Occluded information is restored at preview but not during visual search

Abstract

Objects often appear with some amount of occlusion. We fill in missing information using local shape features even before attending to those objects-a process called amodal completion. Here we explore the possibility that knowledge about common realistic objects can be used to "restore" missing information even in cases where amodal completion is not expected. We systematically varied whether visual search targets were occluded or not, both at preview and in search displays. Button-press responses were longest when the preview was unoccluded and the target was occluded in the search display. This pattern is consistent with a target-verification process that uses the features visible at preview but does not restore missing information in the search display. However, visual search guidance was weakest whenever the target was occluded in the search display, regardless of whether it was occluded at preview. This pattern suggests that information missing during the preview was restored and used to guide search, thereby resulting in a feature mismatch and poor guidance. If this process were preattentive, as with amodal completion, we should have found roughly equivalent search guidance across all conditions because the target would always be unoccluded or restored, resulting in no mismatch. We conclude that realistic objects are restored behind occluders during search target preview, even in situations not prone to amodal completion, and this restoration does not occur preattentively during search.

Figure 1

Restoring missing information from complex objects is likely to involve more processes than amodal completion: Classic examples of cases where amodal completion processes (A) are engaged and (B) are not. In everyday contexts (C), some objects will engage traditional amodal-completion processes, but many will not. We can still, however, behave as though we know what missing information is present for the majority of these objects. These filling-in processes technically meet the definition of amodal completion (completing missing information without a resulting percept) but are qualitatively different from the processes in (A), since it is not linking contours across two sides of an occluder. (D) An extreme case, in which half of an object is occluded and the visible contours cannot relate behind the occluder in a way that would allow the contours to merge (Tse, 1999b). As in (B), low-level completion processes (where contours are matched to linked parts across an occluder) are not expected here. However, through knowledge of the object category, we know what the missing information is likely to be. Experiment 1 in the current study uses stimulus arrangements resembling (D).

Figure 2

The procedure from Experiment 1. The target was always present and always shared the same category (e.g., fish) as the distractors.

Figure 3

Measures that include verification processes suggest that the target was not completed in the search display, as indicated by poor performance in U-O. Overall reaction times, from search display onset to subject response, were longest in the U-O condition in Experiment 1 (A) and longer in U-O relative to U-U in Experiment 2 (B). Verification time, from the first fixation on the target until the subject's manual response, was similarly longest in U-O, both for Experiment 1 (C) and for Experiment 2 (D). Error bars indicate one standard error of the mean. See Figure 4A and 4B for the time-to-target component of reaction time.

Figure 4

Guidance was relatively poor when the target was occluded in the search display (U-O and O-O), suggesting that restoration is not preattentive. In Experiment 1, this was true both for time to target (A) and for initial saccade direction (C). In Experiment 2, no effect was found for time to target (B), but a generally similar pattern to Experiment 1 was found for initial saccade direction (D). Chance is 25% for saccade direction, and guidance was better than chance in all conditions. Error bars indicate one standard error of the mean.

Figure 5

Patterns in initial saccade latencies were not consistent with a speed/accuracy trade-off. Unoccluded displays (where guidance was strongest—see Figure 4) had the shortest initial saccade latencies, in Experiment 1 (A) and Experiment 2 (B). Error bars indicate one standard error of the mean.