Parts of visual form: psychophysical aspects.

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.