Discriminating contrast discontinuities: Asymmetries, dipper functions, and perceptual learning

Abstract

In a visual search task, a target has to be found among distractors. For two given elements A and B, the search difficulty can depend on which of the two elements is defined as the target, a phenomenon called search asymmetry. Here, we study to what degree an element's ability to 'win' in a search asymmetry depends on its absolute contrast (first-stage signal) and to what degree it depends on its contrast difference from the background (second-stage signal). One quadrant contained a target texture (2 × 2 Gabor patches of contrast ctg), and the other three quadrants contained distractor textures (2 × 2 Gabor patches of contrast cdt). These four 'foreground textures' were embedded in a background texture consisting of patches with contrast cbg. The task was to identify which quadrant contained the target. Quadrants are referred to as increments (foreground contrast cfg > cbg), or decrements (cfg < cbg). We found that the second-stage signal determines which element wins the performance asymmetry, i.e. it is easier to find strong increments (decrements) among weak increments (decrements) than vice versa. A comparison of our data with the prediction of the independent-processing model [Vision Res. 40 (2000) 2677] shows that the observed performance asymmetries are in general too large to be attributed to noise differences alone. Rather, asymmetries might reflect a global competition between salient elements. Moreover, performance asymmetries can reverse during practice. We characterize a dipper-shaped nonlinearity on the second stage: discrimination of increment (decrement) signals x and x + Δx first improves for increasing x, and then deteriorates. © 2001 Elsevier Science Ltd. All rights reserved.