Space-based theories of visual attention fall into two broad categories, unitary and distributed. Among the unitary models/metaphors for spatial attention, the most popular has been the Spotlight Metaphor. The Spotlight Metaphor compares attention to a spotlight with its “beam” covering an area of space; objects within the “beam” of the spotlight are more effectively processed than those outside (Posner,
1978; Posner, Snyder, & Davidson,
1980). The spotlight of attention was found to be variable in area, depending on the task to be performed (Jonides,
1983; LaBerge,
1983). Eriksen and Yeh (
1985) proposed that attention resembled a zoom lens more closely than a spotlight; the power of the lens was inversely related to the area over which attention was spread. That proposal was more directly tested and confirmed in Eriksen and St. James (
1986). Subsequent studies showed that not only is the area of the spotlight variable, its shape can be dynamically varied as well (Pan & Eriksen,
1993; Juola, Bouwhuis, Cooper, & Warner,
1991; Eimer,
1999). For example, attention can be directed to an annular region of space, while excluding the region enclosed by the attended region (Egly & Homa,
1984; Eimer,
1999). However, other behavioral and electrophysiological studies suggest that these results have limited generality and attention cannot be arbitrarily allocated across the visual field (Posner et al.,
1980; Eriksen & Yeh,
1985; Kiefer & Siple,
1987; McCormick & Klein,
1990; Heinze et al.,
1994; McCormick, Klein, & Johnston,
1998). In addition, Eriksen and Yeh (
1985) found that if observers were asked to attend simultaneously to a primary location and a secondary location, the time they took to respond to a target at one of these locations was consistent with the observers first directing their attention to the primary location, and then, if necessary, shifting their attention to the secondary location, suggesting that observers were able to attend to only one location at a time. Moving the focus of attention from one location to another has been suggested to be an analogue process (Shulman, Remington, & McLean,
1979). This process is believed to involve three separate phases: the disengaging of attention from the first location, the moving of attention to the second location, and then the reengaging of attention at this location; and these three phases are believed to be controlled by three different anatomical regions of the brain (Posner & Petersen,
1990).