Spatial coding for the Simon effect in visual search.

Two experiments were conducted to examine the Simon effect (i.e., faster responding when irrelevant stimulus location corresponds with response location than when it does not) in visual search tasks. The search items were arranged in a 4 x 4 grid, and grid locations were coded into sets of four, two involving inner columns and two involving outer columns. In experiment 1, three different types of inefficient search tasks were used. The Simon effects were shown to be larger when the target appeared in one of the outer columns than in one of the inner columns ("laterality effect"). This pattern of results was not observed when distractors were absent, suggesting that the laterality effect depends on the operation of selective attention. In experiment 2, a pop-out search task was used, and no significant effect of target location on the Simon effect was found. Interpretations of the results based on the attention-shift account and referential-coding account were discussed.