How does our search engine "see" the world? The case of amodal completion

Abstract

This article illustrates a dissociation between the perceived attributes of an object and the ability of those attributes to guide the deployment of attention in visual search. Orientation is an attribute that guides search. Thus, a vertical line will "pop out" amid horizontal distractors. Amodal completion can create perceptually convincing oriented stimuli when two elements appear to form a complete object partially hidden behind an occluder. Previous work (e.g., Rensink & Enns, Vision Research, 38, 2489-2505, 1998) has shown a preattentive role for amodal completion in search tasks. Here, we show that orientation based on perceptually compelling amodal completion may fail to guide attention. The broader conclusion is that introspection is a poor guide to the capabilities of our internal search engine.

Fig. 1

A black vertical bar (on the left) and a black horizontal bar (on the right), each occluded by a white oblique bar

Fig. 2

Search for the vertical black bar

Fig. 3

After Rensink and Enns (1995). It is much easier to find the long black line segment in the easy search than in the hard search. In the hard search, the presence of a visible occluder causes the segments to complete into lines that are all of the same length

Fig. 4

Search for a vertical black bar occluded by a white bar tilted to the left

Fig. 5

Stimuli and results for Experiment 1. Error bars show ±1 SEM (sometimes hidden by the data point). Search for the occluded target is clearly inefficient

Fig. 6

Stimuli and results for Experiment 2. Error bars are ±1 SEM. Again, search for the occluded target is inefficient

Fig. 7

Stimuli and results for Experiment 3. Error bars are ±1 SEM. Use of large occluders does not produce efficient search for occluded targets

Fig. 8

Occluder conditions in Experiment 4, with slopes for target-present and target-absent trials. Good amodal completion might (square) or might not (diamond) produce efficient search

Fig. 9

Unoccluded conditions in Experiment 4, with slopes for target-present and target-absent trials. Stimuli with just the outline of the occluded figure produce efficient search in the absence of amodal completion

Fig. 10

Gap conditions in Experiment 4

Fig. 11

Stimuli and results for Experiment 5. Error bars are ±1 SEM. Again, search for the occluded target is inefficient