Object Identification 197plish it initially are in a state of heightened readiness for the
second presentation (Bartram, 1974). The priming effect is
defined as the difference between the naming latencies in
the first block of trials and those in the second block of re-
peated pictures. What makes priming experiments informa-
tive about object categorization is that the repetitions in the
second block of trials can differ from the initial presentation
in different ways. For example, repetitions can be of the
same object, but with changes in its position within the vi-
sual field (e.g., left vs. right side), its retinal size (large vs.
small), its mirror-image reflection (as presented initially or
left-right reversed), or the perspective from which the ob-
ject is viewed.
The results of such studies show that the magnitude of the
object priming effect does not diminish when the second pre-
sentation shows the same object in a different position or
reflection (Biederman & Cooper, 1991) or even at a different
size (Biederman & Cooper, 1992). Showing the same object
from a different perspective, however, has been found to
reduce the amount of priming (Bartram, 1974). This perspec-
tive effect is thus consistent with the naming latency results
reported by Palmer et al. (1981) and the recognition results
by Edelman and Bülthoff (1992) and Bülthoff and Edelman
(1992). Later studies on priming with different perspective
views of the same object by Biederman and Gerhardstein
(1993), however, failed to show any significant decrease in
priming effects due to depth rotations.
To explain this apparent contradiction, Biederman and
Gerhardstein (1993) then went on to show that priming ef-
fects did not diminish when the same partswere visible in the
different perspective conditions. This same-part visibility
condition is not necessarily met by the views used in the
other studies, which often include examples in which dif-
ferent parts were visible from different perspectives (see
Figure 7.18). Visibility of the same versus different parts may
thus explain why perspective effects have been found in
some priming experiments but not in others. The results of
these experiments on perspective effects therefore suggest
care in distinguishing two different kinds of changes in per-
spective: those that do not change the set of visible parts, and
those that do.
Orientation Effects
Other effects due to differences in object orientation cannot
be explained in this way, however, because the same parts are
visible in all cases. Orientation effects refer to perceptual dif-
ferences caused by rotating an object about the observer’s
line of sight rather than rotating it in depth. Depth rotations of
the object often change the visibility of different parts of the
object, as just discussed, but orientation changes never do,
and Jolicoeur (1985) has shown that subjects are faster at cat-
egorizing pictures of objects in a normal, upright orientation
than when they are misoriented in the picture plane. Naming
latencies increase with angular deviation from their upright
orientation, as though subjects were mentally rotating the ob-
jects to upright before making their response.
Interestingly, orientation effects diminish considerably
with extended practice. Tarr and Pinker (1989) studied this
effect using novel objects so that the particular orientations at
which subjects saw the objects could be precisely controlled.
When subjects received extensive practice with the objects at
severalorientations, rather than just one, naming latencies
were fast at allthe learned orientations. Moreover, response
times at novel orientations increased with distance from the
nearest familiar orientation. Tarr and Pinker therefore sug-
gested that people may actually store multiple representa-
tions of the same object at different orientations rather than a
single representation that is orientation invariant. This possi-
bility becomes particularly important in the section entitiled
“Theories of Object Identification,” in which view-specific
theories of categorization are considered.Part Structural EffectsThe first half of this chapter developed the idea that per-
ceptual organization is centrally related to the idea that the
perceived world is structured into part-whole hierarchies.
Human bodies have heads, arms, legs, and a torso; tables
have a flat top surface, and legs; an airplane has a fuselage,
two main wings, and several smaller tail fins. The important
question is whether these parts play a significant mediating
role in object identification. The most revealing studies of
this question were performed by Biederman and Cooper
(1991) using a version of the priming paradigm discussed in
this chapter’s section entitled “Perspective Effects.” They
showed that identification of degraded line drawings in
the second (test) block of trials was facilitated when subjects
had seen the same partsof the same objects in the initial
(priming) block, but not when they had seen different parts
of the same object in the priming block. This result implies
that the process of identifying objects is mediated by perceiv-
ing their parts and spatial interrelations—because otherwise,
it is not clear why more priming occurs only when the same
parts were seen again.
The drawings Biederman and Cooper (1991) used were
degraded by deleting half of the contours in each stimulus. In
the first experiment, subjects were shown a priming series of
contour-deleted drawings and then a test series in which they
saw either the identical drawing (Figure 7.20; A), its line