Performance asymmetries in visual search demonstrate failure of independent-processing models

We report psychophysical data from orientation-popout experiments that are inconsistent with a rather general decision model. Stimuli consisted of 121 line segments arranged on an 11 x 11 grid. There were two tasks: in the 1- Singleton Task all lines except one had the same orientation, and observers had to report which quadrant contained the singleton. In the 3-Singleton T ask three quadrants contained orientation singletons and observers had to i dentify the quadrant without singleton. These tasks can be Viewed as asymme tric search tasks, in which either a singleton-quadrant has to be found amo ng three homogeneous quadrants, or a homogeneous quadrant has to be found a mong three singleton-quadrants. Using tools from signal-detection theory we show that the large performance asymmetries between 1-Singleton and 3-Sing leton Tasks are inconsistent with any model that makes two (Very basic and common) assumptions: (1) independent processing of the four quadrants and ( 2) an ideal-observer decision. We conclude that at least one of the two ass umptions is inadequate. As a plausible reason for the model failure we sugg est a global competition between salient elements that reduces popout stren gth when more than one singleton is present. (C) 2000 Elsevier Science Ltd. All rights reserved.