Search for a category target in clutter

An airport security worker searching a suitcase for a weapon is engaging in an especially difficult search task: the target is not well-specified, it is not salient, and it is not predicted by its context. Under these conditions, search may proceed item-by-item. In the experiment reported here we tested whether the items for this form of search are whole familiar objects. Our displays were composed of color photographs of ordinary objects, that were either uniform in color and texture (simple), or had two or more parts with different colors or textures (compound). The observer's task was to detect the presence of a target belonging to a broad category (food). We found that when the objects were presented in a sparse array, search times to find the target were similar for displays composed of simple and compound objects. But when the same objects were presented as dense clutter, search functions were steeper for displays composed of compound objects. We attribute this difference to the difficulty of segmenting compound objects in clutter: compared with simple objects, compound objects are less likely to be organized into a single object by bottom-up grouping processes. Our results indicate that while search rates in a sparse display may be determined by the number of objects, search rates in clutter are also affected by the number of object parts.