It is commonly believed that objects viewed in certain contexts may be more or less salient. Measurements of salience have usually relied on asking observers "How much does this object stand out against the background?". In this study, we measured the salience of objects by assessing the distraction they produce for subjects searching for a different, pre-specified target. Distraction was measured through response times, but changes in response times were not assumed to be a linear measure of distracting potency. The analysis rested on measuring the effects of varying disparities-in size, luminance, and both-between a target object, a key distractor, and other background items. Our results indicate: (1) object salience defined by luminance or size disparity is determined by the ratio between its defining feature value and the corresponding feature value of background items; this finding is congenial to Weber's law for discrimination thresholds. (2) If we define salience as the logarithm of a feature value ratio, then salience increases approximately as fast due to increase in area as due to increase in luminance. (3) The sum of salience arising from object-background disparity in both size and luminance is larger than their vector sum (orthogonal vectors), but smaller than their scalar sum. (c) 2005 Elsevier Ltd. All rights reserved.

When a visual search task is very difficult (as when a small feature difference defines the target), even detection of a unique element may be substantially slowed by increases in display set size. This has been attributed to the influence of attentional capacity limits. We examined the influence of attentional capacity limits on three kinds of search task: difficult feature search (with a subtle featural difference), difficult conjunction search, and spatial-configuration search. In all 3 tasks, each trial contained sixteen items, divided into two eight-item sets. The two sets were presented either successively or simultaneously. Comparison of accuracy in successive versus simultaneous presentations revealed that attentional capacity limitations are present only in the case of spatial configuration search. While the other two types of task were inefficient (as reflected in steep search slopes), no capacity limitations were evident. We conclude that the difficulty of a visual search task affects search efficiency but does not necessarily introduce attentional capacity limits. (C) 2004 Elsevier B.V. All rights reserved.

We studied the effects of expectation and repetition on searching for singletons in very brief displays. In Experiment 1, we found that when a given feature defined the singleton for a whole block-so that in every trial the subject could expect a particular target feature-search accuracy was significantly higher than when the feature setting was randomly redetermined from trial to trial. However, an unexpected repetition triggered almost no advantage. In Experiment 2, we found no advantage for expected alternation. Experiments 3 and 4 demonstrated that there is little or no advantage for perception in conditions allowing only for target-feature facilitation or distractor-feature inhibition. We propose that in singleton search a division of feature space facilitates detection, and that this division works best under conditions of expected repetition. Experiment 5 replicated Experiment 1, but in Experiment 5 we examined response times and long display exposures. Results suggested that previous findings of singleton priming reflect mainly postperceptual factors.

Previous researchers have proposed that there are two types of symmetry detection: one based on crude preattentive symmetry judgments and another based on detailed scrutiny of individual parts (Barlow Reeves, 1979; S. E. Palmer Hemenway, 1978; Royer, 1981). Four experiments were conducted to examine capacity limits in different symmetry judgments. Observers were required to discriminate between random patterns and approximate symmetry (Experiments 1 and 3) or between perfect and approximate symmetry (Experiments 2 and 4). The patterns were divided into two sets of dots, presented either simultaneously or successively. A comparison of accuracy under these two presentation conditions suggested that symmetry detection involves an analysis that is spatially parallel but coarse, regardless of either task difficulty or task type (detecting symmetry vs. detecting asymmetry).