguous stimuli present your perceptual systems with the chal-
lenge of recognizing one unique figure out of several possibilities. One or
another interpretation of the stimulus is correct or incorrect with respect to a
particular context. When your perceptual systems actually deceive you into
experiencing a stimulus pattern in a manner that is demonstrably incorrect, you
are experiencing anillusion.Thewordillusionsharesthesamerootasludi-
crous—bothstemfromtheLatinilludere, which means ‘‘to mock at.’’ Illusions
are shared by most people in the same perceptual situation because of shared
physiology in sensory systems and overlapping experiences of the world. (This
sets illusions apart fromhallucinations.Hallucinations are nonshared perceptual
distortions that individuals experience as a result of unusual physical or mental
states.) Examine the classic illusions in figure 7.7. Although it is most conve-
nient for us to present you with visual illusions, illusions also exist abundantly
in other sensory modalities such as hearing (Bregman, 1981; Shepard & Jordan,
1984) and taste (Todrank & Bartoshuk, 1991).
Since the first scientific analysis of illusions was published by J. J. Oppel in
1854–1855, thousands of articles have been written about illusions in nature,
sensation, perception, and art. Oppel’s modest contribution to the study of
illusions was a simple array of lines that appeared longer when divided into
segments than when only its end lines were present:
versusOppel called his work the study ofgeometrical optical illusions. Illusions point
out the discrepancy between percept and reality. They can demonstrate the
abstract conceptual distinctions between sensation, perceptual organization,
and identification and can help you understand some fundamental properties
of perception (Cohen & Girgus, 1973).
Let’s examine an illusion that works at the sensation level: theHermann grid,
in figure 7.8. As you stare at the center of the grid, dark, fuzzy spots appear
at the intersections of the white bars. How does that happen? The answer lies
in something you read about in the last chapter—lateral inhibition. Assume the
stimulus is registered by ganglion retinal cells, two of which have their recep-
tive fields drawn in the lower corner of the grid. The receptive field at the cen-
ter of the intersection has two white bars projecting through its surround, while
the neighboring receptive field has only one. The cell at the center, therefore,
receives more light and can respond at a lower level because of the greater lat-
eral inhibition by the surround. Its reduced response shows up as a dark spot
142 Philip G. Zimbardo and Richard J. Gerrig