Object Identification 195visible. If this is not the case, then category-mediated object
identification appears to be the only route for perception of
function.
Typicality and Basic-Level Categories
The first fact that must be considered about identifying
objects is that it is an act of classification or categorization.
Although most people typically think of objects as belonging
to just one category—something is either a dog, a house, a
tree, or a book—all objects are actually members of many
categories. Lassie is a dog, but she is also a collie, a mammal,
an animal, a living thing, a pet, a TV star, and so on. The cat-
egories of human perception and cognition are quite complex
and interesting psychological structures (see chapter by
Goldstone and Kersten in this volume).
One of the most important modern discoveries about
human categorization is the fact that our mental categories
do not seem to be defined by sets of necessary and sufficient
conditions,but rather to be structured around so-called best
examples, called prototypes(Rosch, 1973, 1975a, 1975b).
The prototypical bird, for example, would be the “birdiest”
possible bird: probably a standard bird that is average in size,
has a standard neutral sort of coloring, and has the usual
shape of a bird. When Rosch asked participants to rate vari-
ous members of a category, like particular kinds of birds, in
terms of how “good” or “typical” they were as examples
of birds, she found that they systematically rated robins quite
high and penguins and ostriches quite low. These typicality
(orgoodness-of-example) ratings turn out to be good predic-
tors of how quickly subjects can respond “true” or “false” to
verbal statements such as, “A robin is a bird,” versus, “A
penguin is a bird” (Rosch, 1975b). Later studies showed
that it also takes longer to verify that a picture of a penguin
depicts an example of a bird than to verify that a picture of a
robin does (Ober-Thomkins, 1982). Thus, the time required
to identify an object as a member of a category depends
on how typical it is perceived to be as an example of that
category.
Rosch’s other major discovery about the structure of
human categories concerned differences among levels within
the hierarchy. For example, at which level does visual identi-
fication first occur: at some low, specific level (e.g., collie), at
some high, general level (e.g., animal), or at some intermedi-
ate level (e.g., dog)? The answer is that people generally rec-
ognize objects first at an intermediate level in the categorical
hierarchy. Rosch called categories at this level of abstraction
basic level categories(Rosch, Mervis, Gray, Johnson, &
Boyes-Braem, 1976). Later research, however, has shown the
matter to be somewhat more complex.
Jolicoeur, Gluck, and Kosslyn (1984) studied this issue by
having subjects name a wide variety of pictures with the first
verbal label that came to mind. They found that objects that
weretypicalinstances of categories, such as robins or spar-
rows, were indeed identified as members of a basic level cate-
gory, such as birds.Atypicalones, such as penguins and
ostriches, tended to be classified at a lower, subordinate level.
This pattern of naming was not universal for all atypical cate-
gory members, however. It occurs mainly for members of
basic level categories that are relatively diverse. Consider
some basic level categories from the superordinate categories
of fruit (e.g., apples, bananas, and grapes) versus animals (e.g.,
dogs, birds, and monkeys). Most people would agree that the
shape variation within the categories of apples, for instance, is
more constrained than that within the categories of dogs. In-
deed, most people would be hard-pressed to distinguish be-
tween two different kinds of apples, bananas, or grapes from
shape alone, but consider how different dachshunds are from
greyhounds, penguins are from ostriches, and goldfish are
from sharks. Not surprisingly, then, the atypical exemplars
from diverse basic-level categories are the ones that tend to be
named according to their subordinate category. Because the
categories into which objects are initially classified is some-
times different from the basic level, Jolicoeur, Gluck, and
Kosslyn (1984) called thementry-level categories.
It is worth noting that, as in the case of basic-level cate-
gories, the entry-level category of an object can vary over dif-
ferent observers, and perhaps over different contexts as well.
To an ornithologist or even to an avid bird watcher, for in-
stance,birdmay be the entry-level category for very few,
if any, species of bird. Through a lifetime of experience at
discriminating different kinds of birds, their perceptual sys-
tems may become so finely tuned to the distinctive character-
istics of different kinds of birds that they first perceive robins
as robins and sparrows as sparrows rather than just as birds
(Tanaka & Taylor, 1991).Perspective EffectsOne of the seemingly obvious facts about identifying three-
dimensional objects is that people can do it from almost any
viewpoint. The living-room chair, for example, seems to be
easily perceived as such regardless of whether one is looking
at it from the front, side, back, top, or any combination of
these views. Thus, one of the important phenomena that must
be explained by any theory of object classification is how this
is possible.
But given the fact that object categorization is possible
from various perspective views, it is all too easy to jump to
the conclusion that object categorization isinvariantover