Abstract
Some targets in visual search are more difficult to find than others. In particular, a target that is similar to the distractors is more difficult to find than a target that is dissimilar to the distractors. Efficiency differences between easy and difficult searches are manifest not only in target-present trials but also in target-absent trials. In fact, even physically identical displays are searched through with different efficiency depending on the searched-for target. Here, we monitored eye movements in search for a target similar to the distractors (difficult search) versus a target dissimilar to the distractors (easy search). We aimed to examine three hypotheses concerning the causes of differential search efficiencies in target-absent trials: (a) distractor dwelling (b) distractor skipping, and (c) distractor revisiting. Reaction times increased with target similarity which is consistent with existing theories and replicates earlier results. Eye movement data indicated guidance in target trials, even though search was very slow. Dwelling, skipping, and revisiting contributed to low search efficiency in difficult search, with dwelling being the strongest factor. It is argued that differences in dwell time account for a large amount of total search time differences.
Highlights
Visual search is the task of finding a target among non-target distractors
Before we ask how search efficiency in absent trials is determined by distractor skipping, we examine the evidence for attentional guidance by the target in target trials
Even though all three causes contributed to some degree, the central result is that search difficulty causes sizable differences in dwell times which in turn impact response time (RT), a result that is unexplained by popular models of visual search
Summary
Visual search is the task of finding a target among non-target distractors. Search is difficult and the target is found only after inspecting a number of distractors. Search efficiency is a formal characterization of poor and good performance in visual search experiments where the number of non-target stimuli in the display is varied (variation of set size) and response time (RT) for correct decisions on target-absent or target-present is measured. Search efficiency is measured in ms/item, which is given by the slope of the RT/set-size function. Search may be inefficient with a positive slope of the RT/set-size function, indicating that adding more items increases RT. Search efficiency is a continuum, and may assume any number between slightly negative slopes up to more than 100 ms/item per item in target trials (Wolfe, 1998)
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