Attention demands of visual search.

The relation between attention demand and the number of items in the array (array size) was investigated by engaging subjects in a primary search task and measuring spare capacity at different points in time, with a secondary tone task that occurred randomly on half of the trials. The major variables in both tasks were array size (4, 8, or 12 letters) and stimulus onset asynchrony (SOA: -400, -200,0,200,400, and 600 msec). Subjects were able to perform the tasks quite independently, and most of the interference that resulted from nonindependence appeared in tone-task performance. The amount of interference (i.e., maximum tone reaction time) was independent of array size, but the duration of interference (i.e., the number of SOAs at which tone reaction time was elevated) increased with array size. The findings were interpreted as supporting unlimited-capacity models of visual search performance.