Shape-similarity gain: A selection profile for simple objects.

Recent research has examined the selection profile-the pattern of enhancement and suppression of stimuli when allocating attention to a specific feature value. Support for several different selection profiles for simple feature dimensions (e.g., color, orientation) have been identified such as the feature-similarity gain and surround suppression effects. However, it is unclear whether the selection profile for more complex stimuli follows a similar pattern. Using a recently developed circular shape space, the current study assessed the selection profile for these simple shapes as an initial step toward the study of object-based attention. Participants attended to two target shapes while ignoring two distractors, each overlapping with one target. They performed a change identification task, reporting luminance changes of only the target shapes. We systematically varied the difference between the two targets in the shape space. In two experiments covering different ranges in the shape space, performance accuracy peaked when the two target shapes matched and then decreased in a monotonic manner as the two shapes became more different. These results align with a feature-similarity gain model and suggest that an analogous shape-similarity gain effect operates for more complex stimuli. This gain effect suggests that shape-based selection occurs in an intrinsic similarity space which can be realized by top-down attention operating on neural substrates that encode object similarity.