The representation of medial axes in the perception of shape

UnrestrictedAristotle famously said that vision is “to know what is where, by looking”—but that is not the whole story. Vision is also to know what is where relative to everything else. We constantly make use of relative position information, when we draw, build, or read a map or diagram. How does our visual system divide up space, to let us know that one object is above another, or that one part of an object protrudes from the end of another part? Structural description theories of shape recognition hold that our visual system represents objects as collections of parts in particular relations.; The principal focus of this dissertation is to investigate one plausible scheme for the encoding of within-object (between-part) spatial relations: the encoding of medial-axis-relative relations. Medial axes are imaginary lines that pass through the central part of a volume, as a spit through a hot dog. Medial axes within each part of an object can define an invariant, stick-figure like structure of an object, in the sense that the points of attachment and relative angles between the axes (in rigid objects) will not change with the perspective from which the object is seen. A behavioral similarity rating experiment showed that naïve subjects spontaneously judge novel objects to be more similar if the objects share the same medial axis structure, in many cases even if the objects are composed of different-shaped parts and shown from different perspectives. When subjects were asked to distinguish (in a same/different task) categorically-different medial axis structures seen from different perspectives, they showed slower recognition times with greater differences in perspective (though the estimated rates of mental rotation were very fast).; Finally, evidence from a multi-voxel pattern classification fMRI study showed that medial axis structure—as distinct from simple retinotopic orientation—is encoded starting at a very early stage (V3) in the visual cortex. Patterns of activity in V3 were more similar in response to novel objects that shared the same medial axis structure than in response to novel objects that shared the same overall orientation