Article,
Parts of visual form: psychophysical aspects.
Perception, 25 (4): 399--424 (1996)
Abstract
Part-based representations allow for recognition that is robust in
the presence of occlusion, movement, growth, and deletion of portions
of an object, and play an important role in theories of object categorization
and classification. A partitioning theory for visual form is proposed
that is based on two types of parts: limb-based parts arise from
a pair of negative curvature minima with evidence for "good continuation'
of boundaries on one side; neck-based parts arise from narrowings
in shape. The motivation for this model is computational requirements
for recognition. The psychophysical relevance of this model is addressed
by measuring intrasubject and intersubject consistency in partitioning
tasks and comparing perceived and computed parts. A series of experiments
were performed in which subjects were required to partition a variety
of biological and nonsense two-dimensional shapes into perceived
components. Specifically, it was examined (1) whether a subject determines
components consistently across different trials of the same partitioning
task, (2) whether there is evidence for consistency between subjects
for the same partitioning task, and (3) how the perceived parts compare
with limbs and necks resulting from the computational model. The
results are interpreted as suggesting that there are high levels
of both intrasubject and intersubject consistency and that a large
majority of the perceived parts do in fact correspond to the parts
computed on the basis of our model. The implications of our model
are discussed in relation to previous experimental results. Intuitive
observations concerning the relationship between parts of visual
form and their function are then presented. Finally, a role is envisioned
for parts in figure/ground segregation; the notion of a "parts receptive
field' through which parts can serve as an intermediate representation
between local image features, eg edges, and global object models,
is suggested.
