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The human visual cortex starts its analysis of a visual scene with the extraction of low-level features, such as color, con-tour orientation, and spatial frequency; this process, which is performed by neurons with small receptive fields, occurs in parallel across the visual field. Psychologists call this feature-extraction phase preattentive because it is effortless and does not require attention (Julesz, 1981; Riesenhuber & Poggio, 1999a; Tovée, 1994). When people look around, they do not perceive a set of disconnected features, but rather experience coherent and unitary objects comprising many features; more-over, people are very apt in judging where in a picture one object ends and another one begins (Roelfsema & Houtkamp, 2011). The mechanisms responsible for feature grouping and segre-gation are only partially understood. Some studies have found that perceptual grouping is a time-consuming (Jolicoeur, Ullman, & Mackay, 1986) and attention-demanding process (Ben-Av, Sagi, & Braun, 1992; Houtkamp, Spekreijse, & Roelfsema, 2003). For example, studies on a curve-tracing task—in which participants have to decide whether two cues are on the same curve or different curves—have shown that reaction times (RTs) increase linearly with the distance between these cues and that grouping is both time-consuming and attention demanding (Jolicoeur et al., 1986; Pringle & Egeth, 1988). In this task, perceptual grouping of contour ele-ments is associated with the gradual spread of object-based attention over the curve (Houtkamp et al., 2003). These findings contrast with the popular view that perceptual grouping is highly efficient and does not require attention (Julesz, 1981; Treisman & Gelade, 1980). One mechanism that could produce efficient perceptual grouping is the convergence of visual attributes onto single neurons in higher areas in the visual cortex (Riesenhuber & Poggio, 1999b; Roelfsema, 2006; Tovée, 1994). These neurons are selective for object shape, which implies that they are tuned to groups of low-level features in specific spatial configurations (Hung, Kreiman, Poggio, & DiCarlo, 2005; Tanaka, 1993). Thorpe and his col