Top-Down and Bottom-Up Processes for Object Segmentation

M.J. Bravo and H. Farid

Vision Sciences (VSS), Sarasota, FL, 2001

Purpose: Last year, we reported a study of the effects of object knowledge on object segmentation. Here we examine the relationship between this top-down segmentation process and a bottom-up segmentation process based on luminance cues.

Methods: Twelve block-objects were generated by randomly assigning a color (R,G,B,Y) to 7 blocks and then randomly but neatly stacking the blocks next to or on top of one another. Half of these objects were studied by one set of subjects, the other half were studied by a second set. The block-objects were then neatly stacked next to one another to create blocks-scenes. These scenes contained no visual cues marking the boundaries between objects. Also in each scene, stacked between two objects, was a 4-, 5- or 6-block target. The target luminance varied across trials; sometimes it matched the surrounding blocks, at other times it was noticeably darker. The subject's task was to count the number of blocks in the target under three conditions: (1) Top-down: scenes composed of studied objects, target not defined by luminance cue. (2) Bottom-up: non-studied objects, target defined by luminance cue. (3) Both: studied objects and luminance cue.

Results: (1) With only top-down information, subjects were able to accurately (>90%) segment the target blocks from the other 28 blocks in the scene. This was a slow process requiring 10-15 sec. (2) With a target defined by a strong luminance cue, subjects were also accurate but they responded 2-3 times faster. As the luminance cue was reduced, accuracy fell while response times increased to the same level as the top-down condition. (3) When both top-down and weak bottom-up cues were available, some subjects were able to combine the two strategies: accuracy was similar to the top-down only case, but response times were faster.

Conclusion: Object knowledge can be used for object segmentation. Although this top-down process is slow, it can be combined effectively with a faster bottom-up process.