Cognitive psychology: rare items often missed in visual searches

Abstract

Our society relies on accurate performance in visual screening tasks--for example, to detect knives in luggage or tumours in mammograms. These are visual searches for rare targets. We show here that target rarity leads to disturbingly inaccurate performance in target detection: if observers do not find what they are looking for fairly frequently, they often fail to notice it when it does appear.

Figure 1

Stimuli: Observers searched for tools in displays with semi-transparent objects placed randomly on a noisy background.

Figure 2

Figure Two: the effects of target prevalence on search performance. 2a: Error data: When targets were rare (1% prevalence - black bars) observers made more than 4X times the errors made when targets were common (50% prevalence - white bars). Data are averages of 12 observers. Error bars show +/− 1 s.e. for those 12 error rates. Gray bars show 10% prevalence results. 2b: Left: Reaction Times for 50% prevalence. Typical reaction times are longer when the target is absent (open symbols) than when targets a present (closed) Miss error RTs are shown by diamonds. 2c: Right: Reaction Times for 1% prevalence. However, when prevalence is low, observers make “absent” responses that are faster than the “present”. This leads to elevated error rates. For 1c&d, error bars (s.e.) fall within data points.