MIT Encyclopedia of the Cognitive Sciences - Cryptome

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Further Readings
Folstad, I., and A. J. Karter. (1992). Parasites, bright males and the
immunocompetence handicap. American Naturalist 139: 603–
622.
Johnston, V. S. and M. Franklin. (1993). Is beauty in the eye of the
beholder? Ethology and Sociobiology 14: 183–199.
Perrett, D. I., K. A. May, and S. Yoshikawa. (1994). Facial shape
and judgments of female attractiveness. Nature 368: 239–242.
Shape Perception
This article concerns shape perception, so it is natural to
consider what is meant by “shape.” Most readers undoubtedly
have an intuitive feel for the meaning of the term, sensing
its relatedness to such concepts as form and structure.
Nevertheless, a precise definition of shape has proved elusive.
Instead, experimenters have adopted the working definition
that shape is an aspect of a stimulus that remains
invariant despite changes in size, position, and orientation.
For example, 2-D visual stimuli have the same shape if there
exists a transformation of spatial scale (e.g., magnification)
or a rotation in the picture plane that renders them identical.
Similarly, 3-D objects have the same shape if their volumes
can be equated by size changes or a combination of rotations
about three spatial axes.
Shape Perception 753
Although our world is filled with objects and patterns,
most studies of shape perception have used only 2-D stimuli.
This emphasis reflects the belief that perception of 3-D
form depends on the shape of 2-D regions in the retinal
image. The fact that line drawings can evoke vivid percepts
of 3-D form (see figure 1a) supports the idea that 2-D and 3-
D shape perception are related, but how they are related is
still debated (cf. Attneave 1954; GIBSON 1950; Hochberg
1964; Koffka 1935; MARR 1982). Furthermore, variables
that affect LIGHTNESS PERCEPTION (e.g., shading) and
DEPTH PERCEPTION (e.g., binocular disparity and relative
motion) also affect form perception. How these variables
interact with 2-D shape to yield a 3-D percept is not well
understood.
In most viewing conditions, a pattern’s shape is closely
related to the spatial arrangement of its contours. Thus,
shape perception depends in part on feature extraction processes
that encode edges and elementary features in the retinal
image, and on processes that group elements into
higher-order units (see GESTALT PERCEPTION). In the latter
case, we can perceive both the shape of the individual elements,
as well as the global shape of the grouped elements
(figure 1b). Despite the close connection between contours
and shape, simultaneous presentation of contours is neither
necessary nor sufficient for shape perception. It is not necessary
because shape is perceived even when only isolated
parts of contour are presented. For example, in anorthoscopic
stimuli a visual pattern is moved behind a stationary
slit in an opaque screen: although only a small dot or line is
visible at any one time, observers often perceive the shape
of the hidden pattern. Simultaneous presentation of contours
is not sufficient for shape perception because observers do
not necessarily attend to everything in the visual field.
Although some attributes of an unattended object, such as
its presence, location, and color, can be perceived, ATTEN-
TION appears to be required for shape perception (Rock and
Gutman 1981; Rock et al. 1992). Other top-down processes
also affect shape perception, as is shown by ambiguous figures
like the duck/rabbit pattern in figure 1c: The contours
remain constant, but perceived shape depends on the interpretation
of the figure. In some cases, the allocation of
attention to particular stimulus features can bias the percept
of ambiguous figures (Peterson and Gibson 1991).
Although clearly related to HIGH-LEVEL VISION phenomena
such as OBJECT RECOGNITION, shape perception is better
classified as an aspect of MID-LEVEL VISION because the perception
of shape does not require recognition: One can perceive
the shapes of novel stimuli that have no a priori
meaning (figure 1d), and an observer can recognize objects
based on surface properties like TEXTURE or color (Humphrey
et al. 1994; see also SURFACE PERCEPTION). Nevertheless,
shape provides important clues about an object’s
identity, as well as information that is critical to manipulating
objects and determining their functional properties or
AFFORDANCES (e.g., Can I stand on this object?). Thus, it is
not surprising that visual mechanisms encode shape rapidly
and accurately. For example, the response time needed to
name line drawings of familiar objects, or to determine if
two novel polygons have the same shape, is approximately 1
sec (Larsen 1985). Although these response times are short,