Differences between searching among objects and searching among holes.
Hulleman J., Humphreys GW.
We report results from six experiments in which participants had to search for a "C" among "O" distractors. The search items were either holes or objects, defined by motion, contrast, or both. Our main findings were (1) it was easier to search among objects than to search among holes, and (2) the difference between search among objects and search among holes was primarily caused by grouping with the background. The data support the hypothesis that the shape of a hole is only available indirectly. We further note that, in our experiments, search performance for both holes and objects depended on the surface medium used to define the search items.