Attentional modulation of outlier processing

Ensemble perception enables the visual system to function effectively when the number of stimuli in the environment exceeds its capacity. Ensemble representations not only help the limited capacity of visual representations, but they also facilitate the detection and representation of items deviating from the group (i.e., the outlier). This study focuses on how attentional mechanisms modulate outlier processing. In three experiments, we presented participants with an ensemble that was formed by circle stimuli in varying sizes, and the outlier item was distinct in terms of its location. We measured outlier localization performance while manipulating attentional orienting via a spatial cueing paradigm. In Experiment 1, a valid, invalid, or neutral cue was presented before or after the display. Facilitation of outlier localization was most pronounced in the valid precue condition. Experiment 2 included a task to actively engage ensemble perception in addition to outlier localization, and cue validity effect was observed as in Experiment 1. Experiment 3A was carried to directly compare the top-down and bottom-up influences on outlier processing by presenting two spatial outliers—one target and another distractor outlier. The target outlier identity was previously determined and was identical across trials. In Experiment 3B, the target was in red, making it salient among the remaining items. In the invalid trials, where the distractor outlier was cued, responses were closer to the distractor item indicating that outlier processing is cue driven. These experiments overall demonstrate that automaticity of outlier processing can be overridden by cue-driven processes.