194 Visual Perception of Objects
Figure 7.17 Effects of orientation on perceived shape for figures with poor
intrinsic axes (A) versus figures with good intrinsic axes (B). Source: From
Palmer, 1999.
chance for 90° rotations, indicates that subjects often fail to
perceive the equivalence in shape of the presented and tested
figures when they are differently oriented. Further, Rock
(1973) found that tilting the observer’s head reduced recogni-
tion memory less than tilting the objects within the environ-
ment. This suggests that the reference frames for these figures
are environmental rather than retinal.
Why, then, do people seldom fail to recognize, say, a chair
when it is seen lying on its side rather than standing up? The
crucial fact appears to be that objects like chairs have enough
orientational structure that they effectively carry their own
intrinsic, object-centered reference frames along with them.
Roughly speaking, an object-centered reference frameis a
perceptual reference frame that is chosen on the basis of the
intrinsic properties of the to-be-described object, one that
is somehow made to order for that particular object (see
Palmer, 1999, pp. 368–371). For example, if the orientations
of two otherwise identical objects are different, such as an
upright and a tipped-over chair, the orientation of each
object-centered reference frame—for instance, the axis of
elongation that lies in its plane of symmetry—will be defined
such that both objects will have the same shape description
relative to their object-centered frames.
Wiser (1981) used Rock’s memory paradigm to study
shape perception for objects with good intrinsic axes and
found that they are recognized as well when they are pre-
sented and tested in different orientations as when they are
presented and tested in the same orientation (Figure 7.17; B).
In further experiments, she showed that when a well-
structured figure is presented initially so that its axis is not
aligned with gravitational vertical, subsequent recognition is
actually fastest when the figure is tested in its verticalorien-
tation. She interpreted this result to mean that the shape is
stored in memory as though it were upright, relative to its
own object-centered reference frame. This idea is important
in certain theories of object identification, a topic which will
be discussed in this chapter’s section entitled “Theories of
Object Identification.”OBJECT IDENTIFICATIONAfter the image has been organized into a part-whole hier-
archy and partly hidden surfaces have been completed, the
perceptual objects thus defined are very often identified as
instances of known, meaningful types, such as people,
houses, trees, and cars. This process of object identification is
often also referred to as object recognitionorobject catego-
rization.Its presumed goal is the perception of function,
thereby enabling the observer to know, simply by looking,
what objects in the environment are useful for what purposes.
The general idea behind perceiving function via object iden-
tification is to match the perceived properties of a seen object
against internal representations of the properties of known
categories of objects. After the object has been identified, its
function can then be determined by retrieving associations
between the object category and its known uses. This will not
makenoveluses of the object available—additional problem
solving processes are required for that purpose—rather, only
uses that have been previously understood and stored with
that category are retrieved.
Before pursuing the topic of object identification in depth,
it is worth mentioning that there is an alternative approach to
perceiving function. The competing view is Gibson’s (1979)
theory of affordances,in which opportunities for action are
claimed to be perceived directly from visible structure in the
dynamic optic array. Gibson claimed, for example, that peo-
ple can literally see whether an object affords being grasped,
or sat upon, or walked upon, or used for cutting without first
identifying it as, say, a baseball, a chair, a floor, or a knife.
This is possible only if the relation between an object’s form
and its affordance (the function it offers the organism) is
transparent enough that the relevant properties are actually