Searching a Cluttered Scene

M.J. Bravo and H. Farid

Vision Sciences (VSS), Sarasota, FL, 2003

Purpose: In one popular scenario of vision, bottom-up grouping processes organize a scene into objects and then attention selects one of these objects for recognition. A problem with this scenario is that many ordinary objects are composed of multiple distinct parts (e.g., a lamp with a paper shade and a ceramic base), and when these objects are presented in clutter, it may not be possible to group whole objects using only bottom-up processes. To test whether attention selects whole objects or just object parts, we asked observers to search for a category target (food) in cluttered displays composed of single-part and multi-part objects.

Methods: Each display contained twelve color photographs of ordinary objects. The observer's task was to determine whether these objects included a food item. In half of the displays, the distractors were selected from 66 single-part objects; in the other half, the distractors were selected from 66 multi-part objects. While both types of displays were composed of the same number of similarly sized objects, the multi-part displays had many more parts. We also used two types of object arrangements. In the sparse arrangement, the objects were uniformly positioned and well-separated from one another. In the clutter arrangement, the objects were randomly positioned and overlapped one another.

Results: With the sparse arrangement, there was little difference in the search times for displays composed of single-part objects and those composed of multi-part objects. With the clutter arrangement, however, search times for multi-part displays were much slower than those for single-part displays.

Conclusion: These data suggest that with sparse arrangements (the norm in vision research), it is reasonable to suppose that the visual system can select and reject whole objects when searching for a target. With cluttered arrangements (the norm in everyday vision), object parts are likely the initial units of selective attention.