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Visual search for rare targets: Distracter tuning as a mechanism for learning from repeated target-absent searches


Dr Daniel T. Levin, Professor, Department of Psychology and Human Development, Vanderbilt University, Peabody College # 512, 230 Appleton Place, Nashville, TN 37203-5701, USA (e-mail:


In the typical visual search experiment, participants search for targets that are present on half of the trials and absent on the other half. However, many real-world tasks involve targets that are present only occasionally. Given this, it is important to know how people deal with the problem of finding targets they have little experience with. One possibility is that they develop an awareness of the degree to which they have effectively completed a search through complex target-absent scenes. To test this hypothesis, we had participants complete two relatively long search tasks in which only a minority of trials included targets. Stimuli were cluttered real-world scenes, and targets were defined by category. We examined participants' ability to terminate search on the target-absent scenes based on an accurate assessment of scene difficulty. Scene difficulty was estimated by computing the mean correct-trial response time (RT) for each of the target-absent scenes across all participants. These group RTs were then correlated with each participants' individual correct-trial RTs for the same stimuli to assess the degree to which a given participant's search-termination times were correlated with those of the group. These correlations successfully predicted participants' target-detection success in both experiments. These experiments suggest that an integral part of visual search is the need to calibrate search behaviour to scenes of varying levels of complexity even when no targets are present.