In press: Psychological Science EXOGENOUS SPATIAL ATTENTION INFLUENCES FIGURE-GROUND ASSIGNMENT

In a hierarchical stage account of vision, figureground assignment is thought to be completed before the operation of focal spatial attention. Previous results support this account by showing that unpredictive, exogenous spatial precues do not influence figure-ground assignment, although voluntary attention can influence figure-ground assignment. However, in these studies, attention was not summoned directly to a region in a figure-ground display. In three experiments, we address the relationship between figure-ground assignment and visuospatial attention. In Experiment 1, we replicated the finding that exogenous precues do not influence figure-ground assignment when they direct attention outside of a figureground stimulus. In Experiment 2, we demonstrated that exogenous attention can influence figure-ground assignment if it is directed to one of the regions in a figure-ground stimulus. In Experiment 3, we demonstrated that exogenous attention can influence figure-ground assignment in displays that contain a gestalt figure-ground cue, suggesting that figure-ground processes are not entirely completed prior to the operation of focal spatial attention. Exogenous spatial attention acts as a cue for figure-ground assignment and can affect the outcome of figure-ground processes. In visual scenes that contain multiple objects, the ability to visually segregate objects from one another becomes critically important: Without isolating different objects, high-level visual processes such as object recognition would not know where one shape ended and another began. Figure-ground assignment processes allow shapes (figures) to be segregated from backgrounds. This visual process is important because figures form the basis of much visual processing—humans are more likely to recognize and act upon figures than backgrounds. Address correspondence to Shaun P. Vecera, Department of Psychology, E11 Seashore Hall, University of Iowa, Iowa City, IA 52242-1407; email: shaun-vecera@uiowa.edu. The visual cues that influence figure-ground assignment were studied extensively by the Gestalt psychologists (Bahnsen, 1928; Koffka, 1935; Metzger, 1953; Rubin, 1915/1958), who identified the stimulus characteristics that would allow one region to appear as the figure and another to appear as the ground. For example, Rubin (1915/1958) reported that in most cases a smaller region is generally reported as figure and a larger region reported as the ground. Bahnsen (1928) reported that symmetric regions were more likely to be perceived as figure than asymmetric regions. Convex regions are more likely to be perceived as figures than concave regions (Hoffman & Singh, 1997; Kaniza & Gerbino, 1976). Regions that fall below a horizon line are more likely to be perceived as figure than regions that fall above a horizon line (Metzger, 1953; Vecera et al., 2002). Figure-ground assignment is considered a preattentive process that operates in parallel across the visual field before focal attention processes (e.g., Julesz, 1984). What is the relationship between preattentive segregation processes and focal spatial attention? A straightforward account, endorsed by several theories, is a hierarchical stage account in which figureground assignment is completed before other high-level visual processes, such as focal attention or object identification (e.g., Biederman, 1987; Julesz, 1984; Kosslyn, 1987; Marr, 1982; Neisser, 1967). Under this account, spatial attention would not influence figure-ground assignment automatically because attention would not operate until figure-ground processes were completed. Voluntary, or strategic, attention might influence figure-ground assignment, but only when a figure-ground stimulus was ambiguous and either region could be perceived as figure equally. In this latter case, figure-ground processes would fail and allow voluntary spatial attention to resolve the perceptual ambiguity (see Peterson, 1999, for relevant discussion). Recent findings support these predictions and support the hierarchical stage view of segregation and attention. Baylis and Driver (1995; Driver & Baylis, 1996) demonstrated that exogenous (automatic) spatial precues do not influence figure-ground assignment, although endogenous Figure and Ground and Attention 2 (voluntary) precues do influence figure-ground processes. Exogenous and endogenous attention were distinguished by manipulating the predictability of a spatial precue. Previous research has demonstrated that uninformative precues tap exogenous attention whereas predictive precues tap endogenous attention (e.g., Jonides, 1981; Klein et al., 1992). Observers viewed ambiguous figure-ground displays in which either of two regions could be perceived as the figure (similar to the figure-ground stimuli shown in Figure 1). Prior to presenting the figure-ground display, a spatial precue was flashed briefly on one side of where the figure-ground display would appear. The figure-ground display then appeared for a short duration, followed by a blank screen. A test display followed the blank screen; the test display contained two shapes, and observers were asked to decide which of the two contours matched one of those from the figure-ground display. The direction of the test contours was manipulated so that the two contours would match either the cued region (Figure 1A) or the uncued region (Figure 1B). If the attentional precue influenced figure-ground assignment, then the precued region should be perceived as figure, and this region should be discriminated rapidly in the test display. ---------------------------------------Insert Figure 1 About Here ---------------------------------------Observers in Baylis and Driver’s study were faster to discriminate the test shape when the spatial precue predicted the region to be tested (endogenous precue), indicating that endogenous spatial attention influenced figure-ground assignment. When the precue did not predict the to-be-tested region (endogenous precue), spatial attention did not influence figure-ground assignment. Thus, voluntary attention influenced segregation, but this attentional influence did not occur automatically, consistent with a hierarchical stage view of segregation and attention. We ask if exogenous attention might influence figureground assignment if attention was allocated directly to the space occupied by one of the shapes in the figure-ground display. The study discussed above may have obscured the 1 Although these results support a hierarchical stage account, Baylis and Driver (1995; Driver & Baylis, 1996) do not attempt to argue for a strict serial stage relationship between figure-ground assignment and spatial attention. They do state that exogenous attention mechanisms do not influence segregation. effects of exogenous attention in two ways. First, the spatial precues appeared outside the figure-ground stimulus, thereby preventing attention from operating directly on a potential figure. Second, the figure-ground stimulus appeared after the precue; the abrupt onset of the figure-ground stimulus could have captured attention away from the precued location (Yantis, 1998; Yantis & Jonides, 1984). In Experiments 1 and 2, we demonstrated that unpredictive, exogenous spatial precues influence figure-ground assignment when attention is allocated within the boundaries of a potential figure.