The prevalence effect is determined by past experience, not future prospects.

Area of Science:

• Cognitive Psychology
• Visual Search

Background:

• The prevalence effect describes a high miss rate when targets are infrequent, observed in tasks like airport security screening.
• This effect's origin—whether top-down control or bottom-up priming—remains debated.

Purpose of the Study:

• To investigate whether the prevalence effect stems from voluntary top-down control or implicit bottom-up priming.
• To determine the influence of actual versus instructed target prevalence on miss rates.

Main Methods:

• Laboratory experiments simulating X-ray baggage search tasks were conducted.
• Target prevalence was manipulated both by actual frequency and explicit instructions.
• Trial-by-trial prevalence cues (background color) were used in some experiments, with constant and mixed blocks.

Main Results:

• Miss rates were solely determined by the actual prevalence of targets, irrespective of instructions.
• Prevalence significantly impacted miss rates in constant blocks (all high or all low prevalence).
• Prevalence had no significant effect on miss rates in mixed blocks (randomly varied prevalence).

Conclusions:

• The prevalence effect is primarily an outcome of implicit, bottom-up priming from past experience.
• Top-down control related to future prospects does not significantly influence the prevalence effect.
• Understanding this effect is crucial for optimizing search performance in real-world scenarios.