Search

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.

People often have difficulty detecting visual changes in scenes, a phenomenon referred to as 'change blindness'. Although change blindness is usually observed in pictures of objects that are not the focus of attention, it also occurs for attended objects in the real world. Here, we further explore the finding that many participants fail to detect the unexpected substitution of one conversation partner for another. We show that change blindness for a conversation partner occurs in a variety of situations. Furthermore, when tested with a photographic lineup following the change, participants who noticed the substitution showed better memory for both pre- and post-change experimenters than participants who did not detect the change. We conclude that change blindness in this case is associated with relatively ineffective or inaccessible representations of previously attended objects, and we contrast these results with others indicating that change blindness arises from a failure to compare the original and changed object.