Previous research has shown that visual attention can operate in, at least, two ways: one space-based and another object-based (see Scholl, 2001; also Yantis and Serences, 2003, for comprehensive reviews). Visual attention operates on a spatial representation of the visual scene (e.g. Posner, 1980; Eriksen and Yeh, 1985) and also on objects or perceptual groups previously segmented on the basis of gestalt principles (e.g. Duncan, 1984; Yantis, 1992). Evidence for the spatial selection comes mainly from spatial cueing experiments (Posner, 1980; e.g. Blanco and Soto, 2002; Downing, 1988; Muller and Findlay, 1987) and from experiments using the flanker paradigm (e.g. Eriksen and Eriksen, 1974; Eriksen and St. James, 1986; Eriksen and Yeh, 1985). Evidence for object-based attention comes from a great variety of experimental procedures: selective looking (Neisser, 1967), divided attention to object attributes (e.g. Duncan, 1984), cued detection and discrimination tasks (e.g. Brawn and Snowden, 2000; Soto and Blanco, 2004), visual search (Soto, Heinke, Humphreys, and Blanco, 2005; Yeshurun, Kimchi, Sha'shoua, and Carmel, in press), dissociations in neurological patients (e.g. Egly, Driver, and Rafal, 1994; see also Rafal, 1997, for a review), flanker of response-competition experiments (e.g. Kramer and Jacobson, 1991), inhibition of return (e.g. Tipper, Driver, and Weaver, 1991), negative priming (e.g. Tipper, Brehaut, and Driver, 1990), visual illusions (e.g. Hikosaka, Miyauchi, and Shimojo, 1993), attentional blink (e.g. Raymond, 2003), and binocular rivalry (Mitchell, Stoner, and Reynolds, 2004). Nowadays there is a general agreement about existence of object-based effects on visual selection (see Scholl, 2001), but not regarding how many objects can be attended at a time. Results from well-known experiments on divided attention to object attributes of Duncan (1984) and others (Baylis and Driver, 1993; Behrmann, Zemel, and Mozer, 1998; Kim and Cave, 2001; Lavie and Driver, 1996; Watson and Kramer, 1999; though see Davis, Driver, Pavani, and Shepard, 2000, for contrary evidence) suggest that attention can select only one object at a time. However, a different conclusion comes from experiments with the multiple-object tracking (MOT) procedure developed by Pylyshyn and Storm (1988). In MOT experiments, observers must attentively track a number of identical objects moving independently and unpredictably among a larger set of identical moving objects. The results suggest that observers could track a maximum of about 3 of 4 moving objects (Pylyshyn and Storm, 1988; Pylyshyn, 1989; Scholl and Pylyshyn, 1999; Scholl, Pylyshyn, and Feldman, 2001; see also Yantis, 1992).