However, attention and eye movements can also be partly decoupled as indicated by covert attention shifts. Covert attention shifts can be several times faster than overt eye movements (Nakayama & MacKeben,
1989). Well-known metaphors for attention include the spotlight (Posner, Snyder, & Davidson,
1980), zoom lens (Eriksen & James,
1986), and the attentional-gradient model (LaBerge & Brown,
1989). In the spotlight model, covert attention moves in an analog fashion across the visual field using disengage, shift, and engage operations. The zoom-lens model extends the spotlight by a variable-sized focus. Several aspects of the spotlight metaphor have been questioned by later work. For example, the movement of covert attention is probably digital rather than analog, meaning that the spotlight is turned off at one location and turned on at the next without passing over intermediate locations (Chastain,
1992a,
1992b; Gersch, Kowler, & Dosher,
2004). Furthermore, the time to move attention between two locations is rather independent of the distance between the two locations (Eriksen & Webb,
1989; Sagi & Julesz,
1985). Although additional effects of object-based selection have been demonstrated in simple tasks (Duncan,
1984; Egly, Driver, & Rafal,
1994) as well as in scene perception (Malcolm & Shomstein,
2015), one critical feature of the spotlight model that has remained valid is that attentional selection is based on location, mirroring the importance of topological maps in the visuospatial processing stream. The attentional-gradient model emphasizes that the size and the concentration of attention can vary according to task demands. A further innovative feature is the possibility of several peaks, so that independent locations can be enhanced in parallel (e.g., see Engbert, Trukenbrod, Barthelmé, & Wichmann,
2015, for a computational implementation in scene viewing). This also includes the possibility of a ring-shaped allocation of attention as has been experimentally demonstrated (Egly & Homa,
1984; Juola, Bouwhuis, Cooper, & Warner,
1991). When the attention field gets large compared to the size of the target, introducing target location uncertainty, one observes a performance decrement with invalid cues (i.e., withdrawal of attention) but no enhancement with valid cues (Herrmann, Montaser-Kouhsari, Carrasco, & Heeger,
2010). In summary, covert attention can obviously be distinguished from overt attention as indicated by eye-movement execution, but it might still be related or equivalent to eye-movement planning.