Sustained attention to four locations disrupts concurrent subitizing

SHIMOMURA, Tomonari; KUMADA, Takatsune · 2012 · Crossref

DOI: 10.4992/pacjpa.76.0_2pma63

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Summary

This study investigates the functional overlap between visual attention and subitizing, the rapid and accurate enumeration of small numbers of objects (typically up to four). While previous research indicates that humans can sustain attention on approximately four locations simultaneously, it remained unclear whether this capacity limit shares a common mechanism with the subitizing limit. To address this, the authors conducted two experiments to determine if the cognitive load of sustained visual attention disrupts concurrent subitizing performance. In Experiment 1, twelve participants with self-reported normal vision performed a dual-task paradigm combining a cueing task and a counting task. Participants viewed a fixation point, followed by placeholders, then a variable number of cue stimuli (0 to 6) indicating specific locations, and finally a target display. The task required participants to count target lines appearing at locations *not* cued by the preceding stimuli. The number of cues served as a manipulation of top-down attentional load, while the number of targets (1 to 8) measured subitizing capacity. Reaction times were recorded as the primary dependent variable. The results of Experiment 1 revealed significant main effects for both the number of cues and the set size of targets, along with a significant interaction. When no cues were presented, reaction times remained constant for up to four targets, demonstrating intact subitizing. However, as the number of cues increased, the range of efficient counting narrowed. Crucially, when participants had to sustain attention on four cue locations, the subitizing advantage disappeared entirely, and reaction times increased linearly with set size. This indicated that top-down control of visual attention interferes with subitizing. Experiment 2 aimed to distinguish top-down attentional control from bottom-up visual tagging. Seven participants performed a modified task where cues were replaced by horizontal or vertical lines that persisted into the target display, inducing bottom-up attention allocation rather than top-down selection. In this condition, the main effect of cue number was non-significant. Regardless of the number of lines present, participants maintained efficient subitizing for up to four targets, showing no disruption in reaction times. The study concludes that subitizing is disrupted only when top-down visual attention is required, not by bottom-up visual tagging. This suggests that subitizing and sustained visual attention share a common limited-capacity mechanism. The findings imply that the capacity limit of four items is a fundamental constraint of the visual attention system, affecting both the ability to maintain focus on multiple locations and the ability to rapidly enumerate small sets.

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