Cortical representation of medial axis structure.

The identity of an object is not only specified by its parts but also by the relations among the parts. Rearranging parts can produce a completely different object, in the same manner as rearranging the phonemes in "fur" can yield "rough." How does the visual system represent the relative positions of parts? Between-part relations can be characterized by specifying the relations between the medial axes (imaginary lines through the centers) of an object's parts. A functional magnetic resonance imaging multivoxel classification study tested whether the medial axis structure is represented in the human visual system independent of part identity and overall object orientation. Stimuli were line drawings of novel 3-part geometrical objects, which differed in the relations between their parts' medial axes (i.e., in their medial axis structures), the geons that composed each object, and the objects' orientations in plane and in depth. In regions of interest throughout visual cortex, a support vector machine classifier was trained to distinguish objects that shared either the same medial axis structures or the same orientations. By the level of V3, different medial axis structures were more accurately classified than different orientations, indicating a change in the representation of shape compared with earlier visual areas.