B.1.2. Salience

Despite its widespread usage in psychological and neurophysiological literature, the term of salience received varying and sometimes vague definitions, and was sometimes even confounded with relevance (Fecteau & Munoz, 2006). Two characteristics of salience are usually recognized: that salience depends on bottom-up factors and that it is not a characteristic of an item alone, but of an item in a situation, in a context. Indeed, a red item could be salient when surrounded by green distractors, while being not salient, or much less, when presented among burgundy distractors. One definition of salience that might be usually accepted and still useful is the following: the perceptual characteristics of an item in its context that influences attention orienting regardless of the incentive and knowledge of the subject. This stresses the exogenous and contextual nature of salience, and avoids assumptions about the computations or underlying theoretical models. Most authors recognized that salience played an important role in orienting visual selective attention. Many models were furthermore founded on the concept of salience map (Koch & Ullman, 1985), representing merely and strictly the salience activities of the visual scene. These activities were then assumed to control orienting of the attentional focus. The first attentional shift after the display onset would be towards the location or item with the highest activity. Then, search was hypothesized to continue according to salience (e.g. Theeuwes, 1993, p. 111-112; see also Itti & Koch, 2000; Wolfe, 1994). Salience-map models usually claim that visual searches were serial, and that the attentional "spotlight" moved following the decreasing order of saliences. Müller and Van Mühlenen (2000; see also Klein & MacInnes, 1999) also showed that inhibition of return (Klein, 2000; Posner & Cohen, 1984) was involved in visual search. This process might be important in counteracting the activities of salient but irrelevant objects, to continue the search (e.g. Itti & Koch, 2000).