Spatial and temporal separation fails to counteract the effects of low prevalence in visual search

Abstract

Recent research has shown that, in visual search, participants can miss 30–40% of targets when they only appear rarely (i.e., on 1–2% of trials). Low target prevalence alters the behaviour of the searcher. It can lead participants to quit their search prematurely (Wolfe, Horowitz, & Kenner, 2005), to shift their decision criteria (Wolfe et al., 2007), and/or to make motor or response errors (Fleck & Mitroff, 2007). In this paper we examine whether the low prevalence (LP) effect can be ameliorated if we split the search set in two, spreading the task out over space and/or time. Observers searched for the letter “T” among “L”s. In Experiment 1, the left or right half of the display was presented to the participants before the second half. In Experiment 2, items were spatially intermixed but half of the items were presented first, followed by the second half. Experiment 3 followed the methods of Experiment 2 but allowed observers to correct perceived errors. All three experiments produced robust LP effects with higher errors at 2% prevalence than at 50% prevalence. Dividing up the display had no beneficial effect on errors. The opportunity to correct errors reduced but did not eliminate the LP effect. Low prevalence continues to elevate errors even when observers are forced to slow down and permitted to correct errors.

Keywords:

• Attention
• Visual search
• Low prevalence
• Airport screening
• Space and time

Acknowledgements

This research was supported by grants from the National Institute of Health (EY17001) and the Department of Homeland Security to JMW, a National Health & Medical Research Council/Menzies Foundation grant to ANR (No. 359244), and a University of Warwick Research Development Fund to MAK. We wish to thank Michael van Wert and Tom Barry for their assistance with data collection.