Binary categorization of ordered stimuli implicitly guides attention during visual search

Visual search is a fundamental task in the attention research area. One critical factor that affects visual search performance is category information, which shows better search efficiency in the between-category condition than in the within-category condition. However, previous studies have focused primarily on explicit category information (e.g., alphanumeric categories), and whether implicit categorization of sequential information that is not directly observable can also guide attention during visual searches is unclear. To address this question, via five visual search experiments, participants were presented with arrays that consist of either numbers or letters and were instructed to identify a predetermined target item within these arrays. Notably, the results of five experiments consistently showed the implicit category effect with a longer RT in the within-category condition than that in the between-category condition. This advantageous effect was stably observed by controlling the factors of numerical/symbolic distance between the target and distractors, stimulus frequency, and target presence. Further temporal dynamics analysis and cross-experiment analysis consistently showed this implicit category effect gradually increased as the reaction time prolonged, which indicates that this effect is mainly induced by late knowledge-guided category information processing. Our findings underscore the tendency of observers to implicitly exploit their understanding of sequential information to modulate their attentional allocation to facilitate target identification.