Why do extra items on screen reduce distraction?
Adding neutral items to a display only weakened a distractor's interference when those items were within the spread of attention, were real letters like the target, and could not be filtered out by knowing the target's colour.
Source
Perceptual load vs. dilution: the roles of attentional focus, stimulus category, and target predictability
Study at a glance
- Design
- Human experiment — Within-subject letter-search tasks with a central to-be-ignored distractor; cue set size (2 vs 6 locations), display set size (2 vs 6 items) and target-distractor congruency manipulated; Exp 4 compared predictable vs unpredictable target colour between groups
- N
- Exp 1: 19 undergraduates (1 excluded); Exp 2: 20; Exp 3: 16; Exp 4: 20 (1 excluded), each a new sample
- Population
- Undergraduate students at the University of Canterbury, New Zealand
- Outcome
- Congruency effect (reaction time and error difference between incongruent and congruent distractor trials) as an index of distractor processing
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
Students searched for a target letter (H or S) at cued locations around a circle while ignoring a central distractor letter that either matched or conflicted with the target. Across four experiments the authors varied how many locations were cued (2 or 6, a manipulation of attentional focus), how many letters appeared (2 or 6), how new letters appeared (brightening vs dimming), whether the extra items were upright or inverted letters, and whether the target's colour was known in advance. Distraction was measured by the congruency effect: how much slower people were when the distractor conflicted with the target.
What they found
With brightening onsets (Exp 1), more letters reduced interference regardless of cue size, but with dimming onsets (Exp 2) this 'dilution' occurred only when six locations were cued; with two cued locations the effect even reversed. Replacing extra upright letters with inverted letters (Exp 3) removed dilution, suggesting it arises at the level of letter identity rather than simple features. Knowing the target's colour in advance (Exp 4) eliminated dilution, whereas not knowing it preserved a larger congruency effect in 2-letter than 6-letter displays. Contrary to perceptual load theory, the harder 6-cue condition produced more, not less, distractor interference.
The limits
What it doesn't show
Each experiment used small samples (16-20 students, and about 10 per group in Experiment 4), so between-experiment comparisons rely on modest power. The findings come from one artificial letter-search paradigm, and the authors caution that the level at which dilution happens may depend on the task's goals. The paper does not settle the load-versus-dilution debate; it argues that a more complex hybrid account is needed. The supplied text lacks the paper's introduction, so the full background review is not available here.
Key terms
- Perceptual load theory
- The view that distractors are processed only when the main task leaves spare perceptual capacity, so harder (high-load) tasks reduce distraction.
- Dilution account
- The alternative view that extra neutral items reduce distractor interference by competing with and degrading the distractor's representation, not by using up capacity.
- Congruency effect
- The slowing or extra errors when a distractor is associated with a different response than the target, used to measure how much the distractor was processed.
- Attentional focus
- The spatial extent of the region attention is spread over; items inside it receive more processing.
- Flanker-type task
- A task where people respond to a target while ignoring nearby distractors that can match or conflict with it.
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Quiz yourself
What does a larger congruency effect indicate?
Common questions
Why does the 6-cue result count against perceptual load theory?
Six possible target locations made search harder (slower responses), which should mean higher load and less distractor processing, but the congruency effect was actually larger in that condition.
Why did inverted letters not dilute the distractor?
Inverted letters share visual features with letters but do not activate letter identities, so they did not compete with the distractor letter's representation; this suggests dilution happens at the letter or category level.
What is the practical upshot for theories of attention?
Neither pure load theory nor pure dilution explains all results; how much each extra item matters depends on where attention is focused, its category, and whether it can be ignored using known target features.
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