The Depth of Distractor Processing in Search Through Clutter

M.J. Bravo and H. Farid

Vision Sciences (VSS), Sarasota, FL, 2005

Background clutter can make it difficult to segment whole objects. This is especially true for compound objects, which have parts made from different materials (e.g., a table lamp). We reported earlier that when observers search for a category target in dense clutter, search is slower when the distractors are compound objects rather than simple objects. This result is consistent with two interpretations. In the first, observers reject parts, and this process is slow for compound distractors because they have multiple parts. In the second, observers reject whole objects, and this process is slow for compound distractors because they are difficult to segment. In the present search experiment, we used familiar and chimerical distractors to distinguish between these alternatives. Familiar distractors were drawn from a set of 100 color photographs of everyday objects. Each of these objects had at least two clearly delineated parts. Chimerical distractors were created by exchanging parts between objects. Observers searched for a target defined by its membership in a broad category (e.g., animal) or categories (e.g., animal or vehicle). We found that when target uncertainty was high and target recognition was difficult (e.g., the target was partially occluded, randomly rotated or drawn from two categories), search times were significantly slower for chimerical distractors than for normal distractors. This difference suggests that for some search tasks, observers identify and reject whole objects. This difference was greatly reduced, however, when the target was unoccluded, upright and drawn from a single category. For this simpler search task, observers may reject object parts. In sum, the demands of the search task determine the depth of distractor processing required, and this determines whether observers recognize distractor objects.