The impact of task-irrelevant heterogeneity on visual search efficiency and attentional capture

In our capacity-limited perceptual and cognitive systems, multiple stimuli compete for representation. This competition intensifies with increased stimulus heterogeneity, impairing the processing efficiency of search for a specific target among multiple items. While previous studies primarily examined the effect of heterogeneity in task-relevant dimensions on visual search efficiency, the effect of task-irrelevant heterogeneity remains unclear. Here, we investigated how variability in task-irrelevant features, spatial frequency or color, affects search efficiency and attentional capture. Participants searched for a 45-degree tilted grating among vertical and/or horizontal non-targets, with orientation as the task-relevant feature. The non-targets varied in task-irrelevant dimensions, and the degree of heterogeneity in these dimensions was manipulated. Results showed that task-irrelevant heterogeneity significantly impaired search efficiency and weakened attentional capture, but these effects were contingent on the organization of the task-relevant dimension. When the non-targets shared the same orientation, both spatial frequency and color heterogeneity affected search efficiency and attentional capture. However, when orientation varied among non-targets, only spatial frequency heterogeneity continued to exert disruptive effects. These results indicate that the disruptive effects of task-irrelevant heterogeneity depend on both the type of feature and the structure of the display. Spatial frequency exerts a stronger and more consistent impact than color, highlighting the importance of feature-specific processing in guiding visual attention.