Target-flanker similarity effects reflect image segmentation not perceptual grouping.

When responding to the identity of a visual target, nearby stimuli (flankers) that are associated with the same response as the target cause faster and more accurate responding than flankers that are associated with different responses. Because this flanker-congruence effect (FCE) decreases with increasing target-flanker separation, it was thought to reflect limited precision of spatial selection mechanisms. Later studies, however, showed that FCEs are larger when the target and flankers are the same color compared to when they are different colors. This led to the group selection hypothesis, which states that flankers are perceptually grouped with the target and are obligatorily selected along with it, regardless of spatial separation. An alternative hypothesis, the image segmentation hypothesis, states that feature differences facilitate the segmentation of visual information into relevant and irrelevant parts, thereby mitigating the limitations of spatial precision of selection mechanisms. We test between these hypotheses using a design in which targets and flankers are grouped or not grouped, while holding feature differences in the stimulus constant. Contrary to earlier results, we found that same-colored flankers do not yield larger FCEs than different-colored flankers when feature differences are held constant. We conclude that similarity effects on the FCE reflect differential support for image segmentation, on which selection depends, rather than the obligatory selection of perceptually grouped flankers and targets.