Priors, preferences and categorical percepts
Visual perception is the process of inferring world structure from image structure. If the world structure we recover from our images \makes sense" as a plausible world event, then we have a \percept" and can often ooer a concise linguistic description of what we see. For example, in the upper panel of Figure 3.1, if asked, \What do you see?", a typical response might be a pillbox with a handle either erect (left) or at (right). This concise and conndent response suggests that we have identiied a model type that ts the image observation with no residual ambiguities at the level of the description. In contrast, when asked to describe the two lower drawings in Figure 3.1, there is some hesitancy and uncertainty. Is the handle erect or not? Does it have a skewed or rectangular shape? The depiction leaves us somewhat uncertain, as if several options were possible, but none where all aspects of the interpretation collectively support each other. What then, leads us to the certainty in the upper set and to the ambiguity in the lower pair? To be a bit more precise about our goal, let us assume that some Waltz-like algorithm has already identiied the base of the pillbox and the wire-frame handle as separate 3D parts. Even with this decomposition, there remains an innnity of possible interpretations for any of these drawings. Yet we conndently commit to one interpretation in the case of the upper panel, but otherwise for the lower pair. Our aim, then, is to understand why the image structures in the upper panel support the assertion that they must arise only from very particular world structures, whereas the lower two structures seem more ambiguous. Our analysis will consist of three parts: rst we will lay out the domain associated with pillboxes having handles. Then the role of preferences for 80