Perceptual similarity in visual search for multiple targets.

Visual search for multiple targets can cause errors called subsequent search misses (SSM) - a decrease in accuracy at detecting a second target after a first target has been found. One of the possible explanations of SSM errors is perceptual set. After the first target has been found, the subjects become biased to find perceptually similar targets, therefore they are more likely to find perceptually similar targets and less likely to find the targets that are perceptually dissimilar. This study investigated the role of perceptual similarity in SSM errors. The search array in each trial consisted of 20 stimuli (ellipses and crosses, black and white, small and big, oriented horizontally and vertically), which could contain one, two or no targets. In case of two targets, the targets could have two, three or four shared features (in the last case the targets were identical). The error rate decreased with increasing the similarity between the targets. These results state the role of perceptual similarity and have implications for the perceptual set theory.