Citation

Wolfe JM, Van Wert MJ. Curr. Biol. 2010; 20(2): 121-124.

Affiliation

Visual Attention Lab, Brigham and Women's Hospital, 64 Sidney Street, Cambridge, MA 02139, USA; Department of Ophthalmology, Harvard Medical School, 243 Charles Street, Boston, MA 02114, USA.

Copyright

(Copyright © 2010, Elsevier Publishing)

DOI

10.1016/j.cub.2009.11.066

PMID

20079642

PMCID

PMC2818748

Abstract

Target prevalence powerfully influences visual search behavior. In most visual search experiments, targets appear on at least 50% of trials 1-3. However, when targets are rare (as in medical or airport screening), observers shift response criteria, leading to elevated miss error rates 4, 5. Observers also speed target-absent responses and may make more motor errors 6. This could be a speed/accuracy tradeoff with fast, frequent absent responses producing more miss errors. Disproving this hypothesis, our experiment one shows that very high target prevalence (98%) shifts response criteria in the opposite direction, leading to elevated false alarms in a simulated baggage search. However, the very frequent target-present responses are not speeded. Rather, rare target-absent responses are greatly slowed. In experiment two, prevalence was varied sinusoidally over 1000 trials as observers' accuracy and reaction times (RTs) were measured. Observers' criterion and target-absent RTs tracked prevalence. Sensitivity (d') and target-present RTs did not vary with prevalence 7-9. These results support a model in which prevalence influences two parameters: a decision criterion governing the series of perceptual decisions about each attended item, and a quitting threshold that governs the timing of target-absent responses. Models in which target prevalence only influences an overall decision criterion are not supported.

Language: en