A common assumption made by most current models of visual search is saliency summation: Salience is computed in parallel in separate visual dimensions and is then summed onto a master salience map that guides the allocation of focal attention (see, e.g., Found & Müller, 1996;Gao, Mahadevan, & Vasconcelos, 2008;Itti & Koch, 2000;Koch & Ullman, 1985;Müller, Heller, & Ziegler, 1995;Wolfe, 1994;Wolfe, Cave, & Franzel, 1989). However, although it is by now a standard assumption, the empirical evidence for saliency summation is rather sparse (Krummenacher, Müller, & Heller, 2001Nothdurft, 2000;Poirier, Gosselin, & Arguin, 2008), and there are alternative (serial, parallel-independent, or parallel-interactive) processing architectures with various stopping rules (selfterminating or exhaustive search). The aim of the present study was to strengthen the support for the assumption of saliency summation, vis-à-vis the alternative accounts.
Saliency Summation ModelsIn saliency summation models (see, e.g., Itti & Koch, 2000;Koch & Ullman, 1985;Treisman & Gelade, 1980;Wolfe, 1994), feature contrast is a measure of how different a specific location in the field is relative to its surrounding locations with regard to a particular feature. For example, with a red vertical bar surrounded by green vertical bars, feature contrast is high for "red" and low for "vertical." According to saliency summation models, feature contrast signals are pooled into dimension-specific maps (for an overview of dimensions in visual search, see Wolfe, 1998, andWolfe &Horowitz, 2004) and are then summed into a supradimensional saliency or master map of the visual field. Activation at any location on the master map signals the presence of local feature differences, without providing information about the critical dimensions or features that give rise to these differences.
Ludwig-Maximilians-Universität München, Munich, GermanyInfluential models of visual search assume that dimension-specific feature contrast signals are summed into a master saliency map in a coactive fashion. The main source of evidence for coactivation models, and against parallel race models, is violations of the race model inequality (RMI;Miller, 1982) by redundantly defined singleton feature targets. However, RMI violations do not rule out serial exhaustive (Townsend & Nozawa, 1997) or interactive race (Mordkoff & Yantis, 1991) architectures. These alternatives were tested in two experiments. In Experiment 1, we used a double-factorial design with singleton targets defined in two dimensions and at two levels of intensity, to distinguish between serial versus parallel models and self-terminating versus exhaustive stopping rules. In Experiment 2, we manipulated contingency benefits that are expected to affect the magnitude of redundancy gains and/or RMI violations on the assumption of an interactive race. The results of both experiments revealed redundancy gains as well as violations of the RMI, but the data pattern excluded serial-exhaustive and interactive race models as possible ...