Unpredictable singleton distractors in visual search can be subject to second-order suppression

Drisdelle, Brandi Lee; Zivony, Alon; Eimer, Martin · 2025 · Crossref

DOI: 10.3758/s13414-025-03028-3

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Summary

This study investigates whether humans can suppress attentional capture by salient, irrelevant distractors when their features are unpredictable, a phenomenon known as second-order suppression. Previous research suggested that suppression only occurs when distractor features are constant and predictable (first-order suppression). The authors hypothesized that prior failures to observe second-order suppression may have resulted from low feature variability, where the probability of a distractor color matching the target color on subsequent trials was too high, making suppression inefficient. To test this, the researchers developed a "multiframe letter-probe paradigm," where participants counted target shapes across a sequence of displays and reported probe letters superimposed on items in the final display. This design allowed for the assessment of attention allocation to singleton distractors versus non-singleton distractors and targets, including trials where the target was absent. The study comprised three experiments manipulating color variability. Experiment 1 validated the paradigm using constant target and singleton colors, replicating established first-order suppression effects where probe letters at singleton locations were reported less frequently than those at non-singleton locations. Experiment 2 employed two randomly swapped colors for targets and singletons, replicating previous findings of no suppression effect under these conditions. Critically, Experiment 3 introduced eight different random colors for targets and singletons. Under these high-variability conditions, a robust suppression effect reappeared, with significantly fewer probe reports at singleton distractor locations compared to non-singleton locations. This pattern held true regardless of whether a target was present in the final display, indicating that suppression is not driven by direct competition between the target and the singleton. The findings demonstrate that second-order suppression is possible for unpredictable singleton distractors, provided that the feature space is sufficiently large to make specific color-based suppression inefficient. The reappearance of suppression in Experiment 3, but not in Experiment 2, suggests that the lack of second-order suppression in prior studies was likely due to the limited number of colors used, which may have encouraged participants to treat all colors as potential targets. Furthermore, the consistency of suppression effects across target-present and target-absent trials implies that the mechanism operates independently of immediate target-singleton competition. These results challenge the view that suppression is strictly limited to first-order, feature-specific processes and highlight the adaptability of suppressive mechanisms in visual search. The study concludes that the visual system can flexibly apply suppression based on feature discontinuities rather than specific feature values, contingent on the statistical regularities and variability of the visual environment.

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• visual search