Page 251 - Foundations of Cognitive Psychology : Core Readings

Principles of Categorization 257

agreed upon listings of the same movements of the same body part in the same
part of the movement sequence formed the unit of analysis. Results were iden-
tical to those of the attribute listings; basic objects were the most general classes
to have motor sequences in common. For example, there are few motor pro-
grams we carry out to items of furniture in general and several specific motor
programs carried out in regard to sitting down on chairs, but we sit on kitchen
and living-room chairs using essentially the same motor programs.

Similarity in Shapes
Anotheraspectof the meaning ofa classofobjects is theappearance of the
objects in the class. In order to be able to analyze correlational structures by
different but converging methods, it was necessary to find a method of ana-
lyzing similarity in the visual aspects of the objects that was not dependent on
subjects’ descriptions, that was free from effects of the object’s name (which
would not have been the case for subjects’ ratings of similarity), and that went
beyond similarity of analyzable, listable attributes that had already been used
in the first study described. For this purpose, outlines of the shape of two-
dimensional representations of objects were used, an integral aspect of natu-
ral forms. Similarity in shape was measured by the amount of overlap of the
two outlines when the outlines (normalized for size and orientation) were
juxtaposed.
Results showed that the ratio of overlapped to nonoverlapped area when two
objects from the same basic-level category (e.g., two cars) were superimposed
was far greater than when two objects from the same superordinate category
were superimposed (e.g., a car and a motorcycle). Although some gain in ratio
of overlap to nonoverlap also occurred for subordinate category objects (e.g.,
two sports cars), the gain obtained by shifting from basic-level to subordinate
objects was significantly less than the gain obtained by shifting from super-
ordinate to basic-level objects.

Identifiability of Averaged Shapes
If the basic level is the most inclusive level at which shapes of objects of a class
are similar, a possible result of such similarity may be that the basic level is also
the most inclusive level at which an averaged shape of an object can be recog-
nized. To test this hypothesis, the same normalized superimposed shapes used
in the previous experiment were used to draw an averaged outline of the
overlapped figures. Subjects were then asked to identify both the superordinate
category and the specific object depicted. Results showed that basic objects
were the most general and inclusive categories at which the objects depicted
could be identified. Furthermore, overlaps of subordinate objects were no more
identifiable than objects at the basic level.
In summary, our four converging operational definitions of basic objects all
indicated the same level of abstraction to be basic in our taxonomies. Admit-
tedly, the basic level for biological objects was not that predicted by the folk
genus; however, this fact appeared to be simply accounted for by our subjects’
lack of knowledge of the additional depth of real-world attribute structure
available at the level of the folk generic (see Rosch et al. 1976a).