Handbook of Psychology, Volume 4: Experimental Psychology

200 Visual Perception of Objects

found in normally functioning individuals (Palmer et al.,
1981), except that instead of simply taking longer to arrive at
the correct answer, these patients are unable to perform the
task at all, even in unrestricted viewing conditions.
There are many other visual disorders due to brain damage
that are related to visual agnosia. They exhibit a wide variety
of complex symptoms, are caused by a broad range of under-
lying brain pathologies, and are generally not well under-
stood. Still, the case histories of such patients and their
phenomenological descriptions of their symptoms make for
fascinating reading, such as the patient whose agnosia led
neurologist Oliver Sacks (1985) to entitle one of his books,
The Man Who Mistook His Wife for a Hat. The interested
reader is referred to Farah (1990, 2000) for discussions of
these and related disorders.

THEORIES OF OBJECT IDENTIFICATION

Given that people obviously manage to identify visually
perceived objects as members of known, functional classes,
how might this result be achieved? There are many possibil-
ities, but within a modern, computational framework, all of
them require four basic components: (a) The relevant char-
acteristics of the to-be-categorized object must be perceived
and represented within the visual system in anobject repre-
sentation;(b) Each of the set of possible categories must
be represented in memory in acategory representationthat
is accessible to the visual system; (c) There must becom-
parison processesthrough which the object representation is
matched against possible category representations; (d) There
must be adecision processthat uses the results of the com-
parison process to determine the category to which a given
object belongs. This section considers each of these compo-
nents and then describes two contrasting types of theories
that attempt to explain how object identification might be
performed.

Representing Objects and Categories

The problem of how to represent objects and categories is a
difficult one (cf. Palmer, 1978) that lies at the heart of most
theories of object identification. Especially thorny are the
representational issues pertaining to shape, which tends to
be the single most important feature for object identification.
Most proposals about shape representation cluster into three
general classes: templates, feature lists, and structural de-
scriptions, although various hybrids are also possible. Space
limitations prohibit a detailed discussion of these issues, but
the interested reader can consult the more extensive treat-
ment by Palmer (1999, chapter 8).

Templates

The idea behindtemplatesis to represent shape as shape. In

standard presentations of this kind of theory, templates are

specified by the conjunction of the set of receptors on which the

image of the target shape would fall. A template for a square,

for example, can be formed by constructing what is called a

square-detector cell whose receptive field structure consists of

excitation by all receptors that the square would stimulate, plus

inhibition by all nearby receptors around it that it would not

stimulate (Figure 7.23). A white square on a black ground

would maximally excite this square detector because its spatial

structure corresponds optimally to that of its receptive field.

Templates are often ridiculed as grossly inadequate for

representing shape. In fact, however, they are the most obvi-

ous way to convert spatially structured images into symbolic

descriptions. Line- and edge-detector theories of simple cells

in cortical area V1 can be viewed as template representations

for lines and edges. Each line detector cell responds maxi-

mally to a line at a specific position, orientation, and contrast

(light on dark versus dark on light). Whether such a scheme

can be extended to more complex shape representations is

questionable (see following discussion), but recent theories

of object identification have concentrated on view-specific

representations that are template-like in many respects (see

this chapter’s section entitled “View-Specific Theories”).

Some of the most difficult problems associated with tem-

plates as a general scheme for representing shapes of objects

and categories are outlined in the following list:

1.Concreteness:There are many visual factors that have

essentially no impact on perceived shape, yet strongly in-

fluence the matching of template representations, including

Figure 7.23 A template representation of a square. Source: From Palmer,

1999.