Missed targets are more frequent than false alarms: a model for error rates in visual search

In many visual search tasks, reaction times (RTs) for target detection are measured as a function of display size. The corresponding error rates are usually low but increase with increasing display size. Missed-target errors are more common than false alarms. In recent models of visual search, the error rates were attributed to a premature search termination and error rates increasing with display size were interpreted as indicating a speed-accuracy trade-off and an underestimation of search times per item (obtained from RT slopes). A model is described in which errors occur as a result of imperfect rather than incomplete search (i.e., it is assumed that there are task-specific probabilities of categorizing a target or a distractor incorrectly). Signal-detection theory is used to show that the observed error rate properties can be attributed to an optimized decision strategy. "Corrections" of RT data are thus questionable.