Inattentional blindness for a salient target in visual search: Finding a surprisingly easy target can be surprisingly hard

In natural search settings, observers often do not know how difficult it will be to find the next target. The next security threat or the next potential cancer in a lung CT might be quite salient or very subtle. In the lab, visual search experiments typically involve multiple successive trials of constant difficulty. This allows participants to anticipate the salience of the next target. How will observers respond if the current target salience suddenly deviates from the target salience of all preceding trials? It would be unremarkable to find that it is harder to detect a low salience target when the observer expects a high salience one. More interestingly, here we report that observers are impaired when they are surprised with a high salience target after a series of difficult searches. In Experiment 1, observers searched for a hard-to-detect O target (always present, compound search) among C distractors with small gaps for 32 trials. On the 33rd trial, the gaps of the C distractors were large, making the O target much more salient. Yet, search efficiency was considerably more inefficient on this surprise trial (147 msec/item) than on subsequent trials with the same high target-distractor dissimilarity (66 msec/item). In Experiment 2, observers reported the presence or absence of an O in a display with short presentation duration. After multiple hard trials, observers frequently showed inattentional blindness towards an unexpectedly salient target that was reported almost perfectly when the identical target was presented on repeated trials. Gaze data suggests that observers adopted a tightly focused attentional window during the initial, hard search. This strategy made them surprisingly ‘blind’ to targets that were unexpectedly highly detectable. The wrong attentional set may be one explanation for situations where we “look but fail to see” obvious stimuli.