Searching for impossible objects: Processing form and attributes in early vision

In five experiments, we investigated the extent to which form (shape) and m etric attributes (three-dimensional, 3-D, orientation), both defined by rel ations between line elements, are processed in early vision. Search for a t arget defined by an abstract property of form (i.e., impossibility) was slo w and serial. In contrast, search for a 3-D orientation target was consider ably easier. Subsequent experiments suggest that this difference reflects t he fact that 3D orientation is derivable from localized sets of lines, wher eas impossibility is an idiosyncratic property of the complete set of relat ions between lines. We conclude that only "gross" aspects of form are avail able in early vision as the complete set of line relations is not processed preattentively. However, localized processing of line relations is suffici ent to derive 3-D orientation.